The genetic analysis of meiosis in Drosophila melanogaster females

Published on March 22, 2009 Reviewed on May 31, 2018   53 min

Other Talks in the Category: Cell Biology

0:00
Good afternoon. My name is Scott Hawley, from the Stowers Institute of Medical Research in Kansas City, Missouri. And I'm here to do a couple of things. I have an easy job, which is to talk to you about the genetic analysis of meiosis in Drosophila melanogaster females. And I have a hard job, which is to make you care. And the reason it's difficult to make you care, is that whenever I mentioned the word meiosis, I watch people's eyes glaze over. They've had meiosis in Montessori school that had five, six, seven, eight, nine times. And frankly, they're really tired of it. And the reason they're tired of it has to do with this next slide.
0:34
If you look at this next slide, which I took out of a textbook, the main problem with this slide is if you didn't understand meiosis before you saw this slide it's kind of hopeless thereafter. As you see upon the top, there's all this chromosomal fettuccini. We even added a mushroom, which is actually the nucleolus. And just to make sure you could digest it all, we even added a replication fork. I'm not sure there's all those Greek names leptotene, zygotene, pachytene, diplotene, diakinesis, they're all Greek for prophase. And if you look down in the middle of the bottom row, I don't know what that middle chromosome is doing. But it's not metaphase II. In other words, it's really impossible to try and understand what's going on in meiosis, by looking at a complicated, overdrawn slide like this. And that's why I wanna try and simplify meiosis for you before we even begin.
1:23
This slide shows the meiosis the way the people in my laboratory think about it. It's an idea that we've taken from the late Barbara McClintock, who simplified meiosis back in the 1930s, by pointing out that if you get rid of all that Greek, if you get rid of all those complicated diagrams. You can think about meiosis as a process where three things have to happen. Chromosomes have to pair, they have to match up along their length, so that the copy of chromosome one that was obtained from the organism's father, lines up against the copy of chromosome one that was obtained from the organism's mother, and so on. So they have to pair. They have to match by homology along their entire length. Once they pair, they have to undergo exchange or crossing over. If you went to private school or recombination or chiasma formation, I don't care what term you use, as long as you realize that exchange or chiasma formation serves the vital function of interlocking paired homologs. That's what meiosis does it makes sure that what was paired remains stuck together as these chromosomes go throughout the ballet, which is meiosis. And then finally, at the first meiotic division, chromosomes have to segregate from each other. They have to disjoin if you will. So that each homologue goes to opposite poles, and we end up with two daughter cells, each of which contains a haploid complement of chromosomes. If we're talking about humans, that means we have to go from a cell with 46 chromosomes, to two daughter cells each of which have 23. If you're talking about my organism, which is Drosophila melanogaster females, they start with eight chromosomes at the beginning of meiosis and the two products of the first meiotic division, each of which each has four chromosomes, usually one of each homologue if everything has gone correctly. The second meiotic division we're not going to worry about very much it really is a haploid mitosis. Unfortunately, as you're going to see in the next slide, this is rather a simplified description of meiosis, because we can't count on exchange always occurring. In this slide, we look at the frequency of recombination for
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The genetic analysis of meiosis in Drosophila melanogaster females

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