 Now we're going to do something a little different in this lecture because we have the mitosis background We've done all the notes for what happens in mitosis. And as I've said a bajillion for a million times Mitosis and meiosis are really really similar So I'm just going to go directly to our Animation and we're going to look we're going to get ourselves to prophase one in a cell that's about to go through meosis the significant thing that happens in prophase one is that Crossing over takes place crossing over happens between homologs, and we're going to look at what that actually looks like to get us here to prophase one of meiosis Look does this look familiar? We're just going to throw the scrambled cell like I think I changed the color of the DNA just for fun Because you know why not I don't have anything else to do except for change DNA colors We have some arguments to have about that But this looks really familiar doesn't it and if we watch Meiosis happening we can go through all of our stages of interphase. We can be like dude This is exactly the same as in mitosis If we were to peek at our chromosomes, let's just Wind up that DNA just to look you can see that I've actually drawn three different chromosomes here easier to do that When I can copy and paste versus doing it by hand, right? If you have to deal with three different chromosomes, but convince yourself. Do you see my three homologs? Do you see my homologous pairs? Can you predict what might happen? If I showed this to you, would you know that that cell was in G1 of interface? Yeah, there there's no sisters. We haven't duplicated replicated Duplicated our DNA yet So if we go into s at the beginning it looks like a tangle and at the end it looks like a double tangle Can you predict what the chromosomes would look like if we peaked at our chromosomes? They look like little X's totally what we would expect each chromosome now has a sister chromatid attached At the centromere Remember how I told you I don't know. Do you remember things that I tell you? Remember how I told you that you could count? Centromeres to find out how many chromosomes you have in a cell This cell has one two three four five six Total chromosomes even though we doubled our DNA Convince yourself get cozy with that. It's the same over and over and over again Alright G2 we have double the DNA, but how fun? Now we can go into pro phase. What are you going to predict is going to happen in pro phase pro phase one? Okay, you caught me. I Just said we're gonna go into pro phase if I say that to you you say oh We're going through mitosis rigs because pro phase is a stage in mitosis You have to add a one or a two if you're going to talk about meiosis That's enough if you say we're entering pro phase one We know you're going through meiosis. You don't have to say pro phase one of meiosis You can I often do because I want to burn into my brain that I have to keep the number to Match it to Meiosis if I don't have that number on there. It's I'm talking about mitosis. Okay pro phase one This is unique. There are going to be some unique things push pause and Predict what are the things that you think might not be unique that you would be seeing here in pro phase one? well Super significant almost everything's identical The nuclear envelope dissolves the century old start moving The nucleolus disappears and the chromosomes Condense so they become more tightly condensed, but what is something else? That you see That is happening Okay, we'll just go back to there What you are seeing this is not yet crossing over even though I hinted that crossing over was going to take place What's happening? How would you describe this? What is the thing that's different that's happening right now in this visual? the homologues are hooking up and There's no other way. I'm sure there are scientific words to say about how we describe this, but The bottom line is the homologues hook together. They literally connect During pro phase one Because they're connected to each other This next amazing thing can happen, which is crossing over right it down Crossing over occurs in pro phase one. Can you predict what crossing over might be? Crossing over We're talking about homologues just hooked up Well, let's see how Time out not technical foul time out Let's look at crossing over So remember our homologues and this is all review right which is awesome Keep reviewing predict push pause see if you can guess what's gonna happen. We have our egg parent and our sperm parent Which are homologous chromosomes Each one of these guys Has look do you see what arrows I just put in? What is the word that's gonna go under those arrows? Sister chromatids so each homologous chromosome has sister chromatids because of what? s Because of s in interface we doubled our DNA Okay What do you think that is representing? Those are genes so I've just drawn some weird genes on these chromosomes and look we can have genes on both sides and They're homologs. So if we have genes in those places On one homologue, we will have it on the other Same place same trait same gene But Let's throw some letters in there. We have the A gene and the B gene Predict what our sister is gonna look like. I think sisters would come next What letters are we gonna put on our sister chromatids? Identical no matter what that letter was when I first threw it on there. The sister has the identical letter What about the homologue? Does it have to be a big A and a big B on the homologue? Nope. I went Rogue I went diverse. I went genetically diverse and we have a different Allel for the same gene on the sperm parents chromosomes But the sperm parents Sister chromatids have the same identical alleles. Are you good now? What do you think happens with crossing over? Take a look. Do you agree that? Where are the new arrows showed up? Can you see my my mouse my mouse is circling the new arrows that just showed up Something's gonna happen right here Crossing over is going to take place The homologs have to be connected to each other to make this occur. Oh Those are alleles of the same gene. We already talked about that, but what's gonna happen right there? The homologs get very close together and they literally swap alleles That's what crossing over is they do hmm They swap the same genes So they will actually think it's like a true story they like take off one of them takes off the right arm and it's like if my kid were in here and I Gave him my right arm and he gave me his right arm. I would get the better deal on that But cool. We traded right arms. We both still have right arms We both both homologs still have the a gene, but they just swapped out the allele sometimes that swap if both Chromosomes have the same allele that swap isn't gonna increase genetic diversity, but Sometimes it does increase genetic diversity most of the time it does and you swap an entire chunk of DNA and then You end up with a mix of chromosomes from your sperm parent and your egg parent This is wild prophase one Results in that crossing over now. I think I have a few more I want to show it happening here before we get into metaphase There I just made all of them swap a little chunk of DNA I Think it's important to remember that crossing over is Random which also should just blow your mind like It's like well, do you want to switch arms today or do you want to switch legs? Oh, let's do legs. Okay. Again. My kid would get a worse deal I'd be on the better end of the deal if I swapped a leg with my kid That's really my legs are much shorter than my kids. This would not be entirely efficient It's not a great analogy as you can see the chromosomes swap chunks of DNA and you can swap identical chunks All right, how do you feel? What do you think is coming next? I'm just gonna stop our visual here Let's head toward metaphase one and another like mind-blowingly wild source of genetic diversity in the process of meiosis