1 00:00:00,000 --> 00:00:05,000 Just wanted to begin today by pointing out a couple of articles 2 00:00:05,000 --> 00:00:10,000 from today's Boston Globe. There's an article that's on the 3 00:00:10,000 --> 00:00:15,000 financial pages about investors reconsidering gene therapy, 4 00:00:15,000 --> 00:00:20,000 and startups funded as technology evolves. A lot of you have heard 5 00:00:20,000 --> 00:00:25,000 the words gene therapy. A lot of you, at the end of that 6 00:00:25,000 --> 00:00:30,000 bacterial genetics lecture, said you weren't quite sure what 7 00:00:30,000 --> 00:00:35,000 complementation was. But, gene therapy is basically 8 00:00:35,000 --> 00:00:40,000 complementation but to work. The idea is if you got a broken 9 00:00:40,000 --> 00:00:45,000 gene, and you can put in a good copy, then you'll take the cell back to 10 00:00:45,000 --> 00:00:49,000 being normal, which is exactly what we're doing in those phage crosses 11 00:00:49,000 --> 00:00:54,000 where we had two phage, one with the mutant copy of the gene, 12 00:00:54,000 --> 00:00:59,000 and one with the wild type copy of the gene. And they're saying here 13 00:00:59,000 --> 00:01:04,000 that there's a renewed interest. Now, the basic strategy in these 14 00:01:04,000 --> 00:01:09,000 things is to take usually a retrovirus, the same kind of idea 15 00:01:09,000 --> 00:01:14,000 that we heard about with the HIV-1 virus that will make a DNA copy, 16 00:01:14,000 --> 00:01:19,000 and then the copy gets inserted somewhere in the chromosome. 17 00:01:19,000 --> 00:01:24,000 So, the good thing is it puts and a new copy in a chromosome. 18 00:01:24,000 --> 00:01:29,000 The bad thing is you got a piece of DNA being stuck somewhere in the 19 00:01:29,000 --> 00:01:34,000 chromosome. And there are places where it can go where it doesn't 20 00:01:34,000 --> 00:01:40,000 matter, and there are other places where it really does matter. 21 00:01:40,000 --> 00:01:43,000 So, it's been a very tricky business with gene therapy. 22 00:01:43,000 --> 00:01:46,000 Part of the thrust here is they're thinking that maybe they need to 23 00:01:46,000 --> 00:01:49,000 pick targets were the person's going to die anyway so that if there is a 24 00:01:49,000 --> 00:01:52,000 risk of getting cancer or something, it's outweighed by the possible 25 00:01:52,000 --> 00:01:55,000 thing. That's sort of the thrust. You can see the article; it's in 26 00:01:55,000 --> 00:01:58,000 today's paper. It's very timely for what we are 27 00:01:58,000 --> 00:02:01,000 talking about now. And then on Friday I'm going to 28 00:02:01,000 --> 00:02:05,000 start talking about recombinant DNA strategies and stuff. 29 00:02:05,000 --> 00:02:09,000 But there's another article in today's paper. 30 00:02:09,000 --> 00:02:13,000 This is one of those things that I told you about where an article is 31 00:02:13,000 --> 00:02:17,000 published in the, in this case, online in the journal 32 00:02:17,000 --> 00:02:20,000 nature. They have an embargo on a press thing until Thursday or so, 33 00:02:20,000 --> 00:02:24,000 and then the article is on, the hot papers that have just come out show 34 00:02:24,000 --> 00:02:28,000 up in the newspapers, usually either on Friday or Monday. 35 00:02:28,000 --> 00:02:32,000 And this is yet another system that they describe as making a molecular 36 00:02:32,000 --> 00:02:36,000 repair kit that corrects mutations in the cells' DNA. 37 00:02:36,000 --> 00:02:40,000 So far, this has only been done in cells. But it's working well enough, 38 00:02:40,000 --> 00:02:44,000 and the article is saying this approach could become a serious 39 00:02:44,000 --> 00:02:48,000 competitor to conventional gene therapy. In here, 40 00:02:48,000 --> 00:02:52,000 you have sort of a layman's account of this kind of complementation by 41 00:02:52,000 --> 00:02:56,000 gene therapy. We're talking about gene therapy in that field which 42 00:02:56,000 --> 00:03:00,000 attempts to force feed healthy genes into cells hobbled by defective ones 43 00:03:00,000 --> 00:03:04,000 has been plagued by failure, and recently was found to cause 44 00:03:04,000 --> 00:03:07,000 leukemia in some patients. That's probably because of the 45 00:03:07,000 --> 00:03:11,000 insertion events associated with these viruses. 46 00:03:11,000 --> 00:03:14,000 This new system rewrites the small stretch of misspelled genetic code. 47 00:03:14,000 --> 00:03:18,000 That's typically the reason the gene has gone bad. 48 00:03:18,000 --> 00:03:22,000 So it's a different kind of strategy. This is the first report 49 00:03:22,000 --> 00:03:25,000 I've heard of that. Some of the stuff we're talking 50 00:03:25,000 --> 00:03:29,000 about is sort of inescapable because you're going to see 51 00:03:29,000 --> 00:03:32,000 it in the paper. Here, they're telling you the sort 52 00:03:32,000 --> 00:03:35,000 of thing I was just saying. The viruses inject their payloads 53 00:03:35,000 --> 00:03:39,000 at random within the cells tangled mass of DNA sometimes disrupting 54 00:03:39,000 --> 00:03:42,000 normal genes. Even when genes land in good locations, 55 00:03:42,000 --> 00:03:45,000 the molecular machinery that regulates their activity is also 56 00:03:45,000 --> 00:03:48,000 often thrown off, leaving the healthy genes operating 57 00:03:48,000 --> 00:03:52,000 at a level to low to be helpful or functioning in the wrong parts of 58 00:03:52,000 --> 00:03:55,000 the body. It must begin to sound familiar. You stick a random piece 59 00:03:55,000 --> 00:03:58,000 of DNA somewhere, you can disrupt the regulation in 60 00:03:58,000 --> 00:04:02,000 the gene as well as just landing in the open reading frame. 61 00:04:02,000 --> 00:04:05,000 So there's a problem with gene therapy. Now, 62 00:04:05,000 --> 00:04:08,000 they'd like to make you think that this is going to be the promised to 63 00:04:08,000 --> 00:04:11,000 everything, but as always it's more complicated. And so, 64 00:04:11,000 --> 00:04:14,000 I just picked up the last paragraph of this article. 65 00:04:14,000 --> 00:04:17,000 Other scientists said the approach looked promising, 66 00:04:17,000 --> 00:04:20,000 but predicted it would end up struggling with this problem of its 67 00:04:20,000 --> 00:04:23,000 own, James M. Wilson, a gene therapy researcher at the 68 00:04:23,000 --> 00:04:26,000 University of Pennsylvania. Now, there's somebody who's in a 69 00:04:26,000 --> 00:04:29,000 position to criticize, but also has a vested interest 70 00:04:29,000 --> 00:04:32,000 himself and the alternative therapy, noted that the first step of the new 71 00:04:32,000 --> 00:04:35,000 process causes for [zinc fingers?]. 72 00:04:35,000 --> 00:04:39,000 It's a kind of protein that's involved in this, 73 00:04:39,000 --> 00:04:43,000 to make a fresh break in DNA to accommodate the insertion of the 74 00:04:43,000 --> 00:04:47,000 corrected sequence. It's the same kind of break it with 75 00:04:47,000 --> 00:04:51,000 radiation that can lead to cancer. And we're going to talk today about, 76 00:04:51,000 --> 00:04:55,000 you'll see where double-stranded breaks and whatnot can be a real 77 00:04:55,000 --> 00:04:59,000 problem in terms of the integrity of the DNA and genome stability, 78 00:04:59,000 --> 00:05:03,000 which is maintaining gene stability so important in avoiding cancer. 79 00:05:03,000 --> 00:05:07,000 You guys are a tough crowd. Comments ranged from, this was the 80 00:05:07,000 --> 00:05:12,000 best lecture that I'd given to ones that were absolutely the other end 81 00:05:12,000 --> 00:05:17,000 of the spectrum. You're a very hard group. 82 00:05:17,000 --> 00:05:22,000 I truly am going to take, I don't have time really to sit back and 83 00:05:22,000 --> 00:05:27,000 redo all my lectures, but I've a huge amount of input, 84 00:05:27,000 --> 00:05:32,000 and I'm going to go back over all this input and getting, 85 00:05:32,000 --> 00:05:37,000 and take it very seriously in terms of how I go about things next year. 86 00:05:37,000 --> 00:05:41,000 I can see a number of changes and going to make. 87 00:05:41,000 --> 00:05:46,000 Whoever sent me the e-mail that talked about the Dock Street 88 00:05:46,000 --> 00:05:50,000 Bohemian Pilsner brewed in Utica, NY, this classic golden Pilsner, 89 00:05:50,000 --> 00:05:55,000 pungent, perfumy taste and so on, it was a moment of lightness, but if 90 00:05:55,000 --> 00:05:59,000 I got to many things like that it's not going to hell in a contract to 91 00:05:59,000 --> 00:06:04,000 figure out how to do. For the person who asked about how 92 00:06:04,000 --> 00:06:08,000 to avoid accidental pollination, when they're going to do that, they 93 00:06:08,000 --> 00:06:13,000 actually take the stamens off the flowers so you don't get accidental 94 00:06:13,000 --> 00:06:17,000 self fertilization. You actually go out and make ones 95 00:06:17,000 --> 00:06:22,000 that are just male or just female flowers. You can do that rather 96 00:06:22,000 --> 00:06:26,000 easily. OK, so what we got into last time, or what I told you last 97 00:06:26,000 --> 00:06:31,000 time, and I promise you that there's another Mendel lecture coming. 98 00:06:31,000 --> 00:06:34,000 For those of you who thought this was a little slow, 99 00:06:34,000 --> 00:06:38,000 you may find the next one is all too fast. But anyway, 100 00:06:38,000 --> 00:06:42,000 that will be how it goes. Mendel had carried out this 101 00:06:42,000 --> 00:06:46,000 fundamental series of experiments. And I did linger over it because I 102 00:06:46,000 --> 00:06:50,000 think it's so simple you can actually see the scientific process 103 00:06:50,000 --> 00:06:54,000 in action, and it also isn't complicated by any techniques that 104 00:06:54,000 --> 00:06:58,000 we can't see. And it was a tremendously important insight that 105 00:06:58,000 --> 00:07:02,000 genetic information came in units. But as I told you, 106 00:07:02,000 --> 00:07:06,000 it wasn't accepted because for most of the world at the time, 107 00:07:06,000 --> 00:07:10,000 it was a statistical argument. There weren't any entities that 108 00:07:10,000 --> 00:07:14,000 were two of inside a cell, and all that sort of thing. 109 00:07:14,000 --> 00:07:18,000 So, it didn't take over scientific thinking. It wasn't until about 110 00:07:18,000 --> 00:07:23,000 1900 when several other geneticists did finally find the systems that 111 00:07:23,000 --> 00:07:27,000 duplicated Mendel's results, and therefore showed that these were 112 00:07:27,000 --> 00:07:31,000 general inferences, something that Mendel himself, 113 00:07:31,000 --> 00:07:35,000 as I told you, by choosing weeds and bees as follow-up systems 114 00:07:35,000 --> 00:07:40,000 hadn't been able to do. These other geneticists showed them 115 00:07:40,000 --> 00:07:44,000 in other systems. But the other thing that made the 116 00:07:44,000 --> 00:07:48,000 difference at the time, was by this point cytologists, 117 00:07:48,000 --> 00:07:53,000 who were looking at cells under microscopes, had found entities 118 00:07:53,000 --> 00:07:57,000 whose behavior seemed to match those of these particles of information 119 00:07:57,000 --> 00:08:01,000 that Mendel had postulated to explain these patterns of 120 00:08:01,000 --> 00:08:07,000 inheritance that he was seeing. These particles, 121 00:08:07,000 --> 00:08:13,000 because they were called chromosomes, which literally means colored things, 122 00:08:13,000 --> 00:08:19,000 the reason they were colored was not because they were inherently colored, 123 00:08:19,000 --> 00:08:25,000 but because cytologists were staining the cells with a variety of 124 00:08:25,000 --> 00:08:32,000 things. And that was what allowed them to visualize these. 125 00:08:32,000 --> 00:08:41,000 And so, by staring into microscopes, right around this time, around the 126 00:08:41,000 --> 00:08:50,000 turn of the century, 1900s or so, cytologists had come up 127 00:08:50,000 --> 00:08:59,000 with three key observations. One was that chromosomes came in 128 00:08:59,000 --> 00:09:07,000 matched pairs. So, let's say in this cell, 129 00:09:07,000 --> 00:09:15,000 there were too long and too short. So that's a chromosome. And a 130 00:09:15,000 --> 00:09:23,000 correspondence, we now know, to double-stranded DNA 131 00:09:23,000 --> 00:09:31,000 molecule that's been all condensed up. 132 00:09:31,000 --> 00:09:36,000 And there are two copies of each chromosome in this thing. 133 00:09:36,000 --> 00:09:42,000 So: too long, too short, one came from mom, one came from dad. 134 00:09:42,000 --> 00:09:48,000 And for bookkeeping purposes, I'm going to color them so that from 135 00:09:48,000 --> 00:09:54,000 one parent is unshaded, and the other one is shaded. 136 00:09:54,000 --> 00:10:00,000 Most of you know in human cells we have 23 pairs of chromosomes. 137 00:10:00,000 --> 00:10:04,000 This is a now fancier method of visualizing chromosomes called 138 00:10:04,000 --> 00:10:08,000 [peating?] chromosomes where you take little stretches of DNA that 139 00:10:08,000 --> 00:10:12,000 are unique to particular sequences, attached dyes, and then use them to 140 00:10:12,000 --> 00:10:17,000 [stain?] the cells, and so they will color only 141 00:10:17,000 --> 00:10:21,000 particular chromosomes. And we have 22 pairs of identical 142 00:10:21,000 --> 00:10:25,000 chromosomes, and then the sex chromosomes here, 143 00:10:25,000 --> 00:10:30,000 X and Y, this would be a male, or two Y's if we were female. 144 00:10:30,000 --> 00:10:34,000 We'll return to those in the next lecture. Kim Naismith, 145 00:10:34,000 --> 00:10:38,000 who spent his life working on how these chromosomes segregate gave a 146 00:10:38,000 --> 00:10:42,000 talk a year ago. He's in London at a hospital in 147 00:10:42,000 --> 00:10:46,000 London. And then he brought in the next slide. There was an artist who 148 00:10:46,000 --> 00:10:51,000 spent several months visiting with him, trying to find ways to 149 00:10:51,000 --> 00:10:55,000 represent in art some of what she saw going on. And I thought you 150 00:10:55,000 --> 00:10:59,000 might enjoy seeing this, the slide. This is the human 151 00:10:59,000 --> 00:11:03,000 complement of chromosomes, represented in stripy socks that 152 00:11:03,000 --> 00:11:07,000 various staff members from the hospital brought in for 153 00:11:07,000 --> 00:11:12,000 her to do this. So, a little touch of scientist 154 00:11:12,000 --> 00:11:17,000 humor which some of you may think is pretty nerdy, but anyway there it 155 00:11:17,000 --> 00:11:21,000 was for what it's worth. He showed that in our departmental 156 00:11:21,000 --> 00:11:26,000 [colloquium?]. OK, the second thing, 157 00:11:26,000 --> 00:11:31,000 then, that the cytologists noted by staring at this was during ordinary 158 00:11:31,000 --> 00:11:36,000 cell division, which gives two identical daughters 159 00:11:36,000 --> 00:11:41,000 the numbers of chromosomes per cell is preserved. 160 00:11:41,000 --> 00:12:11,000 And this ordinary cell division is 161 00:12:11,000 --> 00:12:19,000 given a special name of mitosis. So if we take this cell, the first 162 00:12:19,000 --> 00:12:27,000 thing that happens, then, is the chromosomes are 163 00:12:27,000 --> 00:12:37,000 duplicated. We've talked a lot about DNA 164 00:12:37,000 --> 00:12:47,000 synthesis at the molecular level. This is looking at it. Excuse me, 165 00:12:47,000 --> 00:12:57,000 and so, there are now two DNA molecules, that each one of these 166 00:12:57,000 --> 00:13:07,000 new DNA molecules is given a special name. 167 00:13:07,000 --> 00:13:13,000 This is called a chromatid. So there was one DNA molecule here. 168 00:13:13,000 --> 00:13:19,000 It's now duplicated, but you'll notice these are still joined 169 00:13:19,000 --> 00:13:25,000 together. The point at which they are joined together is known as the 170 00:13:25,000 --> 00:13:32,000 centrosome. And then, after this, after the DNA's been 171 00:13:32,000 --> 00:13:38,000 copied, these things are lined up at the center of the cell and then they 172 00:13:38,000 --> 00:13:44,000 are pulled apart to give two daughter cells that are just like 173 00:13:44,000 --> 00:13:50,000 what you started with. This part, so this is 2N, 174 00:13:50,000 --> 00:13:54,000 if N is the number of kinds of chromosomes. And then there's two 175 00:13:54,000 --> 00:13:59,000 each. This is 2N. At this point, the DNA contents of 176 00:13:59,000 --> 00:14:04,000 the cell is 4N. This is 2N. This is 2N. 177 00:14:04,000 --> 00:14:09,000 We're back to where we started. This part was invisible to the 178 00:14:09,000 --> 00:14:15,000 cytologists, at least in the sense the chromosomes at this point are 179 00:14:15,000 --> 00:14:20,000 extended. And so, they weren't able to visualize them 180 00:14:20,000 --> 00:14:25,000 by looking through the microscope. But as it came time for cell 181 00:14:25,000 --> 00:14:31,000 division, then the chromosomes condensed up very tightly. 182 00:14:31,000 --> 00:14:35,000 This is probably to avoid tangles. And you need to pull them apart 183 00:14:35,000 --> 00:14:40,000 into the two daughter cells. And then the part that could see 184 00:14:40,000 --> 00:14:44,000 them, this was known as mitosis, this process by which the 185 00:14:44,000 --> 00:14:49,000 chromosomes are pulled apart. So, mitosis is a mechanism for 186 00:14:49,000 --> 00:14:53,000 nuclear division that results in two daughters with identical 187 00:14:53,000 --> 00:15:14,000 genetic information. 188 00:15:14,000 --> 00:15:24,000 When you contrast this to meiosis, which I'll show you in just a minute, 189 00:15:24,000 --> 00:15:34,000 this has been studied for years in a kind of observation way. 190 00:15:34,000 --> 00:15:37,000 I showed you at some point. This is an animal cell where the 191 00:15:37,000 --> 00:15:41,000 chromosomes line up, and then they pull apart. 192 00:15:41,000 --> 00:15:45,000 I think I showed you, and you can see the cells divide once they've 193 00:15:45,000 --> 00:15:49,000 done that. I'm going to show you this next thing, 194 00:15:49,000 --> 00:15:53,000 which everything happens more slow motion. This is a picture of the 195 00:15:53,000 --> 00:15:57,000 blood lily, so it's a plant. Again, the choice of model system 196 00:15:57,000 --> 00:16:01,000 is often, if it has a feature that is good for a particular thing. 197 00:16:01,000 --> 00:16:05,000 This happens, very easy to visualize in these lily cells. 198 00:16:05,000 --> 00:16:10,000 Now here, the cells have duplicated their chromosomes, 199 00:16:10,000 --> 00:16:15,000 and the daughter chromosomes are held together. 200 00:16:15,000 --> 00:16:20,000 They're glued together, so you can tell in this thing that 201 00:16:20,000 --> 00:16:25,000 they've been glued. Now, in order to separate the first 202 00:16:25,000 --> 00:16:30,000 thing they have to do is line up at the equator of the cell. 203 00:16:30,000 --> 00:16:35,000 And then it pulls apart and you can see that each daughter cell gets one 204 00:16:35,000 --> 00:16:41,000 of the duplicated chromosomes. OK, so that's mitosis. 205 00:16:41,000 --> 00:16:48,000 It generates two copies of what you started with. The other observation 206 00:16:48,000 --> 00:16:55,000 that the cytologists made was that cell divisions that produce sex 207 00:16:55,000 --> 00:17:02,000 cells work by a different system. So, the third thing, 208 00:17:02,000 --> 00:17:16,000 And, I'm going to give you the 209 00:17:16,000 --> 00:17:20,000 scientific word now for sex cells. So, they're usually called gametes. 210 00:17:20,000 --> 00:17:24,000 General term, so that would be sperm and egg, 211 00:17:24,000 --> 00:17:28,000 or pollen would be a kind of gamete. 212 00:17:28,000 --> 00:17:37,000 In this process, the number of chromosomes is halved. 213 00:17:37,000 --> 00:17:47,000 And this special type of cell division is known as meiosis. 214 00:17:47,000 --> 00:17:57,000 It's very important, because if we didn't have meiosis we wouldn't 215 00:17:57,000 --> 00:18:04,000 have any progeny. We need to be able to cut the number 216 00:18:04,000 --> 00:18:10,000 of chromosomes we have per cell in half, so as Mendel inferred, 217 00:18:10,000 --> 00:18:16,000 then when each parent made a contribution you'd be back up to the 218 00:18:16,000 --> 00:18:22,000 right number of cells. Now, so this process begins again 219 00:18:22,000 --> 00:18:28,000 by duplication of the DNA. So this would be 2N. 220 00:18:28,000 --> 00:18:37,000 Now we go to, as before, with one interesting difference here. 221 00:18:37,000 --> 00:18:53,000 So we are at 4N in the cell. 222 00:18:53,000 --> 00:19:00,000 But you notice that drawing these duplicated chromosomes with 223 00:19:00,000 --> 00:19:08,000 chromatids overlapping. This point of, I didn't do this too 224 00:19:08,000 --> 00:19:15,000 well. The point of connection: what are the two chromosomes overlap is 225 00:19:15,000 --> 00:19:22,000 known as the chiasma. It's a point of actual physical 226 00:19:22,000 --> 00:19:30,000 interaction between the chromosomes. 227 00:19:30,000 --> 00:19:35,000 And what it allows the cell to do is to have the two homologous 228 00:19:35,000 --> 00:19:40,000 chromosomes, in this case the two long ones, find each other, 229 00:19:40,000 --> 00:19:45,000 and actually physically interact. That's a critical event for the 230 00:19:45,000 --> 00:19:51,000 next step that I'm going to show you. It's also a point, 231 00:19:51,000 --> 00:19:56,000 which you'll see in the next lecture. There is additional genetic 232 00:19:56,000 --> 00:20:02,000 diversity introduced into the system. 233 00:20:02,000 --> 00:20:07,000 So, at this point, what's happened is that the DNA has 234 00:20:07,000 --> 00:20:12,000 doubled, at this point, looks sort of like mitosis except 235 00:20:12,000 --> 00:20:17,000 for the fact that the duplicated chromatids have overlapped. 236 00:20:17,000 --> 00:20:22,000 There's a least one of these for every pair of duplicated chromosomes. 237 00:20:22,000 --> 00:20:27,000 So what happens in the next phase, which is known as meiosis I is that 238 00:20:27,000 --> 00:20:32,000 the pairs of duplicated chromosomes are separated. 239 00:20:32,000 --> 00:20:37,000 So, we get two cells. We might get, say, the unshaded of 240 00:20:37,000 --> 00:20:43,000 this one and the shaded of the little guys. And the other one then 241 00:20:43,000 --> 00:20:49,000 is shaded with a long one, and shaded with this one. This is 242 00:20:49,000 --> 00:20:55,000 2N, and this is 2N. But if you'll take a look at what's 243 00:20:55,000 --> 00:21:01,000 happening over here, you will see that we don't have the 244 00:21:01,000 --> 00:21:06,000 same thing going on. In that case, we were producing 245 00:21:06,000 --> 00:21:11,000 identical copies of the starting cell. In this case, 246 00:21:11,000 --> 00:21:16,000 we've got something different now because we've separated both copies 247 00:21:16,000 --> 00:21:21,000 of, let's say, the chromosome from dad from both 248 00:21:21,000 --> 00:21:26,000 copies of the chromosome that originated with mom, 249 00:21:26,000 --> 00:21:31,000 and the other way over here. These, then, undergo a second round 250 00:21:31,000 --> 00:21:37,000 that's known as meiosis 2, which resembles now mitosis in the 251 00:21:37,000 --> 00:21:44,000 sense that the duplicated chromatids are now pulled apart into daughter 252 00:21:44,000 --> 00:21:51,000 cells so that this one will generate what long, unshaded one, 253 00:21:51,000 --> 00:21:58,000 one short, and there will be two of those. In the cell, 254 00:21:58,000 --> 00:22:06,000 it will generate a shaded long and unshaded like that. 255 00:22:06,000 --> 00:22:11,000 So now, there are four of these. And you'll notice that there is now 256 00:22:11,000 --> 00:22:17,000 an N. So, we have the number of chromosomes during meiosis via a 257 00:22:17,000 --> 00:22:23,000 process by two rounds of division. The other thing you can think of is 258 00:22:23,000 --> 00:22:28,000 you can begin to see, at least, when the evolution of sex 259 00:22:28,000 --> 00:22:34,000 was such a powerful force in driving evolution by introducing so much 260 00:22:34,000 --> 00:22:40,000 variation, because each of us has a copy of each of our 22 chromosomes 261 00:22:40,000 --> 00:22:46,000 where we have homologous pairs, one from mom, one from dad. 262 00:22:46,000 --> 00:22:50,000 That every time we make a sex cell, a sperm or an egg, what happens is 263 00:22:50,000 --> 00:22:55,000 for chromosome 1, you can either get a copy from mom 264 00:22:55,000 --> 00:23:00,000 or the copy from dad. And then the next one: copy from 265 00:23:00,000 --> 00:23:05,000 mom, copy from dad, and so on. You can see the variation is 266 00:23:05,000 --> 00:23:11,000 possible just from that part of the process alone. 267 00:23:11,000 --> 00:23:16,000 And there's going to be more variation that come about from 268 00:23:16,000 --> 00:23:22,000 events that happen where this chiasma is as I'll show you. 269 00:23:22,000 --> 00:23:27,000 So, meiosis then, as I said, is a mechanism for nuclear division 270 00:23:27,000 --> 00:23:33,000 that produces four daughter cells with half the genetic info 271 00:23:33,000 --> 00:23:39,000 of the original. So, how did I write that? 272 00:23:39,000 --> 00:24:19,000 And in this case, 273 00:24:19,000 --> 00:24:31,000 the daughters are not identical. Does somebody have a question? 274 00:24:31,000 --> 00:24:39,000 Yeah? Oh, this part of the process, 275 00:24:39,000 --> 00:24:44,000 the cytologists broke this down when they were observing. 276 00:24:44,000 --> 00:24:49,000 So, it was a part they couldn't see. When they started to be able to see 277 00:24:49,000 --> 00:24:53,000 it, they first saw this one division, and then the second division. 278 00:24:53,000 --> 00:24:58,000 They called the first round or division of meiosis 1, and 279 00:24:58,000 --> 00:25:06,000 the second meiosis 2. So it was just a name given to the 280 00:25:06,000 --> 00:25:16,000 process. Yeah? Oh, sorry, excuse me, 281 00:25:16,000 --> 00:25:26,000 I see the problem. OK, it goes this way. This period of 282 00:25:26,000 --> 00:25:36,000 time is called meiosis 1. That part of the process is called 283 00:25:36,000 --> 00:25:43,000 meiosis 1, meiosis 2, sorry. Yeah, I mean what I wrote down as 284 00:25:43,000 --> 00:25:49,000 arbitrary. You could've had to for mom, and two from dad. 285 00:25:49,000 --> 00:25:54,000 And in fact, I guess if we have 22 pairs we could eventually have a 286 00:25:54,000 --> 00:26:00,000 sperm or egg that had all of the copies from mom, and all 287 00:26:00,000 --> 00:26:05,000 of the copies from dad. You could calculate the statistical 288 00:26:05,000 --> 00:26:09,000 chances of that happening, but what you'll discover as we'll 289 00:26:09,000 --> 00:26:14,000 find next time, is that even if you did that the 290 00:26:14,000 --> 00:26:18,000 chromosomes would no longer be identical to what you got from mom 291 00:26:18,000 --> 00:26:23,000 or dad, because there's a little bit of genetic recombination going on 292 00:26:23,000 --> 00:26:27,000 during that process. I'll also show in a minute that you 293 00:26:27,000 --> 00:26:32,000 can get errors in this process, and that these are important. 294 00:26:32,000 --> 00:26:37,000 So, what happens then is that fertilization, 295 00:26:37,000 --> 00:26:42,000 what the cytologists can see happened was that the number of 296 00:26:42,000 --> 00:26:47,000 chromosomes have been halved. So, you had N plus N giving you 2N 297 00:26:47,000 --> 00:26:52,000 where this might be, for example, the egg, 298 00:26:52,000 --> 00:26:57,000 and this might be the sperm, since gametes have half the number 299 00:26:57,000 --> 00:27:02,000 of genetic content when they fuse, then you go back to the original 300 00:27:02,000 --> 00:27:08,000 number. And this gave rise to what was known 301 00:27:08,000 --> 00:27:16,000 as the chromosomal theory of inheritance. Even though 302 00:27:16,000 --> 00:27:24,000 cytologists didn't know what these colored things were, 303 00:27:24,000 --> 00:27:32,000 they seemed to have the properties that you would expect the carriers 304 00:27:32,000 --> 00:27:38,000 of the genetic information to have. Be duplicated before a cell was 305 00:27:38,000 --> 00:27:42,000 going to divide and then they seemed to be very precisely segregated so 306 00:27:42,000 --> 00:27:46,000 that each daughter cell got one copy. And it made sense, 307 00:27:46,000 --> 00:27:51,000 then, in terms of the formation of sex cells you'd have, 308 00:27:51,000 --> 00:27:55,000 and then it would all come back up. So, the cytologists proposed this 309 00:27:55,000 --> 00:27:59,000 chromosomal theory of inheritance. Chromosomes were the carriers that 310 00:27:59,000 --> 00:28:04,000 carried the genetic information. And you can see now that Mendel's 311 00:28:04,000 --> 00:28:08,000 work was rediscovered, which is more or less what happens 312 00:28:08,000 --> 00:28:13,000 around 1900, how beautifully it mapped on top of this because the 313 00:28:13,000 --> 00:28:17,000 genetic information came in particles. You'd think, 314 00:28:17,000 --> 00:28:22,000 oh, well those particles are sitting on those chromosomes. 315 00:28:22,000 --> 00:28:26,000 They get duplicated. They get separated. They get divided in half. 316 00:28:26,000 --> 00:28:31,000 Their number gets divided in half when you make sex cells. 317 00:28:31,000 --> 00:28:36,000 So, I want to at least point out, let me go onto this next thing here. 318 00:28:36,000 --> 00:28:41,000 Here was one other picture I'd shown you. This was a cancer cell 319 00:28:41,000 --> 00:28:46,000 dividing again. We see the same process at work. 320 00:28:46,000 --> 00:28:52,000 And that just reminds you, when I was talking about that kind of 321 00:28:52,000 --> 00:28:57,000 process, trying to remind you that one cell gives rise to two cells, 322 00:28:57,000 --> 00:29:03,000 and one DNA molecule gives rise to two DNA molecules. 323 00:29:03,000 --> 00:29:06,000 But these phenomena are related [even no one tends? 324 00:29:06,000 --> 00:29:10,000 to talk about in different parts of the courses, and if you've got more 325 00:29:10,000 --> 00:29:14,000 than one DNA molecule per cell, then each one's a chromosome. And 326 00:29:14,000 --> 00:29:18,000 then you have to take care of segregating those as well. 327 00:29:18,000 --> 00:29:22,000 But a point that I'd made is that up until this part at the right 328 00:29:22,000 --> 00:29:26,000 could be a yeast or an E. coli, but if it's going to be 329 00:29:26,000 --> 00:29:30,000 someone, something more complicated that's got different kinds of cells 330 00:29:30,000 --> 00:29:34,000 like us, then we have these ultimately, an adult human 331 00:29:34,000 --> 00:29:39,000 with 1014 cell. So, this process has happened over, 332 00:29:39,000 --> 00:29:45,000 and over, and over again. And it has to be really very accurate. 333 00:29:45,000 --> 00:29:52,000 And, there was remarkable advances have been made in understanding this 334 00:29:52,000 --> 00:29:59,000 process over the last few years. And a lot of this was due to the 335 00:29:59,000 --> 00:30:06,000 work of Lee Hartwell, who carried out experiments, 336 00:30:06,000 --> 00:30:11,000 I guess, that offered some key insights into the basis of the whole 337 00:30:11,000 --> 00:30:16,000 cell cycle, of which this is just part. So they said, 338 00:30:16,000 --> 00:30:21,000 here we can't really see all of this stuff in the microscope. 339 00:30:21,000 --> 00:30:26,000 What cytologists were able to observe was that after the 340 00:30:26,000 --> 00:30:31,000 chromosomes had duplicated, then they condensed up. They became 341 00:30:31,000 --> 00:30:36,000 visible, and they could see that part of the system. 342 00:30:36,000 --> 00:30:41,000 So, a great body of knowledge has been learned over the past few years. 343 00:30:41,000 --> 00:30:46,000 Lee Hartwell, who was a postdoc at MIT with Boris 344 00:30:46,000 --> 00:30:51,000 Magasanik a number of years ago, was one of the key people who led to 345 00:30:51,000 --> 00:30:56,000 this round of insights. And what we now know, the cell 346 00:30:56,000 --> 00:31:02,000 cycle can be basically divided into four parts. 347 00:31:02,000 --> 00:31:07,000 So a newly born cell, if you will, undergoes what's known 348 00:31:07,000 --> 00:31:12,000 as phase of the cell cycle that's known as the G1 phase. 349 00:31:12,000 --> 00:31:18,000 And you could think of this as, amongst other things, sort of 350 00:31:18,000 --> 00:31:23,000 preparation for duplicating the cell's DNA. And then, 351 00:31:23,000 --> 00:31:29,000 there's a phase known as the S-phase, which is what DNA synthesis 352 00:31:29,000 --> 00:31:35,000 takes place. That now gives the cell and 353 00:31:35,000 --> 00:31:42,000 information content of 4N. There was then another phase known 354 00:31:42,000 --> 00:31:48,000 as G2, which you could think of as preparation for pulling the 355 00:31:48,000 --> 00:31:55,000 chromosomes apart, and for the cell dividing. 356 00:31:55,000 --> 00:32:01,000 And that's known as mitosis. And so, this is quite remarkable. 357 00:32:01,000 --> 00:32:07,000 It's quite fundamental. It's a very elegant and very complex system 358 00:32:07,000 --> 00:32:12,000 of controls. And what Lee learned is that there is an elaborate system 359 00:32:12,000 --> 00:32:18,000 of what we now know as checkpoints in the cell cycle. 360 00:32:18,000 --> 00:32:23,000 And the idea is pretty much the same sort of thing that you might do 361 00:32:23,000 --> 00:32:29,000 if you were an engineer and you were setting up a quality control process 362 00:32:29,000 --> 00:32:34,000 that involved a series of stages. And what a checkpoint does is it 363 00:32:34,000 --> 00:32:38,000 basically commands successful completion of a prior phase before 364 00:32:38,000 --> 00:32:42,000 the system is permitted to move to the next stage. 365 00:32:42,000 --> 00:32:46,000 And you can sort of see the problem here, that if you are trying to, 366 00:32:46,000 --> 00:32:50,000 say you're copying your chromosome, your DNA, and you only got part way 367 00:32:50,000 --> 00:32:54,000 through. And then he proceeded to the part where you pulled the 368 00:32:54,000 --> 00:32:58,000 chromosomes apart. There would be all kinds of chaos 369 00:32:58,000 --> 00:33:02,000 inside of the cells because you would be breaking chromosomes. 370 00:33:02,000 --> 00:33:09,000 You wouldn't be getting identical information. And there are three 371 00:33:09,000 --> 00:33:17,000 major checkpoints. There is a checkpoint at the G1 S 372 00:33:17,000 --> 00:33:25,000 boundary. There is actually more than one. But there's an intra-S 373 00:33:25,000 --> 00:33:33,000 phase checkpoint, and then there's another one at the 374 00:33:33,000 --> 00:33:39,000 G2 M boundary as well. And the way that Lee found this out, 375 00:33:39,000 --> 00:33:43,000 I wanted to just take you back because remember when I was talking, 376 00:33:43,000 --> 00:33:47,000 well, first we talked about how would you get an mutant affecting an 377 00:33:47,000 --> 00:33:52,000 essential cell, a gene whose function is essential 378 00:33:52,000 --> 00:33:56,000 for the cell. [He said? you'd have to be conditional in 379 00:33:56,000 --> 00:34:00,000 some kind of way, because the organism would be dead, 380 00:34:00,000 --> 00:34:05,000 and couldn't use genetics to study it. So, when I introduced you to 381 00:34:05,000 --> 00:34:09,000 those phage, and we wanted to study critical functions needed for phage 382 00:34:09,000 --> 00:34:13,000 growth, we needed a conditional mutant. Remember, 383 00:34:13,000 --> 00:34:18,000 we did look for temperature sensitive mutants. 384 00:34:18,000 --> 00:34:22,000 So that's what Lee Hartwell started to do with yeast in 1970 385 00:34:22,000 --> 00:34:27,000 approximately. He just looked for mutations that 386 00:34:27,000 --> 00:34:32,000 were temperature sensitive lethals in yeast. 387 00:34:32,000 --> 00:34:35,000 Now, yeast can be diploid, but they also can be haploid, 388 00:34:35,000 --> 00:34:39,000 which is just one copy of genetic information. So just like the 389 00:34:39,000 --> 00:34:43,000 bacteria [haploid yeast? , if you get a mutation affecting a 390 00:34:43,000 --> 00:34:46,000 gene, then you see its phenotype right away because there isn't a 391 00:34:46,000 --> 00:34:50,000 complication of the second chromosome. And Lee got the Nobel 392 00:34:50,000 --> 00:34:54,000 Prize for his work, just, I forget, three years ago or a 393 00:34:54,000 --> 00:34:58,000 short time ago anyway. And so, this was the process he was 394 00:34:58,000 --> 00:35:01,000 studying. I showed you this movie. 395 00:35:01,000 --> 00:35:05,000 In this case, this is budding yeast, and you can see why it's called 396 00:35:05,000 --> 00:35:09,000 budding yeast is that rather than the still growing and segregating 397 00:35:09,000 --> 00:35:13,000 down the middle, the daughter cells grow as little 398 00:35:13,000 --> 00:35:16,000 buds coming off the side. The DNA is still going to be 399 00:35:16,000 --> 00:35:20,000 duplicated as we got, and then segregated into the 400 00:35:20,000 --> 00:35:24,000 daughter. So that part of the process is there, 401 00:35:24,000 --> 00:35:28,000 but there's yeast growing sped up a bit so you can see it. 402 00:35:28,000 --> 00:35:32,000 So, what we found was that when he looked at his TS mutants, 403 00:35:32,000 --> 00:35:36,000 he found that they tended to arrest at characteristic stages, 404 00:35:36,000 --> 00:35:41,000 and he was able, then, to figure out that there must be some sort of 405 00:35:41,000 --> 00:35:45,000 checkpoint system that is something went wrong during one of the faces, 406 00:35:45,000 --> 00:35:50,000 then the cells with all pile up at a particular characteristic stage in 407 00:35:50,000 --> 00:35:54,000 the cell cycle. And this was controlled by these 408 00:35:54,000 --> 00:35:59,000 checkpoints. These checkpoints are of enormous 409 00:35:59,000 --> 00:36:04,000 importance because if something goes wrong with them, 410 00:36:04,000 --> 00:36:08,000 as happens often in cancer cells, then you get a huge increase in 411 00:36:08,000 --> 00:36:13,000 instability of the genome very similar to the way, 412 00:36:13,000 --> 00:36:18,000 using, for example, mismatch repair causes instability of the genome and 413 00:36:18,000 --> 00:36:22,000 therefore things that can cause a cell to forget to divide when it 414 00:36:22,000 --> 00:36:27,000 stopped dividing when it hits its neighbors, which part of the cell it 415 00:36:27,000 --> 00:36:32,000 belongs to and so on. Those kinds of changes will come 416 00:36:32,000 --> 00:36:36,000 with much greater frequency. Another thing that was completely 417 00:36:36,000 --> 00:36:41,000 remarkable was because it was yeast it was relatively easy to find genes 418 00:36:41,000 --> 00:36:46,000 that were broken. Going back to complementation, 419 00:36:46,000 --> 00:36:50,000 leaving aside the technicalities, if we had a mutant that's a TS 420 00:36:50,000 --> 00:36:55,000 lethal and it's dead, if we put a good copy we'll know it 421 00:36:55,000 --> 00:37:00,000 because the cell won't die at a high temperature anymore. 422 00:37:00,000 --> 00:37:03,000 So, you can sort of see from the first principle it was relatively 423 00:37:03,000 --> 00:37:06,000 straightforward to find the genes in yeast that are necessary for cell 424 00:37:06,000 --> 00:37:10,000 cycle control. The remarkable thing is that 425 00:37:10,000 --> 00:37:13,000 they've been concerns all the way up to humans that the proteins that are 426 00:37:13,000 --> 00:37:17,000 involved in the cell cycle control are virtually identical in humans. 427 00:37:17,000 --> 00:37:20,000 So, the system, again, is probably locked into place because it works 428 00:37:20,000 --> 00:37:24,000 so well. And once it was working, then evolution didn't have a lot of 429 00:37:24,000 --> 00:37:27,000 space. I won't say it's completely identical in human cells, 430 00:37:27,000 --> 00:37:31,000 for example, have a few more wrinkles that they throw away. 431 00:37:31,000 --> 00:37:36,000 And fundamentally, you can see evolutionarily that the 432 00:37:36,000 --> 00:37:41,000 system [involved once? . One thing I just want to comment 433 00:37:41,000 --> 00:37:46,000 on, and here's just an example of the sort of thing that can happen if 434 00:37:46,000 --> 00:37:51,000 you get a problem with pulling chromosomes apart when they're not 435 00:37:51,000 --> 00:37:56,000 ready, or something happens that make a break in a chromosome. 436 00:37:56,000 --> 00:38:01,000 You will get a double strand break, and cells don't like that, and they 437 00:38:01,000 --> 00:38:06,000 try and fix it up. I think in this case, 438 00:38:06,000 --> 00:38:10,000 you can see the chromosome 9 has picked up a piece of a couple of 439 00:38:10,000 --> 00:38:14,000 other chromosomes. This piece here came from 440 00:38:14,000 --> 00:38:18,000 chromosome 8 over here, and this piece here came from 441 00:38:18,000 --> 00:38:22,000 chromosome 22. And so, this cell now has things 442 00:38:22,000 --> 00:38:26,000 kind of messed up. And this particular chromosome's 443 00:38:26,000 --> 00:38:30,000 got parts of two other chromosomes. And so, you can start to think 444 00:38:30,000 --> 00:38:35,000 about the consequences of that. You'll see when you start 445 00:38:35,000 --> 00:38:39,000 segregating chromosomes, there are issues. And also, 446 00:38:39,000 --> 00:38:43,000 the junctions themselves can sometimes do things like creating 447 00:38:43,000 --> 00:38:48,000 fusion proteins that have aberrant properties, for example, 448 00:38:48,000 --> 00:38:52,000 a signaling molecule that's now locked permanently into the on 449 00:38:52,000 --> 00:38:57,000 position or something like that. Those sorts of things can happen by 450 00:38:57,000 --> 00:39:01,000 these as well. So, here's a very abbreviated form, 451 00:39:01,000 --> 00:39:05,000 the principal that happens, or what happens so that the chromosomes 452 00:39:05,000 --> 00:39:10,000 become visible in the microscope. There is about a couple of meters of 453 00:39:10,000 --> 00:39:14,000 DNA in every human cell. So, you can see almost from first 454 00:39:14,000 --> 00:39:18,000 principles, it almost has to be packaged in some way to fit in that 455 00:39:18,000 --> 00:39:22,000 little, tiny area. And down at the basic level, 456 00:39:22,000 --> 00:39:26,000 the DNA is looped around a little cluster of proteins called histones. 457 00:39:26,000 --> 00:39:30,000 And then, as the cells approach the mitotic stage, 458 00:39:30,000 --> 00:39:34,000 they package these and higher, and higher, and higher levels of 459 00:39:34,000 --> 00:39:38,000 packaging so that the chromosomes get more and more compact, 460 00:39:38,000 --> 00:39:42,000 and therefore less like spaghetti, and less likely to get tangled when 461 00:39:42,000 --> 00:39:46,000 they are pulled apart. Now, this teaching as part of the 462 00:39:46,000 --> 00:39:50,000 course. It's in the book. It's a little like glycolysis, 463 00:39:50,000 --> 00:39:54,000 though, and there's so much detail. It's hard to notice at all. And 464 00:39:54,000 --> 00:39:58,000 what I've tried to do here is just sort of take a step back, 465 00:39:58,000 --> 00:40:02,000 focus you on the really big parts of the process. 466 00:40:02,000 --> 00:40:06,000 But you can see this is a sort of typical textbook thing showing you 467 00:40:06,000 --> 00:40:10,000 the various sub stages of this process that cytologists have given 468 00:40:10,000 --> 00:40:14,000 names to each part of this. For example, you can see the 469 00:40:14,000 --> 00:40:19,000 chromosomes lining up at the equatorial phase just before they're 470 00:40:19,000 --> 00:40:23,000 going to be pulled apart. One little detail which I glossed 471 00:40:23,000 --> 00:40:27,000 over at this point as you can see that the chromosomes are, 472 00:40:27,000 --> 00:40:32,000 of course, surrounded by the nuclear membrane. 473 00:40:32,000 --> 00:40:35,000 And, at least in many cells, what happens is just before the 474 00:40:35,000 --> 00:40:39,000 chromosomes are going to be pulled apart, this nuclear membrane in a 475 00:40:39,000 --> 00:40:42,000 matter of 20 or 30 seconds goes from being a membrane into sort of 476 00:40:42,000 --> 00:40:46,000 breaking into a whole little series of membrane sacs. 477 00:40:46,000 --> 00:40:50,000 And then, after the chromosomes have pulled apart, 478 00:40:50,000 --> 00:40:53,000 now you reassemble the nuclear membrane around each set of 479 00:40:53,000 --> 00:40:57,000 chromosomes. And I think there's still an awful lot to be learned 480 00:40:57,000 --> 00:41:01,000 about how that particular trick is done. And that's a 481 00:41:01,000 --> 00:41:05,000 textbook diagram. These are the kinds of things the 482 00:41:05,000 --> 00:41:09,000 cytologists were looking at as they were describing it. 483 00:41:09,000 --> 00:41:14,000 And one thing you can certainly see, I think, is that there are a lot of, 484 00:41:14,000 --> 00:41:18,000 if you want to think of them as cables, this whole process is done 485 00:41:18,000 --> 00:41:23,000 by molecular machinery. And in fact, one of the key things 486 00:41:23,000 --> 00:41:27,000 for ensuring that the chromosomes are all lined up is each are 487 00:41:27,000 --> 00:41:33,000 attached to each poll of the cell. So, an individual chromosome feels 488 00:41:33,000 --> 00:41:39,000 tension on both sides, and there's pretty good evidence at 489 00:41:39,000 --> 00:41:45,000 this point that says that what has to happen before mitosis is allowed 490 00:41:45,000 --> 00:41:51,000 to proceed is that each duplicated chromosome must feel the tension on 491 00:41:51,000 --> 00:41:57,000 both sides. And if one of them is only attached on one side, 492 00:41:57,000 --> 00:42:02,000 it's not allowed to go ahead. Here are a couple more pictures. 493 00:42:02,000 --> 00:42:08,000 The chromosomes are in blue, and all this cabling is in green. 494 00:42:08,000 --> 00:42:14,000 You can see a molecular machine's basically at work. 495 00:42:14,000 --> 00:42:19,000 And I wanted to show you this picture because this is important. 496 00:42:19,000 --> 00:42:25,000 There's two parts to this. Julie, could you hit? So, this is a normal 497 00:42:25,000 --> 00:42:31,000 mitosis. These are in lung cells from a newt, which are very flat. 498 00:42:31,000 --> 00:42:34,000 So it's easy to visualize them. I believe this is by Nomarski, a 499 00:42:34,000 --> 00:42:38,000 kind of optics called Nomarksi optics. What they're doing, 500 00:42:38,000 --> 00:42:42,000 the duplicated chromosomes are getting lined up at the equatorial 501 00:42:42,000 --> 00:42:46,000 position in the cell, and you get a spindle attached to 502 00:42:46,000 --> 00:42:50,000 one side, and then a spindle attached to the other. 503 00:42:50,000 --> 00:42:54,000 And once that's happened, then you can see the chromosomes. 504 00:42:54,000 --> 00:42:58,000 Watch this one over here, for example. It just got attached, 505 00:42:58,000 --> 00:43:02,000 and it's being pulled into the middle. 506 00:43:02,000 --> 00:43:07,000 And once the cells feel that all the attention, there's tension on all of 507 00:43:07,000 --> 00:43:12,000 them, and then it enables the next stage of the process. 508 00:43:12,000 --> 00:43:17,000 And you will see each of the daughter chromatids segregate before. 509 00:43:17,000 --> 00:43:22,000 And, I just want to impress upon you it's complicated enough just 510 00:43:22,000 --> 00:43:27,000 replicating the DNA at the [fidelity? . We talked about an awful lot of 511 00:43:27,000 --> 00:43:32,000 other stuff has to go on. And it would be a reasonable 512 00:43:32,000 --> 00:43:36,000 question: is there ever a mistake? And there is. This one actually 513 00:43:36,000 --> 00:43:40,000 shows another cell where there's going to be a mistake. 514 00:43:40,000 --> 00:43:44,000 You can't see it yet, but you can see the chromosomes are all 515 00:43:44,000 --> 00:43:48,000 condensed up. And they are beginning to get these spindles 516 00:43:48,000 --> 00:43:52,000 attached. And as they get attached to the site then they get localized 517 00:43:52,000 --> 00:43:56,000 in the central part right down the equatorial plane of the cell. 518 00:43:56,000 --> 00:44:00,000 But, I think you'll start to see the problem emerging at the moment. 519 00:44:00,000 --> 00:44:04,000 I think it's going to be this guy right here. I think a little arrow 520 00:44:04,000 --> 00:44:08,000 will probably appear before too long. But what's happened is this one 521 00:44:08,000 --> 00:44:12,000 didn't get properly connected on both sides. But, 522 00:44:12,000 --> 00:44:16,000 the cell's going to go ahead anyway. Watch. The one up here is going to 523 00:44:16,000 --> 00:44:20,000 get pulled in. There it goes. 524 00:44:20,000 --> 00:44:24,000 It's getting pulled in. And then, once it's in there, 525 00:44:24,000 --> 00:44:28,000 then the cell is going to go ahead and divide anyway. 526 00:44:28,000 --> 00:44:32,000 So you can see what the problem is. This one's got to over here, 527 00:44:32,000 --> 00:44:38,000 and this one over here is missing it. This process is known as 528 00:44:38,000 --> 00:44:43,000 non-disjunction. This is the sort of thing that 529 00:44:43,000 --> 00:44:49,000 happens in cancer cells that have broken, where the checkpoint gene is 530 00:44:49,000 --> 00:44:54,000 broken. And then, if you have that, then the cells go 531 00:44:54,000 --> 00:45:00,000 on. The same problem happens in meiosis, and it's up here. 532 00:45:00,000 --> 00:45:06,000 It's the same principle that if you end up with, if you don't segregate 533 00:45:06,000 --> 00:45:12,000 the cells, the chromosomes correctly, then you can end up with a sex cell 534 00:45:12,000 --> 00:45:18,000 that has two copies of one of them. And the other one has none. And if 535 00:45:18,000 --> 00:45:24,000 you have two copies, or 2N plus 1N, then you end up with 536 00:45:24,000 --> 00:45:30,000 a trisomy. And if you have zero plus N, you end up with what's known 537 00:45:30,000 --> 00:45:35,000 as a monosomy. So, these would be from defects in 538 00:45:35,000 --> 00:45:41,000 meiosis. I'll just close by saying the one that you probably have heard 539 00:45:41,000 --> 00:45:47,000 of is Down's syndrome which is trisomy 21. That comes about from 540 00:45:47,000 --> 00:45:53,000 this sort of process. It's a very debilitating genetic 541 00:45:53,000 --> 00:45:59,000 deficiency. Most of you are probably familiar with it. 542 00:45:59,000 --> 00:46:04,000 This process happens. Things go wrong at meiosis, 543 00:46:04,000 --> 00:46:08,000 about one in every [fifth?] detectable human pregnancy doesn't 544 00:46:08,000 --> 00:46:13,000 go to completion for the most part because of an underlying problem 545 00:46:13,000 --> 00:46:17,000 like this, that the body is set up [to need?] the pairs of each 546 00:46:17,000 --> 00:46:21,000 chromosome. If you have one too many or one too few it's usually 547 00:46:21,000 --> 00:46:26,000 lethal. So, if one in every five human pregnancies has that happened, 548 00:46:26,000 --> 00:46:30,000 you end up with a miscarriage. My wife and I have been through it. 549 00:46:30,000 --> 00:46:34,000 It's a devastating experience. I think you can just look around and 550 00:46:34,000 --> 00:46:38,000 figure the odds. A significant number of the people 551 00:46:38,000 --> 00:46:41,000 in this room are going to have to cope with this. 552 00:46:41,000 --> 00:46:45,000 And I'm just bringing this up for your attention right now is it 553 00:46:45,000 --> 00:46:49,000 something that in our society we tend not to talk about very much 554 00:46:49,000 --> 00:46:52,000 unless maybe you're a Hollywood movie star and it makes it into the 555 00:46:52,000 --> 00:46:56,000 tabloids or something. But it's a very common human 556 00:46:56,000 --> 00:46:59,000 experience, and very difficult to cope with. But there it is, 557 00:46:59,000 --> 00:47:03,000 for most of those pregnancies that terminate very early, 558 00:47:03,000 --> 00:47:06,000 usually the underlying cause is a problem and traces back to the kind 559 00:47:06,000 --> 00:47:10,000 of stuff we see here where something went wrong during the segregation 560 00:47:10,000 --> 00:47:14,000 process of making the sex chromosomes. 561 00:47:14,000 --> 00:47:18,000 And as I say, if it happens in just our ordinary cells, 562 00:47:18,000 --> 00:47:23,000 that's one of the thinks that leads the progression to cancer. 563 00:47:23,000 --> 00:47:26,000 OK, so I'll see you on Wednesday.