 Welcome to the fifth lecture of Conference 35. It's my pleasure to present to you my colleague, Dr. Colleen Jax, who is an associate professor of biology in this college, and as it turns out, the very successful chairperson of the Nobel Conference 35. Thank you, Dick. The term telomere was first used by the famous fly geneticist Hermann Muller in 1940 to describe the ends of the linear chromosomes observed in plant and animal cells. Microscopic studies of broken chromosomes demonstrated that these telomeres had special properties, protecting the chromosomes from destruction by the cell. Much of Elizabeth Blackburn's scientific career has been spent in the biochemical characterization of telomeres to better understand their unique makeup and how the cell maintains their structure. Her work in this area allowed the development of the yeast artificial chromosomes used to clone the long segments of DNA necessary for genome sequencing and has provided tantalizing possibilities for new cancer therapies and for slowing the aging process. One publication referred to her as the Queen of the Telomeres in recognition of her groundbreaking work in this area. Dr. Blackburn obtained her bachelor's and master's degrees from the University of Melbourne in Australia and her PhD in molecular biology at the University of Cambridge, England, working in the MRC laboratory of Fred Sanger at the time that the DNA sequencing techniques were developed in his lab. While at Yale University as a postdoctoral fellow, Dr. Blackburn isolated the repeating DNA sequence that makes up the telomeres of the single-celled organism tetrahymina. She continued her telomere research as a molecular biology faculty member at the University of California, Berkeley, where she and coworkers isolated and characterized the enzyme telomerase, an unusual enzyme made of both RNA and protein and responsible for the maintenance of the telomeres. In 1990, she moved across the bay to the University of California, San Francisco, where she has served as professor and chair in the Department of Microbiology and Immunology since 1993. During her career, Dr. Blackburn has served on the editorial boards or as an editor of many scientific journals, organized international conferences, and still found time to develop program materials for winding your way through DNA at the Exploratorium in San Francisco. Her numerous awards, many of which are listed in your program, include the Eli Lilly Research Award for Microbiology and Immunology, an election to the Royal Society of London, the National Academy of Sciences, and president of the American Society of Cell Biology. Please welcome Dr. Blackburn, who will speak this afternoon on telomerase, Dr. Jekyll or Mr. Hyde. Thank you, Colleen. It's truly a delight to be here, to participate in this very stimulating, very exciting Nobel conference. I'm especially happy because this is also my very first visit to Minnesota. I don't count the time spent in the airport on the way through. So this has been a especially wonderful visit for me. Now, we've been hearing about enormous numbers of genes, the whole panoply of genes, that is presented by our genomes and the genomes of many other creatures. But this afternoon, I'm going to talk really only about very few genes, some genes, and I'm going to talk about a special part of the chromosome. In fact, not really a gene at all. Now, that special part is the telomere, and it's the end of the chromosome. And the genes and their products that I'll be talking about, focusing on are those of telomerase, which makes the DNA of the telomeres in the cell. And telomeres and telomerase profoundly affect how the chromosomes do their jobs. Now, we've been hearing about the chromosomes as the carriers of the genetic information of all the genes of our genetic blueprint. But the job of the chromosome is also to make sure that those genes get faithfully transmitted, all of them in their entirety, intact from cell to daughter cell and from generation to generation. Here depicted on the first slide, can you see that? Can you, is it going to gradually appear like the rising sun? Is this lightening up at all? Okay, good. Okay, so what's shown here on the left is the progress of a cell as it's dividing. And I just put one chromosome in it for demonstration purposes. And the chromosome has two long arms filled with lots of DNA and a little nipped in waste called the centromere, which I'll describe briefly, and it has the two ends. And right away, even before this cell is starting to divide, a lot of evidence indicates that the telomeres help to hold the chromosome in the right places in the nucleus. And we don't understand what that accomplishes for the cell, but we just notice that that does happen. Now, all of that DNA has to become replicated if it's going to be faithfully passed on from one cell to its daughter cells. And so one of the things that the telomere has to do as I'll describe in a moment, is to ensure that the DNA is actually completely replicated all the way to the ends. So let's say the DNA has become replicated. The next job is for that now-duplicated chromosome to partition both of its copies correctly to each of what will become its two daughter cells on the bottom. And that involves the chromosome being pulled apart from those two little pieces shown here. They're called the centromeres, and I'm not going to say anything more about them, excepting a chromosome can only have one centromere and work properly in this process of pulling apart. And if you have two centromeres on the same chromosome, they get into a fight. And the chromosome can get ripped apart and other things can happen. And I'll return to that later in the talk. Now, of course, as you pull those chromosomes apart, they have to completely come apart. And the telomeres, of course, also have to separate in a timely manner, because now the cell is going to divide down the middle, and we don't want any DNA kept caught in the middle. So telomeres, it turns out, are acting in different stages in this cycle of cell division. So as I said, they position the chromosomes. They completely ensure its replication. They completely ensure proper separation of all of the chromosome as the chromosomes double. Now, this is just a very schematic cell. Let's look at a real cell, the way it would appear under the microscope after a suitable staining in one of the cells of our body as it's dividing. And remember, as we start from our very beginnings, all the way to growing up to an adult, well, of course, we know the cells have to divide and divide and become the approximate, the 100 million million cells of our adult body. And then a lot of our cells, as we're adults, also have to keep on multiplying. Think of the cells of our skin that's continually self-renewing of our intestinal lining, our hair follicles, our immune system cells that have to turn out new cells all through our lives. So cell multiplication and growth doesn't stop just because we've grown to our full adult size. And that's something that I'll be returning to over and over again in this talk. So here is our set, our blueprint, that set of genetic information. And those blue sausages are the human chromosomes. And they're shown here in a cell that's just replicated as DNA. So there's two copies, and you can see the two copies lying side by side. And the cell is just about to divide and send those two copies to the two newly forming daughter cells. One copy of DNA to one, one copy of the DNA to the other. Now the blue color actually comes from the staining of the actual DNA itself. And then in addition, using a very special molecular fluorescent probe, what's lit up at the ends of all the chromosomes as these exaggeratedly large yellow spots are the telomeres. And they're lit up by a molecular probe that goes in and seeks out and finds the telomeres themselves, and here they are. And one point that's very visually obvious is that all chromosomes have telomeres, the special probes find telomeres at both ends of every chromosome. And they're crucially important for the protection of these linear DNA molecules. Now before I forget to mention this, everything I'm going to say is about this great branch of organisms which are called the eukaryotes. It doesn't include the bacteria. And we don't know whether bacteria are smarter or dumber than this, but they have circular chromosomal DNAs and they don't have to mess with any of this telomere stuff, okay? So what I'm going to talk about applies to us and to a great many different creatures, but just remember, not the bacteria. Now of course we're very interested in this process of how our genetic material gets transmitted. Obviously we care about it as the human race, we want it to be transmitted from generation to generation. It's also very important that it get transmitted intact from cell to cell in those mini dividing cells that continue to divide in our bodies. One consequence of the failure of proper transmission of genetic material from cell to cell in our bodies is that such cells can become cancerous because genes are in the wrong balance for example. Other things can also cause cells to become cancerous, but this is one part of it. So the faithful transmission of genetic material also is important in cells not becoming cancerous. Now I'm going to tell you a lot of complicated science. It's not going to be too complicated, but some science, but I'll give you a take home message right now. You need the telomere to keep the chromosome from being lost. In fact, we can say lose the telomere, lose a chromosome and that's pretty accurate. So what I'm going to do in this talk is I'm going to talk first about the science of what we've learned about these very important chromosome parts, the telomeres. And I'm going to tell you some very new results that really have given us a much better picture, a better way of thinking about telomeres and what it is they're doing. And then I'm going to speculate and I really hope I provoke you to think about, I'm going to speculate on what we can project from the current state of knowledge and I'm going to get you up to that current state of knowledge. What can we project about how using that knowledge might impact on our lives? Okay, so our lives, as I say, we hand genetic information from generation to generation. I like this picture, because it's my husband and our son on a beach and I guess some genetic information has been handed on. They're certainly both very interested in digging and I don't know what sort of behavioral characteristic that tells us about. But they also look very different from this creature. Now this creature, which looks as if it might possibly have just come in from outer space, has probably been on our planet for many, many billions of years, evolving to what it considers its state of highly evolved perfection. And this is actually a creature that lives in, of all things, pond scum. So it's very tiny. I've shown it here as a scanning electron micrograph. Maybe to help think of the scale, you could think of it as several hundred of this of these creatures could fit on the head of a pin, okay? This single cells, they lived in pond scum, they're related to that more familiar, slipper-shaped pond organism called paramecium that some of you might remember from beginning biology classes. And it was in this creature who looks awfully different from us that we discovered the molecular nature first of telomeres and then also discovered telomerase. And that was for technical reasons. Basically, the essence being that this creature happens to have a lot of telomeres in its tiny cell and I was interested in those. So you might ask, well, what does looking at this rather far out looking pond scum creature tell us about us? And so I was reminded by what was said by one of the scientists participants speaking on the first, at the first Nobel conference held here at Gustavus Adolphus College in 1965. This was the conference on genetics and the future of man. And we've come, as we've talked about earlier in this conference, we've come full circle again thinking still about this. To the extent to which we learn how to manipulate genetic change in microorganisms, we should also be able to do so with higher multicellular organisms, including man. So figuring out what telomeres and telomerase are good for in this lowly pond scum creature, it turns out has really greatly informed us about what they do in us. And furthermore, what we might do, which might impact on human health and maybe human lives. So I mentioned before the many functions of telomeres. I'm going to express them in a somewhat different way. You can really distill them all into these two statements that the major roles of telomeres are to protect or to cap chromosome ends. And they also have to ensure the complete replication of the very ends of the chromosomes. And it's this idea of a cap which we've much more recently put a very sort of molecular set of understandings to. So this idea of a cap is a very critical one in telomeres and how telomeres work. Now, telomeres function as a reservoir of replenishable DNA. And that actually relates more to this second function, the complete replication of the chromosomal ends. And here I've depicted what happens in very diagrammatic form to the telomeric DNA as a cell goes through a series of cell divisions. So that's shown in that downward arrow here. Now, the blue just represents the outermost bit of some chromosome. And the red is the telomeric DNA, which I'll describe in a little more detail in a moment for you. And as the cell keeps on dividing and replicating its chromosomal DNA, the telomeres, the red DNA, doesn't stay the same. It changes. It gets a bit longer. It gets a bit shorter, longer, shorter. But on average, it doesn't get lost. And in fact, that cell is able to keep on dividing pretty much indefinitely barring other accidents. So let's look a little more closely at this DNA. What is its nature? So up in the top, I've depicted a chromosome in blue, so the little red, little waist in the middle, the centromere. I can't see this thing pointing properly. And I guess it's going to get focused a little better. Thank you. Okay. And then we've blown up the very last portion of that chromosome. It's not all quite to scale here. And what this DNA is made up of is very, very simple DNA sequences. Now in Tetrahymena, it happens to be the building blocks T, T, G, G, G, G. And they're repeated over and over to make up a stretch of a few hundred bases of these very simple repeat sequences. In humans, it's just one tiny chemical change difference, one base difference. One G is now an A in Orvalchromosomal telomeres. So there's a very tiny difference here. And I put some other examples, slightly more exotic examples, of the DNA sequence that's found repeated over and over again at the ends of various yeast chromosomes, which have turned out to be really great molecular, sorry, organism models for studying telomeres. But the point is that they're very simple sequences. Contrast that with the wealth of information that's contained in the sequences in the genes. And I like to think of these sequences as they're veritable symphonies of information, huge levels of complex, marvellous information is contained in the genetic blueprint of the genes that are making the RNAs and the proteins of our cells and the products that then arise from those, right? And here's the telomere, which in contrast to a symphony, it's a little bit like somebody who's singing the first line of Mary had a little lamb over and over and over again, right? Very simple. So what's the power here? The power is the repetitiveness that that highly repeated sequence in effect is a very, very concentrated set of landing pads on which special proteins attracted to that particular sequence and no other combined and because they combined in repeated numbers they set up structures that function to make the telomere do its job. So the actual DNA sequence of the telomere looks disarmingly simple, but it's that concentration of repeats over and over again that allows many proteins, the same ones over and over again to come together and make higher order structures. And I'm not going to talk in detail about those proteins because we're really just going to focus on the DNA but it's that the fact that the DNA does that is what makes the telomeres do their thing. And we discovered that these sequences are made in a very unusual way. Now, almost all of the rest of the DNA of the chromosome, in fact in any normal cell cycle, all of it is made by copying the DNA of the pre-existing parental chromosome. And as you probably all know, the two strands of the double helix come apart and each parental DNA strand is copied into another strand using each parental strand as a template to make the new strand. So you end up again with two perfectly duplicated DNA strands. However, these enzymes that do this job which are a marvel of complex and well-regulated cellular machinery, the enzymes that copy all the rest of our genes have a fatal glitch, a fatal flaw in them. They're built so they cannot copy the very, very ends of the linear chromosomal DNAs. It just, nature happened, it worked that way. The machines got built in that way and they just can't completely replicate the ends of the chromosomes. And so instead of telomeric DNA being both decided and the sequence being decided and being made by the process of copying DNA into DNA, telomeric DNA sequences decided in a very different way by a very special enzyme. And this was discovered in the 1980s by me and my graduate student, Carol Greider when we were then at Berkeley and we discovered this enzyme which we called telomerase and whose job, at least in part, is to add extra DNA, the telomeric repeats, to the ends of the chromosome. So I'm going to show you a kind of a diagrammatic depiction of this and condensed in this is really a great deal of information about telomeres but I'll just tell you the most and telomerase, I'll just tell you the really salient points. So this blob of red and this purplish line with some bases in it, that's the telomerase ribonuclear protein enzyme made of RNA and protein and over on the left and running partly across the telomerase is the end of the chromosomal DNA. Now before I go into the mechanism of what telomerase is doing, just let me tell you a little bit about this end. Now the DNA of the chromosomal end is mostly double-stranded and you can see I've drawn the two strands up here but the top strand, the one with the G's and the T's in it sort of protrudes a little bit from the end. And it turns out that this particular region of the chromosome, the very, very end is an extraordinarily important part of the chromosome and I want to give you a kind of a scale for this. So if we look at this as you project it on the screen and you can see that little sequence out at the end that I've shown with those few T2G4 repeats sequence that's a certain size projected on the screen and my back of the envelope calculation is that if you drew the whole chromosome on the same scale, it would be about a thousand miles long. So it's this tiny, tiny little bit of material at the very end of the chromosomal DNA that basically will determine whether this chromosome is going to be able to do its job. So all these wonderful genes are for naught if something goes wrong at this very, very tip of the chromosome because then the chromosome won't be able to be transmitted properly. Okay, so what does telomerase do? It adds DNA to the end of the chromosome. It copies a small region of bases shown as those A's and C's in the large red blob which is depicting the telomerase enzyme. It's a small region of bases within a larger RNA that's a part and parcel of the telomerase enzyme and it simply places the DNA tip on corresponding bases on that little tiny template and then incorporates one at a time the G's and the T's copying the C's and the A's respectively in the template and that's what it does. It's not exactly your Einstein of polymerases. This is not a huge amount of genetic information to be copying but it can copy it over and over and over again and make repeated sequences. In fact, if you look at it, you can see that the last two bases, TTG that gets put in, if you now took that three prime end, that right-hand end and moved it over, you could align it again with the AAC on the left of the template and do another cycle of copying and that's what happens. It gets copied over and over again. So that's the simple view of what telomerase does but I just want to take a moment and tell you about a couple things we found out about this enzyme because they were very surprising and interesting to us and I don't really know deeply what it all means yet but first of all, I've said telomerase is a catalyst. It's like other enzymes that has a job that does polymerization, polymerizing DNA to the ends of chromosomes but this is actually very surprising because I just said that DNA usually gets copied into DNA and in fact there's what used to be called the central dogma of molecular biology that DNA was only copied into RNA whose information was then decoded into proteins by the translation machinery of the cell and in fact it was thought that this process by which RNA, those A's and C's here, were copied into DNA which is a property of reverse transcriptases. It was thought that that was a property that was the purview of things that we don't like a whole lot. Retroviruses of which the most notorious is HIV which causes AIDS. So it was thought that this process of RNA being copied into DNA was somehow associated with the more pathological aspects of biology if we take it from our human centric point of view. So it was a very surprising thing to find an enzyme that is a normal part of our healthy dividing cells, not all of them as I'll describe but in those cells where there is telomerase it's using this process of reverse transcriptase. So first of all reverse transcriptases are a normal part. This reverse transcriptase anyway is a normal part of our cells and indeed all eukaryotic cells. Now the second point that I just quickly will allude to is that I've shown you the part of the RNA that carries out the function called the templating. It's simply copied into complimentary DNA but that's not all that the RNA does and by making a lot of changes in the RNA one base, nucleotide at a time and then examining the effects on how this enzyme does its job as an enzyme we found out that the RNA and the protein are collaborating together to make this enzyme work. The RNA is not just simply providing a simple tiny passive template it actually stands up and gets noticed and in fact it probably helps build up that enzymatic active site along with the protein residues that are the essential part of this enzyme as well. So that's just a curious thing and that's interesting because such RNA containing enzymes are not all that frequent in the modern world but they do appear in certain essential components such as the ribosome that does the decoding of the genetic information and turns it into the string of amino acids, the proteins that Lee Hood talked about this morning. That looks as if it's again a collaboration in which RNA and protein pay critically important roles as opposed to the normal situation where usually it's proteins by themselves that do much of the enzymatic work of enzymes. And these kinds of things are thought to be perhaps hallmarks of what was an earlier form of life in which RNA was a much more predominant catalytic component of what we might think of as pre-life. And so one thinks of these things as perhaps being molecular relics of very ancient forms of life and curiously enough telomerase is one of these things. So that to us has made it a very interesting enzyme but I don't know what the deep if any meaning of that is. Well, anyway, fundamentally life forms are all alike and I've been talking interchangeably about telomerase in this particular situation and I've drawn the specifics here for tetrahimina because that's where we discovered telomerase but it wasn't very surprising when telomerase was then found in all sorts of other eukaryotic creatures as well and that included ourselves. Okay, so before I talk about telomerase and telomeres in us, I just want to tell you a little bit about some of the recent science because we're really changing many of our ideas about what it is that's important about telomeres and what they're doing. Let me just tell you about one simple kind of experiment which was done to see if we could learn more about how this enzyme was working but what it ended up doing was telling us something about what telomeres do. Now, what it did was it emphasized the importance of having not just any telomeric DNA but it had to be the right one. That means the specific one for that organism whose proteins have evolved to recognize just that sequence and not other ones and what we did was we made use of the fact that the RNA of telomerase is copied into the telomeric DNA. So you can predict that if you made a little change in that tiny sequence in the RNA that's copied into DNA then the corresponding change would appear in the telomeric DNA repeats over and over that were made in the cell expressing that mutated RNA. And here are two specific examples. Here we've just made a single base exchange. Here we've actually just inserted a couple of extra residues and in each case we get a novel telomeric DNA sequence that is both predicted and is made in the cells. What does this do to the cells? To show you that I first of all have to show you what a whole lot of normal dividing tetraheminar cells look like. And this slide here shows you just a whole swarm of them under the microscope. Now we're not at such high magnification as in that scanning electron micrograph that black and white one I showed you. So each one of these big sort of bluish objects would be a whole cell. So that would correspond to that one whole cell that I showed you in the scanning electron micrograph. And the blue material is the genetic material, the DNA all bundled up in the nuclei of these cells. And so we can see a few cells here that are dividing and so here's a cell for example is dividing. It's nicely separating out its DNA. Now, if we make that telomerase which has a change in the template that causes a change in the telomeric DNA sequence, nothing wrong with the enzyme. The enzyme's working fine. It's churning out the sequence. It's just the wrong sequence. This is what happens. This is a fairly similar magnification. You'd find it hard to believe. And now these cells have become absolutely enormous. You can see there's enormous amounts of DNA now in that cells. It's become huge and swollen. The DNA has been bravely replicating and replicating and replicating but it just can't separate. And it gets stuck between the cells and they quite quickly cease dividing as you might imagine. If you go from looking like the previous slide to looking like this, these are pretty unhappy cells. Okay, despite their desperate attempts to keep them, they keep copying the DNA but they desperately trying to separate it and they can't do it. What's wrong with these cells? And I already, I think alluded to the fact they can't separate the DNA and we did very high resolution light microscopy on cells in which we'd done the same kind of experiment and found out that these cells got stuck by their telomeres. So let's look on the left of the normal progression of what happens as chromosomes separate. They, here we have replicated chromosomes. They get pulled apart by their centromeres and the telomeres normally pull apart in a nice timely fashion and the chromosomes come apart and then each of those chromosomes can go happily to its daughter's cell. But in our mutant what we found was that the telomeres were stuck together. The consequence of that is various. There are things that the cell can do in response to this. One is that one centromere just gives up and the DNA oldest ends up in one cell and that's probably what we were seeing a lot of in those very sick looking cells I just showed you. Another thing that can happen is that the chromosomes can get ripped apart as those centromeres are bound to the cellulule machinery that's pulling them apart and now we get new broken ends of chromosomes and those wreak a lot of havoc in cells. And we get different distributions of genetic material to different cells and so genic imbalances arise. So having stuck telomeres is very bad news for cells. So this was scientifically satisfying for us because that was telling us that very quickly having the wrong sequence at the end of chromosomes had a very deleterious fast effect on cells. At the molecular level we could look a little bit more closely. So here's depicted just the tract of DNA at the end of the chromosome and I've shown in should be green, the wild type situation this is find and then we've popped in this butated telomerase RNA gene. I've just depicted it as causing pink mutant repeats to be added to the chromosome end and even just a tiny bit of mutant DNA added to the end of these telomeres is enough for these cells or these telomeres to become what we call uncapped. And this idea of a cap is something that I'm really going to keep reiterating because we think that's very important for thinking about the telomeres. These cells don't lack telomeric DNA. It's the wrong kind of DNA at the end. It can't attract the right proteins that bind it to the end and make a telomere function as a stable end and what happens in fact is that the cell thinks that this is a broken DNA end. Now if there's one thing cells just hate it's broken DNA and they have all sorts of mechanisms to fix it but the end of a chromosome is a legitimate end shouldn't be fixed but the cell tries to fix it and it fixes with the nearest thing to hand which is another uncapped telomere and now you fuse those two chromosomes together. Bad news. So this is one way to make a chromosome behave as though it's functionally uncapped. The ends become very sticky because they're not capped and they fuse with each other. Now I've shown you sort of the worst thing that can happen. Cells also have another response before they get to this devastating stage of having fused their telomeres wrongly here this uncapped end can also do different things to a cell. The cells are not stupid they don't just blindly go into this pathway here. An uncapped end sets off an exquisite set of alarm signals in the cell. The cell is poised to detect DNA ends that shouldn't be there and it thinks this is one of these ends because it's going to try and fix them and normally that's good. If you've broken something inside a chromosome that's good to fix those two ends but it's a mistake to fix them here but nevertheless this end is appearing like a broken end and the chromosome sends a signal to the cell and the cell does certain things. One thing it does is it stops. It's division. It stops dead. The alarm has gone off. It says, wet. Gotta fix that end and if it's a normal inside end that's fine it'll fix it up. Then after a while it'll go on and divide anyway. And as I say sometimes if it's fixed the wrong thing then that's bad. The other thing that cells do in response to an uncapped end is also very interesting. They commit suicide. They literally eat themselves up in response to this alarm signal that went off because it said your genetic blueprint is imperiled. So it's reading this end as a broken DNA end and that says the genome is imperil. Stop everything. But sometimes the whole multicellular organism considers that it's best and I'm being very anthropomorphic and you have to forgive me about this but basically the idea is that it's thought to be better for the organism to just get rid of such a cell rather than to try what might become a hopeless or a dangerous job of trying to fix up something. And so the chromosome eats itself up and there's a special name given to that called apoptosis. But it is a response to an uncapped telomere end. Okay, so I told you all this and I've described the situation as we did it as an experiment in this model organism tetrahemina. But remember our 1965 Nobel Symposium speaker to the extent to which we learn how to manipulate genetic change in microorganisms we should be able to do so with higher multicellular organisms including man. And so of course the question is well can we do this in man? And in particular we've been asking recently can we do this in cells that we would like to have deleterious effects on and those are cancer cells. And the attraction of this particular idea is that it's fast. Very quickly a telomere becomes uncapped. You don't have to put very many mutant repeats on the end at all before it becomes uncapped and basically starts doing things like what we saw in the couple of slides back. So we're very interested to say can we use this kind of aspect of telomere biology that we've learned in these simpler systems like the pond organism tetrahemina. We've also learned similar things in yeast that I haven't told you about but we've synthesized information from that. Can we use that in a way that might be useful in some way for killing cells we don't like of which we put cancer cells pretty high up on the list. Okay, so this experiment, so that was an application idea but this experiment, this kind of experiment where you put different sequences at the ends of chromosomes. They don't always have to lead to disaster like this. And in fact they've been very, very informative for learning things about how telomeres work in the cell. And one of the most striking things is just how dynamic the structure is. And I've just drawn you in a very simple diagrammatic form. What happens at the end of the chromosomes? We sort of think of the chromosomal material as being relatively static. You make that DNA and it sits there. But at the end of the chromosome there's a continual building up and losing and building up and losing. And we can watch that process very vividly in real time if we make a mark in the template of telomerase. So I've shown it by the X here and that represents a base change in that template. And now they'll get copied into the DNA at the chromosome. So now the X is now in the chromosome end. First of all, just one X appears. And then quite quickly lots of Xs appear but the end of the chromosome may not change its length a whole lot. It gets a bit bigger, smaller and so on depending on the sequence. And so we can follow this dynamic cycle of addition of DNA by telomerase. And if you don't have telomerase, then DNA gets lost. So telomerase is always adding DNA that's getting lost because of this process of incomplete replication which is a consequence of the inability of the DNA replication machine to completely replicate chromosomes. So it's a continual adding and losing and adding and losing. Sounds very inefficient, doesn't it? You're just adding and losing DNA all the time. But I think cells have evolved mechanisms which allow them a lot of control and this mechanism is under exquisite control. It's remarkable what sort of lengths, to use a bad pun, the chromosome will go to protect the integrity of its telomeric DNA. Unless some nasty experimenter comes in and makes a mutation at the end of the telomere, repeats like I just showed you but that doesn't normally happen in nature. So normally the chromosome end keeps replenishing itself because it keeps attracting telomerase to itself. Now if you attracted too much telomerase then chromosomes would just get infinitely long. So this process is regulated and there are proteins that bind the telomeric DNA sequence and regulate the process. And I'm just going to show you in the most diagrammatic form possible how we think of that. So what I've shown here is the DNA stretch with the little red balls all over it and those are the telomeric repeats and they've each attracted a red protein to it. It binds that sequence and only that sequence. And then those proteins in turn attract pink proteins that attract other proteins, orange proteins and then the inner part of the chromosome is those blue blobs which is just the normal part of the chromosome. So we build up this higher order structure that began on the platform of DNA that had all those repeats that all these red proteins could sit down on. Now this big object at the end is telomerase. Telomerase happens to be a dimer protein, I didn't mention that before so I've shown it dimer. And it also, as I'll describe more it also is sitting at the end. Now telomerase and this DNA protein complex they sort of in a constant balanced dual here. The proteins are attracting the action of telomerase if the telomere gets short and if the telomere gets long then they don't welcome telomerase's action. So telomerase acts more often on short telomeres than it does on long telomeres. It's very clever because that's the way it can keep the length of the telomeres normally within bounds that are okay for the telomere to function, to form enough of a structure. So if you just get this a simple idea that the telomeric DNA is loaded with lots of proteins and the number of proteins if the telomere is short is going to be smaller and that makes telomerase very active at the telomeres but that makes the telomeres get longer. As it gets longer you get more and more places for the proteins to bind the telomere gets longer and now it damps down the action of telomerase. So we call this a telomere homeostasis and it's acting in cells in our bodies and particularly it's been, we and others have looked at it in cells for example of our immune system. Now remember I said that we have cells in our body that throughout adulthood have telomerase because they keep on having to multiply. We have to keep making more and more of these immune cells throughout our life. These are the cells some of which Lee Hood talked about this morning. You have to replenish our immune cells, keep them multiplying so they can respond to challenges of pathogens and other challenges. And so we know they have to have intact telomeres intact chromosomes and what's been very striking is that the length of these telomeres can actually change quite a lot throughout our lifetime but in ways that were not expected. Okay, now I'll get later to what had been expected but here's some actual data here and what this shows is the average telomere length and this is taken in lots of individuals of our white blood cells which are cells of the immune system and they can pick on telomerase anytime they want and they do and it's shown through our life span. So here zero is newborn and so newborn babies have fairly long white blood cell telomeres, okay. Now the next point shown is when, this is again lots of individuals and we've averaged them here, you haven't quite gone to kindergarten at this stage, okay. But look what's happened to your telomeric DNA it's plunged down, right. But you're fine. You haven't had any deleterious effect from this plunging down. It's just that the balance of the adding and the losing has been thrown off a little bit for some reason in that phase of your life, okay. So now you go to kindergarten and elementary school, middle school, high school, you around the time the last college tuition check is paid, we look at the telomeres again and they're not an awful lot difference. Now you did an awful lot of growing from preschool to this stage here as you did from newborn to preschool stage it's about the same amount of growing in fact the same amount of cell divisions are expected but the telomeres stayed very steady and then through a later life there is a gentle decline in the telomeric DNA length. So the point is that telomere length per se is a complex output of many things. Turns out in this case in the case of cells that have telomerase it's the total net vector if you will of the action of telomerase the proteins that are protecting the telomeres from too much action of telomerase and so forth. So we just get these lengths which frankly taken by themselves don't mean an awful lot. What actually really matters is that these telomeres are healthy and functional and so I've decided we have to have a new paradigm for thinking about telomeres and so I've taken the road sign terminology as our guide here, okay. Length is not really the issue much of the time. What really counts is that this telomere is quote capped and part of this capping that I refer to in this almost metaphysical sense in fact involves having the right proteins at telomeres and we've put a lot of names to the molecular components of the cap and I won't bore you with them but I can assure you that they're pretty concrete sets of proteins and so forth and they even include telomerase as I'll get to later, okay. So the important point is that the telomere is what's called functionally capped that means it has to have at least some minimal amount of telomeric repeats in the right proteins and so forth around, okay. So that's what happens in our bodies in terms of just one cell type. Now, we don't have telomerase on in many of our cells, okay. So now we're going to talk about the converse situation from what I've been talking about till up to now and that is what about when we don't have telomerase, okay. Now the prediction from many years of knowing about the DNA replication machinery and the fact that chromosomes couldn't be completely replicated predicted that as cells went through divisions their chromosomes would just become progressively shortened and shortened and shortened and eventually cells would cease to be able to divide because they would lose some essential gene from the end of the chromosome, let's say. So that was a theoretical idea and it's sort of right but in fact cells never get further than about say this third line here or so. In other words, telomeres shorten some distance in the absence of telomerase and then cells respond to that in various ways, okay. So by 1990 we knew that telomerase was absolutely essential for maintenance of telomeres again in our pond organism and the way we'd done that was to make a very tiny change in the telomerase and it was just a single base change in a particular part of the RNA that turned out to matter. Remember I said that the RNA and the protein collaborate to make the enzyme activity and either one of those little base changes I've depicted up the top here was sufficient to stop telomerase from functioning. The result of that was that the telomeres shown by these blocks depicting the repeats shortened down and after about 20 cell divisions the cells ceased to divide. Now, tetraheminar is normally effectively immortal. If you give it good food and don't treat it mean it'll just keep dividing and dividing and dividing essentially pretty much forever. So you can think of them as immortal cells and we made them into mortal cells. They stopped dividing after a while. Well that kind of thing has sparked a lot of interest. This is a graphical representation of this is the year and the bottom axis here and in the y-axis here has shown the number of publications on the subject of telomeres and you can see there's been this very large growth in the number of papers published on telomeres or having telomeres somewhere as their subject matter. So why? Why has there been such sustained and increasing interest in this? Well clearly because our bodies are not immortal. Now I did just say that the self-renewing parts of our bodies those cells that do have to keep dividing like immune system cells they do have active telomerase and they often have plenty of it and apparently they can keep on turning it on through life as far as we know. Also our germ cells have active telomerase well that's good because we'd like our species to keep on going. Cancer cells do often have telomerase and I'm gonna get back to that. But telomerase doesn't normally seem to stay active in all the cells of our body and in a very broad brushway we can represent the situation like this. As I said in germ cells and in cancer cells they have telomerase and they keep on dividing pretty much effectively forever if you will. But most somatic human cells not the immune cells, but most of the cells in our bodies actually don't have active telomerase as far as we can see. Now we also notice in such cells that if you take them out of the body and look at their telomere length over age remember these are not the cells without telomerase then the telomere is gradually shortened. And so a very striking experiment was done by Jerry Shea and Woody Wright at the University of Texas collaborating with some scientists in Geron Corporation in California and what they did was they said well what if we put active telomerase back or reactivate telomerase in cells that don't have it and which don't keep dividing indefinitely. So they did this with cells grown in the laboratory in dishes and I want to emphasize where these experiments have done. So in the lab in dishes when you put telomerase into these cells very strikingly they became mortal. So there's a sort of striking symmetry with the experiment we did with tetrahimina immortal cells turned off telomerase cells became mortal. Human cells, normal cells grown in the laboratory dish normally mortal put into telomerase become immortal. And so this is raised much very interested speculation and one of my favorites is a science fiction book written by Bruce Sterling it's called Holy Fire. I don't know if you can read it all but the essence of it is that this 90 year old woman decides that she will have this tavesin telomerase and trick her cells into becoming young again and then the story is all about what happens. Okay, well science fiction or is this a possibility? Now I'm going to get to that but I just briefly want to just tell you another little bit about the science of what I just described. What is it about these telomeres that are in cells that don't have telomerase? Now I'm going to pick the picture on the left. So here we have telomeres and they're normally capped and I've shown them sort of looking happy with little caps faces at the end, right? And then as they shorten as they divide then they become uncapped and that's the red star sending alarm signals to the cell and the cell stops dividing. That's its response and that's a good response to not having a capped telomere. And then on the right hand side is what we discovered happens even in normal cells when we put telomerase in them. And what we found is that the telomeres even though they have telomerase which is now shown as the little yellow cap with the visor and bill on it, that's got telomerase. But actually these telomeres can keep on shortening and they're still okay. So that told us something else about telomerase. It says its job is not just to extend the telomeres and make them longer. It actually physically seems to protect the end as well. The point is that even a very short telomere, not too short, I mean it can't be too short, can't be left of that red arrow here, but even pretty short telomeres if they have telomerase are okay and we say that they're capped again. So the wrinkle is that shortening telomeres in the absence of telomerase as I said become uncapped and the cells then respond by ceasing to divide. If you put telomerase in you can experimentally see situations in which the telomeres are absolutely fine. They're functionally capped, but their length is actually sometimes even less than the length of those telomeres on the left that were uncapped. It just says that length is not the only gain in town. You can't just look at length and deduce anything without knowing a lot of other things about the cells. So again, that applies again here. Now we're talking about not making weird DNA sequences but just shortening telomeres and largely length is not a big problem. It's whether you have a functionally capped telomere. So that's the sort of new wrinkle if you will on the science here. So what does this all mean here? I've depicted the situation and let's look at the right branch here. So here's this normal kind of cell I've shown you. It doesn't normally have telomerase and in at least some cell types, not all, but some at least adding to telomerase is sufficient to convert that cell to a perfectly growing seemingly normal cell as far as anyone can tell and it apparently can divide indefinitely. So we say it's become immortalized. And so that's the branch on the right of the slide here. Now, do we want all cells to be able to renew? And so this brings me to the Dr. Jekyll and Mr. Hyde nature of telomerase. Because there are cells, clearly we don't want to be renewing indefinitely and those are cancer cells predominant. Oh, well certainly one prominent example is cancer cells. So remember Dr. Jekyll and Mr. Hyde, they were the same person but they were manifestations of the good side and the evil side of the same person. And I think this is an interesting analogy to think about for telomerase in various ways. So okay, so let's think about telomerase in an experimental way. What we did was we looked at the effect of telomerase on cells that were not nice normal cells like the one stone on the right going to the green cells but cells that had taken a number of steps down that stairway to becoming cancerous. And it's only going to take a few more shugs before those cells become full blown cancerous. So what we did was we put telomerase into those cells and here's an actual experiment here, okay? Here's a growth curve. Now the cells that, I can't see, the cells that keep growing for a little while and then they cease to grow. So this is population doublings. This is number of days in culture. These cells have been made predisposed to be cancerous. We've put a virus oncogene into them and we've made them on the way to becoming cancerous cells and those cells cease dividing. And what's experimentally said is that they go into what's called crisis as they cease dividing. You probably can't see too well but those of them on the bottom, they're pretty unhappy. And these are the control cells for our experiment. And our experiment we put in a gene, it's called the H-tert gene, I didn't get into names but it basically kicks the dormant telomerase in the cell on. And now these cells just keep on dividing and those multiple cell lines, clonal lines very efficiently or 90% of the cell lines just keep on dividing indefinitely and they look fine. They're growing just fine. So what happened to these cells? Well we made them effectively, we think, immortalized. But what kind of cells are these cells? These are cells that are not good cells. So let's think about what's happening to their DNA. And this is really the Dr. Jekyll, mysticide side of telomerase or aspect of telomerase. If we look at the chromosomes of the cells that are in the process of ceasing to divide, we find that they have a lot of peculiarities and the arrows point to chromosomes that have fused their telomeres. Now what's the consequence of that? Cancer cells can get by with this because they have plenty of extra chromosomes much of the time. So they have the genetic material to keep on growing but these fused chromosomes are a hallmark of genome changes that happen in cells as they get more and more cancerous. So these cells are showing signs of what's called genomic instability of which this is just one manifestation and one type which is going to be likely to make them more likely to go down those steps towards full blown cancerousness. Strikingly at the very same stage in our control cells, here's just some examples and you probably can't look at them all in detail, but these are perfectly okay looking chromosomes and these are the ones that got the activated telomerase and they kept on dividing. So we save their bacon, but wait a minute, you know whose bacon did we save here? Okay, so let's think about this because there are two opposite implications in terms of progression to cancer. The first thing that telomerase did was that it allowed a population of cells that are prone to cancerousness greatly multiply. Now that means that there's more of a pool of cells in which a chance of more genetic events going wrong and which would lead to cancer can take place. So if you have, you know, a few cells, the chances are low. If you have millions and millions of cells, the chances are higher. So that's definitely telomerase on the cancer promoting side of things, what we call Mr. Hyde. On the other hand, I just told you that telomerase actually was acting as a somewhat protective mechanism. It was protecting those chromosome ends from fusing and therefore it was actually stopping one kind, not all kinds, but one kind of genomic instability and that would be actually a somewhat protective mechanism. So what telomerase is doing in terms of our anthropomorphic and disease-oriented viewpoint is that it's the same enzyme but it's doing something different depending on the context. You know, maybe it's like being Mr. Hyde, you know, who was only Mr. Hyde at night and Dr. Jekyll, who is Dr. Jekyll in the daytime. It depends on the context. Same person, same enzyme, but the context are different and the results are very, very different. So that is, oops, just, so again, we see the cat idea that's playing out in these cells and so now if we take that liftwood pathway, now we could make, as I said, cells that have at least the potential to become cancerous divide large numbers of times. And so my concluding remarks, I want to now draw together some of this in thinking about what that might mean to us and I'm going to be immediately mindful of the dangers of predicting science and I love the fact that we're at the end of this millennium because I think it was about the time of the end of the last century that physicists confidently predicted that really there was not a great deal of need to study more physics because pretty much everything was known and some of you might remember Einstein and a few other interesting things that happened in physics in the 20th century. So perhaps while I certainly don't think studying biology is over by any means, I think that perhaps we do have a somewhat analogous situation. We've learned so much and we've answered so many questions and yet it only takes a very little thought to realize, well, how many mysteries of life really are still remaining? How much we have yet to understand and let alone how to decide how to use that information and knowledge. Now, we were asking questions about telomeres in cells because we were interested in these tiny single-celled organisms because they would tell us about telomeres but at that time we were not setting out to try to cure cancer or to look at the ill effects of physical aging. So we don't know where new findings are going to come from and I think it's going to be very hard for us to make really confident predictions about where new findings are going to come from. I bet they're going to come from very unexpected directions. Now, what should we do with this information? Can we do anything with it? Well, technologically we're probably a ways from really being able to do anything with it but I don't think that is a reason to not think very hard about it. So let's think about aging. Now, I think no really honest person would say that they like the physical ravages of aging or cancer. I mean, if you're really honest, we can talk about the wisdom and the perspective that old age brings but the physical ravages, I think honestly, people don't think much of them. Okay, now, what are other things we don't like? Well, we don't like infectious diseases, for example and so we improve hygiene and we vaccinate and we forestall infectious diseases and we consider that as a very appropriate thing to do. Well, I think perhaps we can say, well, can we forestall the equally natural and as unwelcome wear and tear of aging? And I'll include cancer in that because many cancers, not all, many cancers are diseases of aging and again, we sit around sort of waiting for the blow of cancer to fall. We don't usually actively step in and forestall it although there are some exceptions but usually we're passively waiting here and so we get asked a very focused question. We say, well, could we imagine utilizing telomerase, utilizing activating it in certain contexts? Could that be helpful in either of these aspects of aging, the wear and tear and the cancer related aspects? So let me just examine a little bit about the logic of this. Now, first of all, this idea of cells keeping replenishing themselves. Well, what I've described for you is what's been described in the laboratory for human cells grown in dishes in the lab. It does the same kind of thing that I've described actually happen in the human body as we age. That is, does the increase in cells whose telomase is unprotected, does that lead to wear and tear as our bodies get older because cells can't renew themselves? It's a very reasonable, plausible hypothesis but we don't actually know for sure. And we also don't really know if such wear and tear is the major contributor to aging. I suspect it probably is, but we also should note that there's other aspects of aging and then the nematode, the roundworm that we heard referred to this morning by Lee, for example. In that creature, aging is defined by certain genes which are programmed to affect lifespan. Now, what's striking here is that the action of those genes is exerted in non-dividing cells. And everything I've been telling you about is relevant to the situation for dividing cells, multiplying cells, okay. So I think that's important to think about that in fact aging may very well be getting together of programmed and wear and tear and damage aspects of what goes on. And so it might not be at all clear what is the most important in our bodies as we age. We can look at model the organisms but we live very long times and we don't really know what is the preeminent mechanism of aging. So that's a question, but I'd also ask another question. Secondly, could you use the uncapping of telomeres? Does that lead to cells becoming cancerous and could we maybe protect from cancers? So I'm really speculating here, you have to realize, but could we protect from certain cancers by having telomerase protect from a certain kind of genomic instability? Now this is not all kinds of genome damage by any means but could it be used in that way? And again, I'm really speculating here, but I think it's never too early to think about these possibilities. And in particular, to think about the possibilities with respect to human lifespan because I think that the use of such a technology of potentially expanding the healthy lifespan would have very large ramifications. Just think of a few. Well, how long would we like that to be, right? Do we want this to happen? I think that there's very interesting things to start thinking about once we free ourselves of the idea that we have a sort of required lifespan. Do we have a lot of untapped potential that frustrates us that we cannot realize because time is too short? You know, we all said at one time or another, oh, I would have loved to have become a, you know, insert your favorite thing, a writer or a musician. You know, we often think of things that we might have done but we know we have a lifespan and so we, to achieve things in. And so we sort of realistically say, well, this is what I can do in my healthy lifespan. And that's another point. Here we are, we gain in our wisdom and our experience even as our bodies are losing their capability of executing the fruits of that wisdom and experience. And that might be something that we would think about as perhaps we do want to have that possibility. Would we regard time differently if we had a different expectation of what might be reasonable barring accidents to expect as our productive, creative, healthy lifespan? We might do things with the same tempo, for example. I don't know. Does this really change then our nature of how we think about ourselves? So I think all of these questions are very critical questions, which it's not too soon to begin thinking hard about because I suspect at some stage the possibilities will be there. And I can't say next year or 20 years from now but I suspect more the latter. But I think we should think about it. And I'm going to end with a marvelous quote from Robert Louis Stevenson's Dr. Jekyll and Mr. Hyde published in 1886. That was when the novel was published. And what I like about this quote is that you can take it either way you want, any way you choose. Okay, here's Dr. Jekyll. I hesitated long before I put this theory to the test of practice. I knew well that I risked death. For any drug that's so potently controlled and shook the very fortress of identity might by the least scruple of an overdose utterly block out that immaterial tabernacle which I looked to it to change. But the temptation of a discovery so singular and profound at last overcame the suggestions of alarm. End quote, end lecture. And too long. What the hell? Yeah. I'm just nodding. I'm still on. Oh. Oh well, now they know I've messed up. There's coffee and cider out over there. Oh, you want me to say there's coffee and cider or you know there's coffee and cider out over there? There should be. There should be. Say LV says there's coffee and cider out over there and if there isn't, I'm retiring. If there's not, we'll cut your telomers. Just unkempt. Unkempt. I'm trying to introduce the new telomers. We'll take about 10 minutes for questions and comments today and then there'll be coffee or LV says there'll be coffee and cider out on Ekman Mall following our question and answer period. You can send your questions up and I'll ask first whether any of the participants have comments or questions that they want to address to Dr. Blackburn. Dr. Venter. I was gonna ask you what you think of the approaches of companies like Generon where they're using telomerase to try and immortalize stem cells to use as stem cell therapy. If they're truly immortalized isn't that equivalent to putting in a cancer cell? I think we should separate cancerousness from immortality and unfortunately we can because cancerous cells have lost many of the checks and balances that tell normal cells when to divide and when not to divide and even though, and so for example, they're more impervious to certain kinds of genetic damage than a normal cell. So if a normal cell is truly normal and that is a very important question to raising if it stays normal then it may be a relatively benign type of cell. If it shows any propensity to become cancerous then I think we have serious concerns because now as I said the numbers of these cells can get very high and so chances of further changes towards cancerousness could go on. So I think like with any technology the cautions with which one must approach this are very, very great. The potentials are high and so it's not a clear cut indication of what we should do. So that's why I think choosing a society, what we want to do with these potentially quite powerful interventions is so critical. Dr. Venner again. Obviously the news about Dolly has been reported to have shortened telomeres that wasn't reported whether they were capped or not. Yes, well my bet is they were capped. And I don't know what they're being shorter means and that I think is actually where our knowledge of the basic science is incomplete. And to say telomeres are shorter and that is interpretable is premature at this stage. I have a question from the audience. What are the possibilities that adding telomerase to a cell from a specific organ or tissue of the body to increase replication would result in a new organ or tissue for the use in transplants and burn victims? And do you foresee such an undertaking to be possible in the future? I think the question has two aspects and one is at the level of the cells and their proliferation. And if indeed the reason that particular cells have ceased dividing is to do with telomere uncappy, then the prediction is that adding telomerase would then obviate that block to their division. But there are also many other reasons why cells may cease dividing. One could imagine that one could add telomerase but still have to add many other things to get things going. The second issue is, so I just talked about cells multiplying, but there's another very important part of this question and that is that the cells should differentiate properly into the required new organ that they should have all the appropriate interactions and that again is something where we're really in pretty uncharted areas. So, but I would say it's worth trying because it's quite reasonable that that is one aspect of why cells are not proliferating, but only one. I'm not trying to say that it's the one and only but it may be one part of what's going on to limit such growth and perhaps one could overcome certain barriers to continued proliferation by adding it back in. Circumstantly, one would like to think that one would do it in a way that might be very controllable. We talk about adding back a gene but perhaps one doesn't have to add back a gene. Telomerase genes are still in all our cells. They're just tightly shut down, they're dormant. There might be a way with a drug, a small drug that could be administered in a controlled way over a limited period to kick the telomerase into action when you want it to and then turn it off. So I don't think one has to think this should be done only by some gene introduction method but rather a gene awakening method. Another question is telomerase the same invertebrates and protists? How closely is it conserved? And then a second part to the question is Dali the clone sheep is said to be old. Is this due to her telomerase? Okay, the first part is that like many of the genes that we heard about in the previous talks, there are variations in the structure, the detailed structures of the proteins and in this case the RNA from organism to organism but they end up folding up into a three dimensional functional shape which is pretty much conserved. Now there can be subtleties and differences in how the interfaces with the protein is with other parts of the cell and I'm sure those details vary. Could you put a protozoan telomerase into a pond organism protozoa telomerase into a human cell and expect it to work? It might work at some very crude level but it probably wouldn't be subject to all of the fine regulatory controls that such enzymes are in cells so I think it might work in a simple way but it wouldn't work in a good way but fundamentally we've found no significant important difference in how they work between such very, very different creatures. Dali, I might have answered the Dali question, the question was is Dali quote old? And Dali was judged to be quote old by the fact that the telomeres in Dali were shorter than one might have expected but the problem is we don't know if that means, if short telomeres by itself means something. We need to know the status of the telomerase, we need to know the status of all the proteins that bind the telomeres, control, et cetera. So it's a parameter that in isolation is not easy to interpret so I'm not sure that Dali is old. I think one would look for some other much more significant indicators of age than something as fickle as telomere lengths. So telomere lengths, if you look at people who are 20 and people who are 90 in the cells that don't have telomerase, there's enormous variation in length. Individuals with very long telomeres at 90, they can have much longer telomeres at 90 than another individual does who's 20. It just says there are many, many genetic factors and perhaps environmental for all we know that play into that final readout which is telomere length. It's just an easy thing to measure and we got a little bit carried away when we thought about length because it was the only lamp post light under which one could look for quite a while. So length was an easy thing to look for but now there are much more sophisticated things that one should look for as a way of saying is something old or not old. Might it be that a limited lifespan is necessary for a species to prevail? I think those are very, very good questions and I think that's one very good question but that's absolutely a crucial question to ask. The fact that the roundworm, the nematode, the free living nematode, has a gene whose normal form is to keep the lifespan short and it's only in its mutated form that lifespan extends. That already I think is telling us something about how this whole complex interplay of organisms, their environment, their populations play out. I think it's only as humans who are very much into manipulating our fates that we can think about this but I suspect there've probably been very good selective reasons over the years why lifespans are not very, very long for organisms. And this will be the last question but we've had several versions of this. The question has to do with neural cells and the state of their telomeres and telomerase. Neural cells. Yeah, neurons. So the general understanding is that neurons are of the class of cells that have ceased dividing. There's some exceptions to that which I believe are emerging but as a generalization, there are many cells that don't keep dividing. And in that case, the issue of telomeres and telomerase is probably perhaps even not relevant. It's certainly not relevant in any way that anybody has been able to discern. And that is why I wanted to make the point that there's only one aspect of aging that we can think reasonably has to do with dividing cells. There might very well be aspects of aging which have to do with things that go on in non-dividing cells. And DNA damage accumulation is one obvious, one that I didn't touch on but which has been much thought about. Dr. Renner has a comment. I think the notion of brain cells being non-dividing and static is rapidly going away. That's what I understood. I didn't know for how many, whether it was for, Lee's not here, but I don't know for how many of the cells, that's true, but I know people have started now to find evidence of cell division even in what was thought to be the non-dividing brain. But some of them may be dividing fairly slowly. Some of the neurosurgeons at NIH upon doing surgery for epilepsy would put a sponge in the part of the brain they removed and they would go back later and found the sponge was loaded with neurons. So they can. It's a very smart sponge. It's not a sponge. Right. Right. So they can invade given that rather unusual situation we don't all have sponges in our brains at least most of us not much of the time. But I think it's worth pointing out that Craig is also pointing out an experimental situation in which you did something rather unusual. You know, it's not your average brain that has a sponge put in it. So does it normally replicate? What you're saying is it certainly has the potential to renew and to replicate. Well, I'd like to thank Dr. Blackburn one more time and then we'll meet back here at 315 for music and at 330, Dr. Baker will give his lecture. Thank you.