0:00:00.385,0:00:01.557 - [Voiceover] In the last video, 0:00:01.557,0:00:04.459 we had just started to get into meiosis, 0:00:04.459,0:00:06.398 and to be more precise, meiosis I, 0:00:06.398,0:00:08.964 and to be even more precise[br]than that, prophase I, 0:00:08.964,0:00:11.018 but we spent a good bit[br]of time on prophase I 0:00:11.018,0:00:12.783 because some interesting things happened. 0:00:12.783,0:00:15.198 Some things happened just[br]like prophase in mitosis where 0:00:15.198,0:00:19.226 the nuclear envelope disappears[br]or starts to disappear, 0:00:19.226,0:00:23.022 you have the chromosomes[br]going into their dense form 0:00:23.022,0:00:24.705 that has kinda this classic shape that you 0:00:24.705,0:00:26.505 could see from a microscope, 0:00:26.505,0:00:29.395 but what was unique or[br]what was interesting 0:00:29.395,0:00:32.820 about meiosis I and[br]prophase I in particular 0:00:32.820,0:00:35.479 is that you have this[br]chromosomal crossover, 0:00:35.479,0:00:39.484 that is a pretty typical[br]thing to happen in meiosis I, 0:00:39.484,0:00:43.303 and it tends to happen[br]in a fairly clean way 0:00:43.303,0:00:47.238 where homologous sections[br]of these homologous pairs 0:00:47.238,0:00:51.329 crossover, so these[br]sections of the chromosome 0:00:51.329,0:00:54.139 tend to code for the same genes. 0:00:54.139,0:00:55.892 They're just different[br]variants of those same genes. 0:00:55.892,0:00:57.343 They might have different alleles, 0:00:57.343,0:00:59.653 and then once again, this[br]just adds more variation 0:00:59.653,0:01:02.277 as we get into sexual reproduction, 0:01:02.277,0:01:04.970 so it's a kind of neat[br]thing that happens here. 0:01:04.970,0:01:07.036 But now let's continue with meiosis, 0:01:07.036,0:01:09.718 and in particular meiosis[br]I, and you could guess 0:01:09.718,0:01:11.993 what the next phase is going to be called. 0:01:11.993,0:01:16.993 It is metaphase I, metaphase, metaphase I, 0:01:17.554,0:01:21.814 and it has some similarities[br]with metaphase in mitosis. 0:01:21.814,0:01:24.943 So in metaphase I, let me draw my cell, 0:01:25.727,0:01:30.231 so this is the cellular[br]membrane right over there. 0:01:30.231,0:01:32.413 I have my centrosomes,[br]which are now going to play 0:01:32.413,0:01:34.800 more significant roles. 0:01:35.153,0:01:38.032 The nuclear membrane is now gone, 0:01:38.032,0:01:41.527 and just like in metaphase in mitosis, 0:01:41.527,0:01:43.674 my chromosomes are going to line up 0:01:43.674,0:01:48.225 along the, here I'll draw it,[br]kind of this up, down axis. 0:01:48.225,0:01:49.595 So let's do that. 0:01:49.595,0:01:52.956 So you have this one right over here. 0:01:53.774,0:01:57.182 This is one chromosome,[br]two sister chromatids, 0:01:57.245,0:01:58.905 and we had the chromosomal crossover, 0:01:58.905,0:02:00.507 so it has a little bit of pink here. 0:02:00.507,0:02:01.285 I'm gonna take a little bit of time 0:02:01.285,0:02:03.189 to switch colors a little[br]bit more frequently. 0:02:03.189,0:02:05.685 And then you have the one,[br]at least most of which 0:02:05.685,0:02:07.792 you got from your mother, 0:02:07.972,0:02:09.063 yeah but there's been a little bit 0:02:09.063,0:02:12.134 of chromosomal crossover here as well. 0:02:12.290,0:02:14.096 So let me draw that. 0:02:14.403,0:02:15.587 Let me draw that. 0:02:15.587,0:02:17.004 And then you have this one, 0:02:17.004,0:02:19.082 and just for the sake of, 0:02:19.082,0:02:23.221 so you have this one, this[br]chromosome from your father. 0:02:23.621,0:02:26.692 It has replicated, so it's[br]now two sister chromatids. 0:02:26.720,0:02:27.661 And this one from your mother, 0:02:27.661,0:02:30.226 and I'm not gonna show the[br]chromosomal crossover here. 0:02:30.226,0:02:32.171 Maybe it didn't happen over here. 0:02:32.525,0:02:34.818 No homologous recombination over here. 0:02:35.311,0:02:37.360 So these are, I guess, shorter. 0:02:38.248,0:02:40.379 Now let me draw the centromeres. 0:02:40.605,0:02:43.095 The centromeres I started[br]doing in this blue color. 0:02:43.449,0:02:47.442 So the centromeres, the centromeres, 0:02:47.442,0:02:48.824 and then the centrosomes, 0:02:48.824,0:02:51.262 you have these microtubules that start, 0:02:51.262,0:02:54.303 they can push the centrosomes[br]away from each other. 0:02:54.303,0:02:58.529 But they also attach at the[br]kinetochores to the chromosomes, 0:02:58.529,0:03:01.629 to the chromosomes, just like that. 0:03:01.629,0:03:05.146 And these are, the microtubules,[br]you'll see people talk 0:03:05.146,0:03:07.398 about oh these connect, and they're able 0:03:07.398,0:03:08.223 to move things around, 0:03:08.223,0:03:10.579 but I find this incredible[br]that you just have 0:03:10.579,0:03:13.447 a bunch of proteins through[br]just kind of chemical 0:03:13.447,0:03:17.986 and thermodynamic processes, 0:03:17.986,0:03:19.901 are able to do really interesting things 0:03:19.901,0:03:24.684 like move chromosomes to[br]different parts of the cell, 0:03:24.684,0:03:27.401 so that we eventually can[br]get these gametes that can 0:03:27.401,0:03:30.965 participate in sexual reproduction. 0:03:30.965,0:03:32.300 This is an amazing thing, 0:03:32.300,0:03:37.300 and it's developed over[br]billions of years of evolution, 0:03:38.325,0:03:41.157 but it's just mind boggling[br]to think about the complexity, 0:03:41.157,0:03:42.504 and not all of this is[br]completely understood 0:03:42.504,0:03:43.874 exactly how all of this works. 0:03:43.874,0:03:45.836 I mean you have these[br]kind of motor proteins 0:03:45.836,0:03:50.836 that help move the chromosomes[br]along, these microtubules 0:03:51.199,0:03:53.753 can elongate and shorten[br]in interesting ways. 0:03:53.753,0:03:55.761 So it's a really fascinating process. 0:03:55.761,0:03:58.733 But anyway, this is what's[br]happening in metaphase I. 0:03:58.733,0:04:01.252 Now you can probably guess[br]what happens after that. 0:04:01.252,0:04:03.632 We then move to anaphase I. 0:04:03.632,0:04:06.581 So let me, we now go to anaphase I. 0:04:06.581,0:04:08.004 I'll write that over here. 0:04:08.113,0:04:11.533 Anaphase, anaphase I, 0:04:12.014,0:04:14.881 and just like anaphase in mitosis, 0:04:14.881,0:04:19.072 over here, the chromosomes[br]start getting pulled apart, 0:04:19.072,0:04:21.893 except for one significant difference, 0:04:21.893,0:04:24.349 and this is actually a very[br]significant difference. 0:04:24.378,0:04:29.378 In mitosis, the sister[br]chromatids get pulled apart. 0:04:30.159,0:04:32.121 The sister chromatids get pulled apart 0:04:32.121,0:04:34.779 to become two daughter chromosomes. 0:04:34.779,0:04:37.554 That does not happen in anaphase I. 0:04:37.554,0:04:41.849 In anaphase I, the sister[br]chromatids stay together. 0:04:41.849,0:04:45.942 It's the homologous pairs[br]that get pulled apart. 0:04:45.970,0:04:47.370 So let me draw that. 0:04:47.549,0:04:49.952 So this homologous pair[br]up here gets pulled apart. 0:04:49.952,0:04:52.959 The two sister chromatids do[br]not get pulled apart here. 0:04:52.959,0:04:56.901 So you have this one is[br]getting pulled onto this side. 0:04:57.533,0:05:00.017 So this one's getting[br]pulled onto this side. 0:05:00.017,0:05:03.395 It has a little bit from the original, 0:05:03.395,0:05:06.037 so a little bit of that right over there. 0:05:06.077,0:05:09.102 And then you have this one[br]getting pulled on this side. 0:05:09.548,0:05:14.517 So draw it the best I can, the colors, 0:05:14.517,0:05:15.515 alright, so it looks like that, 0:05:15.515,0:05:17.628 although it's nice to have,[br]it's kinda easy to keep track of 0:05:17.628,0:05:19.787 cause these switch colors like that. 0:05:19.787,0:05:22.824 And then you have this one[br]getting pulled on this side. 0:05:23.375,0:05:25.035 This one getting pulled on this side. 0:05:25.035,0:05:30.035 And finally finally this one[br]getting pulled onto that side. 0:05:32.372,0:05:34.728 And let me draw the centrosomes. 0:05:34.728,0:05:37.410 So that's my, oops, centrosome, 0:05:37.410,0:05:39.290 and once again, it's pulling, 0:05:39.290,0:05:41.844 or I guess you could say the[br]chromosomes are being moved 0:05:41.844,0:05:43.667 and these things are[br]pushing each other apart. 0:05:43.667,0:05:45.072 The two centrosomes might be pushing apart 0:05:45.072,0:05:48.671 to get to the opposite[br]ends of the actual cell, 0:05:48.671,0:05:50.609 but they're bringing, 0:05:50.609,0:05:52.165 there's all sorts of[br]interesting mechanisms 0:05:52.165,0:05:54.313 that are bringing along[br]these microtubules, 0:05:54.313,0:05:56.530 bringing the chromosomes, 0:05:56.530,0:05:58.794 once again splitting the homologous pairs. 0:05:58.794,0:06:00.732 And how they split is random. 0:06:00.732,0:06:02.926 You know, this pink one could[br]have been on the right side, 0:06:02.926,0:06:04.087 this orange one could have[br]been on the left side, 0:06:04.087,0:06:05.388 or vice versa, and once again, 0:06:05.388,0:06:10.388 this adds more variation[br]amongst the gametes, 0:06:12.365,0:06:15.685 so even all of the resulting[br]gametes that get produced, 0:06:15.685,0:06:19.423 they all will have different[br]genetic information. 0:06:19.423,0:06:20.932 So this is anaphase I. 0:06:20.932,0:06:22.209 You're pulling these apart, and 0:06:22.209,0:06:25.315 then you could imagine what[br]happens in telophase I. 0:06:25.332,0:06:30.332 So telophase I, telophase, telophase I. 0:06:32.564,0:06:36.553 Telophase I, and this is fairly analogous 0:06:36.553,0:06:39.502 to what happens in mitosis in telophase. 0:06:39.502,0:06:44.221 So now you have your[br]cytokinesis is beginning, 0:06:44.412,0:06:47.001 and actually, it might even begin earlier, 0:06:47.001,0:06:49.811 in mitosis it happens[br]as early as anaphase, 0:06:49.811,0:06:51.540 at least the cytokinesis is starting, 0:06:51.540,0:06:53.119 but you're starting to see that. 0:06:53.119,0:06:58.119 The homologous pairs[br]are fully split apart, 0:06:58.378,0:06:59.794 and they're at opposite ends, 0:06:59.794,0:07:03.451 and actually they can begin to unravel 0:07:03.451,0:07:04.856 into their chromatin state, 0:07:04.856,0:07:09.856 so this one began to unravel[br]into its chromatin state. 0:07:10.359,0:07:12.327 It has a little bit of the magenta. 0:07:12.390,0:07:16.877 Oops, it has a little bit of[br]the magenta right over here. 0:07:17.278,0:07:19.530 This is unravelling as well. 0:07:19.530,0:07:22.177 This is unravelling like that, 0:07:22.177,0:07:24.127 once it gets into its chromatin state. 0:07:24.127,0:07:28.376 The cellular, and let me[br]do the other ones as well. 0:07:28.376,0:07:32.207 So this is this one right over here. 0:07:32.207,0:07:34.686 It's beginning to unravel. 0:07:35.248,0:07:39.114 This one over here, beginning to unravel. 0:07:39.114,0:07:41.384 It's got a bit of orange on it. 0:07:41.668,0:07:42.968 It's got a little bit of orange on it. 0:07:42.968,0:07:45.847 The nuclear membrane begins to form again. 0:07:45.847,0:07:50.822 The nuclear membrane begins to form again. 0:07:51.791,0:07:54.368 In some ways, it's reversing[br]what happened in prophase I 0:07:54.368,0:07:55.750 where the nuclear membrane disappeared, 0:07:55.750,0:07:58.159 and the chromosomes condensed. 0:07:58.524,0:08:03.522 And let me draw, let me[br]draw the centrosomes, 0:08:03.632,0:08:06.732 which are outside the nuclear[br]membrane, just like that. 0:08:06.732,0:08:09.664 And the microtubules are also dissolving. 0:08:09.866,0:08:11.747 The microtubules are also dissolving. 0:08:11.747,0:08:14.591 And you have your cytokinesis. 0:08:14.591,0:08:17.006 So your cytokinesis, so these separate. 0:08:17.006,0:08:20.379 These separate into two cells. 0:08:20.512,0:08:22.311 So once again, when we did the overview 0:08:22.311,0:08:26.406 of meiosis, we said look,[br]the first phase of meisosis, 0:08:26.406,0:08:31.406 you go from a diploid germ[br]cell to two haploid cells. 0:08:32.250,0:08:35.283 And these aren't quite[br]our end product yet. 0:08:35.283,0:08:37.546 This right over here, what[br]we have just gone through, 0:08:37.546,0:08:39.323 what we have just gone through, 0:08:39.323,0:08:42.608 all of this combined that[br]we have just gone through, 0:08:42.608,0:08:44.268 this is meiosis I. 0:08:44.268,0:08:48.482 And in the next video, we're[br]gonna go through meiosis II. 0:08:48.482,0:08:50.775 Whoops, I didn't mean to do that. 0:08:51.141,0:08:56.141 This is, so let's see,[br]all of this is meiosis I. 0:08:56.620,0:08:59.610 Let me write that in a[br]different color, in bold. 0:09:00.103,0:09:04.712 So this is all meiosis, meiosis I here, 0:09:04.712,0:09:07.242 and you can see each of these cells now 0:09:07.242,0:09:08.740 have a haploid number. 0:09:08.740,0:09:13.186 They now have a haploid, haploid number 0:09:13.186,0:09:15.183 of two chromosomes each. 0:09:15.183,0:09:16.460 Now each of those two chromosomes 0:09:16.460,0:09:20.418 do have two sister chromatids, 0:09:20.418,0:09:22.543 and as we'll see in meiosis II, 0:09:22.543,0:09:24.598 which is very similar to mitosis, 0:09:24.598,0:09:27.419 is going to split up the sister chromatids 0:09:27.419,0:09:28.661 from each of these chromosomes, 0:09:28.661,0:09:32.120 which gives us two daughter chromosomes. 0:09:32.120,0:09:33.583 So we're gonna see that over here. 0:09:33.583,0:09:35.278 So your haploid number here is two. 0:09:35.278,0:09:37.008 You have two chromosomes here and 0:09:37.008,0:09:38.517 you have two chromosomes there. 0:09:38.517,0:09:42.017 And we'll explore meiosis[br]II in the next video.