 Now let's talk a little bit about the anatomy and the vocabulary that comes with describing chromosomes or describing DNA in a nucleus and keeping track of it. And we're going to be keeping track of it for the next entire set of lectures until our next exam because we're going to be talking about heredity, we're going to be talking about genes, we're going to be talking about how you inherit characteristics from your mom and why you didn't inherit that one from great grandpa and all of those things. So we need to keep track of some terminology. First of all, I'm telling you right now and I've already told you, we're diploid. And what does that mean? It means you have two copies of each chromosome. Okay, I'm going to show you a picture. This is a picture, this is a karyotype. And a karyotype, this is, we're going to see these a lot as well. A karyotype is a true actual picture of your chromosomes. They basically squish a cell in the middle of it going through mitosis when it's DNA, the nucleus, the DNA inside the nucleus of this cell is all packaged up in a nice little bundle, chromosome bundle. That bundle, you can see. And then you can look at it and you can sort them. Now take a look at this. I told you that we're diploid. We have two copies of each chromosome. Well, here's chromosome number one. And if you look at it really closely, you can see that it's actually striped the same. It has the same length. It has, the shape isn't quite the same because you can imagine that these are very small cells and when you squish a nucleus, like, stuff happens. Things get folded, things get wonky. I mean, that's just normal. But it's, I mean, you can totally see. Basically the size is the same and the striping is the same. So if I gave you this pile of chromosomes, you could go through it and sort them into pairs. And that's how you know that this is a diploid critter. And you can see that we actually have 23 pairs of chromosomes. And every single one of your cells has 23 pairs of chromosomes. And what are the chromosomes? Nothing more than a molecule of DNA wound around some histone proteins into a nice little bundle. Why do we want a bundle? So we can divide them up if we make copies. Okay, diploid means we have two copies of each chromosome. I'm going to draw a diploid critter. Are you ready for this? Because it's going to blow your mind. It's diploid. How do you know? I have two colors. Can you tell how many chromosomes I have in here? Do I have 23 pairs? Dude, who the heck knows? I mean, no one knows. I don't even know. In chromatin form, we can tell nothing about our chromosomes. It's a tangle. How many things of yarn are in here? Nobody knows. It was pure madness in my house when we made that. Even the dog got involved and the cat was in the middle of it. Pure madness. But if we were to condense this, once we condense the chromosomes, the chromatin into a chromosome log, then we can see some stuff. Then, let's say that you see something like this. Now, is that all that I would find in here? There's something red going on in there. And I'm telling you right now that if I condensed it down, look, here are my chromosomes. Now, there's some anatomy. The little dots in the middle are called centromeres. And the centromeres are important in this process. So keep that in mind. What do you notice? How many copies? Let's just name this chromosome A, or chromosome number one, and we'll name this chromosome number two, just for the heck of it. Do you agree that they're different? They're totally different shapes. What would you think the colors represent? The colors actually represent what came from your mom. If these are your chromosomes, these came from your mom. And the red ones came from your dad. That just means that all the red chromosomes in my little picture, the way I'm depicting it, they're not really red and green. You can't really tell when you look at this thing. You know that one of these had to come from your mom. You have come from mom. And one of these had to come from dad. But you can't tell which is which unless you join me in Windy Land where we color code our chromosomes because it's fun. And then you know that, okay, let's just arbitrarily say that all the green ones came from mom. A green chromosome number one and a green chromosome number two came from mom. And a red chromosome number one and a red chromosome number two came from dad. Take a deep breath because on these chromosomes, do you agree that we could say, well, they're just made of strings of DNA. We know DNA codes for proteins. So we can say that there are genes on these chromosomes. Do you agree? And I'm going to tell you right now, just for the fun of it, this is the eye color gene. And I'm going to draw another gene on this little chromosome down here. And you tell me. I'm totally making this up. Eye color is coded for by multiple genes and I'm going to pretend like it's coded for by one just for demonstration purposes. So all of my examples are completely inaccurate. Chromosome number two, let's say, we'll call it the hair color gene because would you have an eye color gene on chromosome number one and an eye color gene, the same eye color gene on chromosome number two? No. We'd have different genes on chromosome number two. Does that work for you? Like chromosome number 22 has a set of genes on it and we can tell you what those genes are. We've totally mapped that out. I haven't. Someone in the world has totally mapped it out and can tell you exactly what those genes are and your chromosome 22 has that gene on it and my chromosome 22 has that gene on it and my kid's chromosome 22 has the gene on it and all my dog pounds in YouTube land, your chromosome 22 has that gene on it. But chromosome number 21 has different genes on it. Now, you're cool with that, right? These chromosomes, when they are the same chromosomes, but they might have different genes on it, these are called homologous chromosomes. Homologous chromosomes. Homologous chromosomes are the same. They contain the same genes, but they can have different forms of the gene. For example, you can have different alleles. Alleles are forms of a gene. What? Seriously. So in the eye color gene, dad has brown-eyed allele and mom has the blue-eyed allele. Again, totally genetically inaccurate, however, just for example purposes. Those two different alleles, it's the same gene, but they have different forms. We could have the same form. Mom could have blue eyes and dad could have blue eyes and they both donate blue-eyed alleles. Awesome. And this is, like, we're headed here to the world of heredity. But we kind of have to have a sense of, like, why do we even keep track of this stuff in mitosis and meiosis? Well, it matters because we're going to keep track of our chromosomes. We're also going to keep track of something else that happens. At some point in the process, we actually have to double our DNA. At some point in, as we prepare to go through mitosis, we have to double the amount of DNA we have. And so, at some point, your chromosomes are actually going to end up looking like this. Dun, dun, dun, dun. From this guy, we got this. And all we did is made a copy of the DNA. And I'm telling you, because I love you. These two, what pieces of this chromosome are sister chromatids. Chrom-a-tid-z. Here's one sister chromatid and here's the other sister chromatid. For example, are there sister chromatids here? No, those are homologous chromosomes. They're the same, but different. They contain different alleles. Sister chromatids are identical. If we have the blue-eyed allele on one sister, we have the blue-eyed allele on the other sister. They're identical to each other, which is very significant because all we did was copy our DNA. We didn't do any mixing. We didn't come from somebody else. We just made a copy of it and we know from DNA replication that the copy is identical. Okay. How many centromeres do you see over here? There's a centromere. There's a centromere. How many chromosomes is that? Buh-boh-ing two. If you have two centromeres, you have two chromosomes. If you have five centromeres, you have two sister chromosomes. If you have 30 centromeres, you have 30 chromosomes. How many centromeres do you see over here? Connecting sister chromatids, dog pounds of a feather. That's one. One centromere, which means even though we have twice as much DNA, how many chromosomes do we have here? That's one chromosome. One chromosome with two sister chromatids. Oh. We could go on all day long talking about chromosome anatomy. And the sooner you're like, okay, I'm cool, like this is going to be a process. Chromosome anatomy is not easy and it's kind of weird, especially since everything is like the same but different. Right? Makes perfect sense. So we look at the whole cell cycle where we change our DNA to prepare to go through mitosis. Cell cycle coming up.