0:00:00.500,0:00:03.940 In its most popular sense,[br]when people talk about 0:00:03.940,0:00:08.310 mitosis, they're referring to[br]a cell, a diploid cell. 0:00:08.310,0:00:10.670 So diploid just means it has[br]its full complement of 0:00:10.670,0:00:13.780 chromosomes, so it has[br]2N chromosomes. 0:00:13.780,0:00:15.950 So that's the nucleus. 0:00:15.950,0:00:17.410 This is the whole cell. 0:00:17.410,0:00:19.960 And so most people are saying,[br]look, the cell itself 0:00:19.960,0:00:23.590 replicates into two diploid[br]cells, so it turns into two 0:00:23.590,0:00:29.710 cells, each that have a full[br]complement of chromosomes, 2N 0:00:29.710,0:00:30.640 chromosomes. 0:00:30.640,0:00:33.400 And so when people say a cell[br]has experienced mitosis, they 0:00:33.400,0:00:34.580 normally mean this. 0:00:34.580,0:00:37.850 But I want to make one slight[br]clarification, that formally, 0:00:37.850,0:00:42.120 mitosis only refers to the[br]process of the replication of 0:00:42.120,0:00:45.350 the genetic material[br]and the nucleus. 0:00:45.350,0:00:50.180 So, for example, if I were to[br]draw this-- let me draw the 0:00:50.180,0:00:54.300 cell-- and it has now two[br]nucleuses, each with the 0:00:54.300,0:00:57.770 diploid number of chromosomes,[br]this cell 0:00:57.770,0:00:59.040 has experienced mitosis. 0:00:59.040,0:01:01.870 0:01:01.870,0:01:05.030 It has not experienced[br]cytokinesis, which we will 0:01:05.030,0:01:07.730 talk about in a few moments, but[br]that's the process of the 0:01:07.730,0:01:12.430 actual cytoplasm of the[br]cell being split into 0:01:12.430,0:01:13.490 two different cells. 0:01:13.490,0:01:17.160 And just as a clarity, the[br]cytoplasm is all the stuff 0:01:17.160,0:01:21.630 outside of the nucleus. 0:01:21.630,0:01:23.590 So I'll talk about that in a[br]second, but just know in 0:01:23.590,0:01:25.680 everyday usage, this is normally[br]the case when people 0:01:25.680,0:01:26.670 talk about mitosis. 0:01:26.670,0:01:28.860 But if you've got a teacher[br]that likes to get you on a 0:01:28.860,0:01:31.730 technicality, this is[br]technically what mitosis is. 0:01:31.730,0:01:35.030 It's the splitting of the[br]nucleus or the replication of 0:01:35.030,0:01:37.460 the nucleus into two[br]separate nucleuses. 0:01:37.460,0:01:42.820 That's normally accompanied[br]by cytokinesis where the 0:01:42.820,0:01:45.930 cytoplasms of the cells[br]actually separate. 0:01:45.930,0:01:51.250 Now, with that said, let's go[br]into the mechanics of mitosis. 0:01:51.250,0:01:55.150 So the first steps that are[br]really necessary for mitosis 0:01:55.150,0:01:58.920 actually occur outside of[br]mitosis when the cell is just 0:01:58.920,0:02:02.883 doing its day-to-day life, and[br]that's during the interphase. 0:02:02.883,0:02:07.570 0:02:07.570,0:02:10.449 And the interphase, literally[br]it's not a phase of mitosis. 0:02:10.449,0:02:14.810 It's literally when the[br]cell is just living. 0:02:14.810,0:02:17.730 Let's say we have[br]some new cell. 0:02:17.730,0:02:20.460 Let me do it in green. 0:02:20.460,0:02:22.430 That's a new cell here. 0:02:22.430,0:02:24.780 Maybe this is its nucleus. 0:02:24.780,0:02:29.630 It's got 2N chromosomes,[br]and then it grows. 0:02:29.630,0:02:32.790 It brings in nutrients from[br]the outside and builds 0:02:32.790,0:02:37.660 proteins and does whatever,[br]and so it grows a bit. 0:02:37.660,0:02:42.610 It's obviously got its full[br]chromosomal complement still. 0:02:42.610,0:02:46.080 And then at some point during[br]this life cycle, and I'll 0:02:46.080,0:02:49.220 label these actually, so this[br]phase in interphase, and this 0:02:49.220,0:02:52.460 might not even be covered in[br]some biology classes, but they 0:02:52.460,0:02:53.070 give it a label. 0:02:53.070,0:02:56.650 They call it G1, which[br]is really just 0:02:56.650,0:02:58.380 when the cell is growing. 0:02:58.380,0:03:00.840 It's just growing, accumulating[br]materials and 0:03:00.840,0:03:05.990 building itself out, and then[br]it actually replicates its 0:03:05.990,0:03:06.770 chromosomes. 0:03:06.770,0:03:09.760 So you still have a diploid[br]number of chromosomes. 0:03:09.760,0:03:11.390 So let me zoom in. 0:03:11.390,0:03:12.160 So let me draw this. 0:03:12.160,0:03:16.490 This is called the S phase of[br]interphase, so this is S. 0:03:16.490,0:03:19.190 And S is where you have[br]replication of the actual 0:03:19.190,0:03:19.430 chromosomes. 0:03:19.430,0:03:22.340 Once again, we're not[br]even in mitosis yet. 0:03:22.340,0:03:28.910 So S, you have replication[br]of your chromosomes. 0:03:28.910,0:03:34.770 So if I were to zoom in on the[br]nucleus during the S phase, if 0:03:34.770,0:03:38.230 I were to start off-- let me[br]just start with some organism 0:03:38.230,0:03:40.940 that has two chromosomes. 0:03:40.940,0:03:45.210 So let's say that at the[br]beginning of S phase, and I'll 0:03:45.210,0:03:48.060 draw things as chromosomes just[br]to make it clear that 0:03:48.060,0:03:49.300 things are being replicated. 0:03:49.300,0:03:54.010 So let me say it has this[br]chromosome right here and then 0:03:54.010,0:03:57.400 let's say it has this chromosome[br]right here. 0:03:57.400,0:04:01.180 As it goes through S phase,[br]these chromosomes replicate. 0:04:01.180,0:04:02.430 And I'm just drawing[br]the nucleus here. 0:04:02.430,0:04:05.740 I've zoomed in on just this part[br]right here, where N is 1, 0:04:05.740,0:04:10.050 where our full diploid[br]complement is two chomosomes. 0:04:10.050,0:04:16.010 During S phase, our chromosomes[br]will replicate and 0:04:16.010,0:04:21.269 will have-- so that green one[br]will completely replicate and 0:04:21.269,0:04:24.220 generate a copy of itself, and[br]we've learned this a little 0:04:24.220,0:04:26.380 bit, they're connected[br]at the centromere. 0:04:26.380,0:04:30.360 Now, each of those copies are[br]called chromatids, and that 0:04:30.360,0:04:33.350 magenta one will do[br]the same thing. 0:04:33.350,0:04:38.260 Even though we have two[br]chromatids, one for each 0:04:38.260,0:04:41.170 chromosome, now we have four[br]chromatids, two for each 0:04:41.170,0:04:45.120 chromosome, we still say we[br]only have two chromosomes. 0:04:45.120,0:04:47.000 That's its centromere[br]right there. 0:04:47.000,0:04:52.410 This occurs in the S phase, and[br]then the cell will just 0:04:52.410,0:04:53.710 continue to grow more. 0:04:53.710,0:04:59.160 So the cell was already big--[br]I'll focus on the cell again. 0:04:59.160,0:05:02.720 The cell was already big[br]and it gets bigger. 0:05:02.720,0:05:05.930 It gets bigger, and that's[br]during the G2 phase, so it's 0:05:05.930,0:05:09.380 just growing more. 0:05:09.380,0:05:12.400 Now, there's another little part[br]of the cell we haven't 0:05:12.400,0:05:13.900 even talked about yet,[br]but I'll talk 0:05:13.900,0:05:14.800 about it a little bit. 0:05:14.800,0:05:17.580 It's not super-duper important,[br]but it's the idea 0:05:17.580,0:05:19.010 of these centrosomes. 0:05:19.010,0:05:21.530 These are going to be very[br]important later on when the 0:05:21.530,0:05:25.170 cell is actually dividing,[br]and those also duplicate. 0:05:25.170,0:05:27.270 So let's say I have a little[br]centrosome here. 0:05:27.270,0:05:29.860 0:05:29.860,0:05:31.940 It has centrioles inside it. 0:05:31.940,0:05:33.820 You don't have to worry too much[br]about that, but they're 0:05:33.820,0:05:36.370 these little cylindrical-looking[br]things. 0:05:36.370,0:05:38.560 But I just want to-- so you[br]don't get confused if you see 0:05:38.560,0:05:41.710 the word centriole and[br]centrosomes, not to be 0:05:41.710,0:05:44.690 confused with centromeres, which[br]are these little points 0:05:44.690,0:05:46.760 where the two chromatids[br]attach. 0:05:46.760,0:05:51.640 Unfortunately, they named many[br]things in this process very 0:05:51.640,0:05:53.520 similarly, or a lot[br]of the parts 0:05:53.520,0:05:55.590 of a cell very similarly. 0:05:55.590,0:05:57.320 But you have these things called[br]centrosomes that are 0:05:57.320,0:05:59.720 going to enter the picture very[br]soon, that are sitting 0:05:59.720,0:06:05.260 outside of the nucleus, and[br]they also replicate. 0:06:05.260,0:06:08.280 They also replicate during[br]the interphase. 0:06:08.280,0:06:11.100 So you had one before, now[br]you have two of them. 0:06:11.100,0:06:13.530 And, of course, they each have[br]their two little centrioles 0:06:13.530,0:06:15.930 inside, but we're not going[br]to focus too much 0:06:15.930,0:06:17.020 on those just yet. 0:06:17.020,0:06:20.460 So that's what happened[br]in the interphase. 0:06:20.460,0:06:23.280 This is most of the cell's life,[br]and it's kind of growing 0:06:23.280,0:06:24.420 and doing what it wants. 0:06:24.420,0:06:26.200 Actually, I'll make a[br]slight point here. 0:06:26.200,0:06:29.310 When I drew the DNA here, I[br]drew them as chromosomes. 0:06:29.310,0:06:32.140 But the reality is when we're[br]sitting in the interphase, 0:06:32.140,0:06:34.760 this is not what the DNA would[br]actually look like. 0:06:34.760,0:06:37.930 The DNA, if I were to actually[br]draw this, it's in its 0:06:37.930,0:06:40.580 chromatin form. 0:06:40.580,0:06:43.670 It's not all tightly wound[br]like I drew it here. 0:06:43.670,0:06:46.050 I drew it tightly wound so that[br]you can see that it got 0:06:46.050,0:06:51.920 replicated, but the reality is[br]that that green chromosome 0:06:51.920,0:06:53.980 would actually be all unwound,[br]and if you were looking in a 0:06:53.980,0:06:56.330 microscope, you would even[br]have trouble seeing it. 0:06:56.330,0:06:59.470 This is its chromatin form. 0:06:59.470,0:07:02.370 We'll talk a little bit about[br]where it actually organizes 0:07:02.370,0:07:06.050 itself back into a chromosome,[br]but in its chromatin form, 0:07:06.050,0:07:09.270 it's just a bunch of DNA and[br]proteins that the DNA is 0:07:09.270,0:07:11.610 wrapped around a little bit,[br]so you might have some 0:07:11.610,0:07:14.990 proteins here that the DNA is[br]wrapped around a little bit. 0:07:14.990,0:07:16.560 But if you're looking at it in[br]a microscope, it just looks 0:07:16.560,0:07:19.210 like a big blur of[br]DNA and proteins. 0:07:19.210,0:07:22.090 Same thing for the[br]magenta molecule. 0:07:22.090,0:07:24.010 Really, for DNA to[br]do anything, it 0:07:24.010,0:07:25.080 has to be like this. 0:07:25.080,0:07:29.150 It has to be open to its[br]environment in order for the 0:07:29.150,0:07:33.960 mRNA and the different types of[br]helper proteins to really 0:07:33.960,0:07:34.790 be able to function with it. 0:07:34.790,0:07:37.260 And even for it to be able to[br]replicate, it has to be 0:07:37.260,0:07:39.360 unwound like this in order[br]for it to function. 0:07:39.360,0:07:41.600 It only gets tightly wound[br]like this later on. 0:07:41.600,0:07:44.630 I just drew it like this, so[br]really it had one green one, 0:07:44.630,0:07:47.080 and it's going to replicate to[br]form another green one, and 0:07:47.080,0:07:49.010 they're going to be attached[br]at some point. 0:07:49.010,0:07:51.260 That magenta one is going to[br]replicate to form another 0:07:51.260,0:07:53.440 magenta one, and they'll be[br]attached at some point, but 0:07:53.440,0:07:54.680 it's not going to be clear. 0:07:54.680,0:07:57.060 I just drew it this way to show[br]that it really happened. 0:07:57.060,0:07:58.320 This is the reality. 0:07:58.320,0:07:59.870 It's in its chromatin form. 0:07:59.870,0:08:03.070 0:08:03.070,0:08:06.930 Now, we're ready for mitosis. 0:08:06.930,0:08:09.060 So the first stage[br]of mitosis is 0:08:09.060,0:08:12.420 essentially-- let me draw this. 0:08:12.420,0:08:17.200 So I'll draw the[br]cell in green. 0:08:17.200,0:08:20.790 I'm going to draw the nucleus a[br]lot bigger than it normally 0:08:20.790,0:08:23.470 is relative to the cell just[br]because, at least right now, a 0:08:23.470,0:08:25.820 lot of the action is going[br]in the nucleus. 0:08:25.820,0:08:28.890 So the first stage of mitosis[br]is the prophase. 0:08:28.890,0:08:37.559 0:08:37.559,0:08:40.809 These are somewhat arbitrary[br]names that were assigned. 0:08:40.809,0:08:41.890 People looked in a microscope. 0:08:41.890,0:08:45.380 Oh, that's a certain type of[br]step that we always see when a 0:08:45.380,0:08:48.100 nucleus is dividing so we'll[br]call this the prophase. 0:08:48.100,0:08:55.210 What happens in the prophase is[br]that the actual chromatin 0:08:55.210,0:08:58.050 starts actually turning into[br]this type of form. 0:08:58.050,0:09:02.330 So as I just said, when we're[br]in the interphase, the DNA's 0:09:02.330,0:09:04.750 in this form where it's all[br]separated and unwound. 0:09:04.750,0:09:08.220 It actually starts to wind[br]together, so this is where 0:09:08.220,0:09:09.940 you'll actually have-- and[br]remember, it's already 0:09:09.940,0:09:10.480 replicated. 0:09:10.480,0:09:13.990 The replication happened[br]before mitosis begins. 0:09:13.990,0:09:16.840 So I had that one chromosome[br]there, and then I 0:09:16.840,0:09:18.400 have another one here. 0:09:18.400,0:09:21.740 It has two sister chromatids[br]that we'll see 0:09:21.740,0:09:24.270 soon get pulled apart. 0:09:24.270,0:09:30.000 Now, during prophase, you[br]also start to have these 0:09:30.000,0:09:35.060 centromeres appear that I[br]was touching on before. 0:09:35.060,0:09:40.380 These guys over here, they[br]start to facilitate the 0:09:40.380,0:09:44.060 generation of what you call[br]microtubules, and you can kind 0:09:44.060,0:09:46.770 of view these as these sticks or[br]these ropes that are going 0:09:46.770,0:09:50.890 to be key in moving things[br]around as we divide the cell. 0:09:50.890,0:09:52.160 All of this is pretty amazing. 0:09:52.160,0:09:54.630 I mean, you think of a cell, you[br]think of something that's 0:09:54.630,0:09:56.090 inherently pretty simple. 0:09:56.090,0:10:01.940 It's the most basic living[br]structure in us or in life. 0:10:01.940,0:10:05.750 But even here, you have these[br]complex mechanics going on, 0:10:05.750,0:10:07.360 and a lot of it still[br]isn't understood. 0:10:07.360,0:10:09.820 I mean, we can observe it, but[br]we really don't know what's 0:10:09.820,0:10:14.140 happening at the atomic level[br]or at the protein level that 0:10:14.140,0:10:17.720 allows these things to move[br]around in such a nicely 0:10:17.720,0:10:18.760 choreographed way. 0:10:18.760,0:10:21.480 It's still an area[br]of research. 0:10:21.480,0:10:23.520 Some of this is understood,[br]some of it isn't. 0:10:23.520,0:10:26.910 But you have these two[br]centrosomes, and they 0:10:26.910,0:10:30.970 facilitate the development of[br]these microtubules, which are 0:10:30.970,0:10:32.740 literally like these little[br]microstructures. 0:10:32.740,0:10:40.310 You can view them as tubes[br]or as some type of rope. 0:10:40.310,0:10:44.230 Now as prophase progresses, it[br]eventually gets to the point 0:10:44.230,0:10:45.720 where-- let me do it. 0:10:45.720,0:10:47.955 I don't want this word[br]replication written here. 0:10:47.955,0:10:49.030 It makes it confusing. 0:10:49.030,0:10:49.950 Let me delete that. 0:10:49.950,0:10:51.775 Let me get rid of this[br]replication. 0:10:51.775,0:10:54.460 0:10:54.460,0:10:58.690 So as prophase progresses, the[br]nuclear envelope actually 0:10:58.690,0:10:59.430 disappears. 0:10:59.430,0:11:01.435 So let me redraw this. 0:11:01.435,0:11:03.855 Let me copy and paste what[br]I've done before. 0:11:03.855,0:11:06.820 0:11:06.820,0:11:08.536 Put it there. 0:11:08.536,0:11:15.760 So as prophase progresses-- the[br]nuclear envelope actually 0:11:15.760,0:11:18.790 starts to disassemble. 0:11:18.790,0:11:24.400 So this starts to actually[br]dissolve and disassemble, and 0:11:24.400,0:11:29.330 then these things start to grow[br]and attach themselves to 0:11:29.330,0:11:29.840 the centromere. 0:11:29.840,0:11:31.290 So actually, let me do that. 0:11:31.290,0:11:34.230 So this is all during[br]prophase. 0:11:34.230,0:11:37.790 0:11:37.790,0:11:40.430 Since all of this happens during[br]prophase, this latter 0:11:40.430,0:11:43.390 part of prophase, sometimes[br]they'll call it late prophase, 0:11:43.390,0:11:44.820 sometimes it'll be called[br]prometaphase. 0:11:44.820,0:11:52.070 0:11:52.070,0:11:54.140 Sometimes it's considered-- I[br]don't think there's a hyphen 0:11:54.140,0:11:55.390 really there. 0:11:55.390,0:11:57.745 0:11:57.745,0:12:00.890 So sometimes it's actually[br]considered a separate phase of 0:12:00.890,0:12:02.700 mitosis, although when I learned[br]it in school, they 0:12:02.700,0:12:04.180 didn't bother with[br]prometaphase. 0:12:04.180,0:12:06.620 They just called it[br]all prophase. 0:12:06.620,0:12:09.620 But by the end of prophase,[br]or actually by the end of 0:12:09.620,0:12:13.220 prometaphase, depending on how[br]you want to view it, the whole 0:12:13.220,0:12:15.510 situation is going to look[br]something like this. 0:12:15.510,0:12:17.390 You have your overall cell. 0:12:17.390,0:12:20.460 The nuclear envelope has[br]disassembled, so to some 0:12:20.460,0:12:21.980 degree, it doesn't[br]exist anymore. 0:12:21.980,0:12:24.380 Although the proteins that[br]formed it are still there and 0:12:24.380,0:12:26.350 they're going to be[br]used later on. 0:12:26.350,0:12:29.840 And you have your two[br]chromosomes in this case. 0:12:29.840,0:12:32.800 In a human's case, you would[br]have 46 of them. 0:12:32.800,0:12:35.320 You have your two chomosomes,[br]each made with sister 0:12:35.320,0:12:41.050 chromatids, each made with[br]two sister chromatids. 0:12:41.050,0:12:42.190 Two chromosomes. 0:12:42.190,0:12:46.900 They, of course, have their[br]centromeres right there, and 0:12:46.900,0:12:55.160 then these centrosomes will[br]have migrated roughly on 0:12:55.160,0:12:59.930 opposite sides of what[br]was once the nucleus. 0:12:59.930,0:13:03.370 And these things have kind[br]of spread apart, these 0:13:03.370,0:13:06.740 microtubules, so they're doing[br]two functions, really. 0:13:06.740,0:13:08.270 At this point, they're[br]kind of pushing these 0:13:08.270,0:13:10.470 two centrosomes apart. 0:13:10.470,0:13:12.370 So you have all of these things,[br]and they're connecting 0:13:12.370,0:13:13.880 the-- you know, some of them[br]are coming from this 0:13:13.880,0:13:15.760 centrosome, some are coming from[br]this centrosome, some are 0:13:15.760,0:13:17.120 connecting the two. 0:13:17.120,0:13:20.900 And then some of these[br]microtubules, these tubes or 0:13:20.900,0:13:23.210 these ropes, however you want[br]to view them, attach 0:13:23.210,0:13:31.340 themselves to the centromeres of[br]the actual chromosomes, and 0:13:31.340,0:13:34.610 the protein structure that they[br]attach them to is called 0:13:34.610,0:13:36.570 the kinetochore. 0:13:36.570,0:13:39.160 So there's the kinetochore[br]there, and that may or may not 0:13:39.160,0:13:41.040 be-- kinetochore. 0:13:41.040,0:13:42.430 It's a protein structure. 0:13:42.430,0:13:43.940 It's actually fascinating. 0:13:43.940,0:13:46.980 It's still an open area of[br]research on how exactly the 0:13:46.980,0:13:49.350 microtubule attaches to the[br]kinetochore, and as we'll see 0:13:49.350,0:13:53.740 in a second, it's at the[br]kinetochore that the 0:13:53.740,0:13:58.400 microtubules essentially start[br]to pull at the two separate 0:13:58.400,0:14:02.670 sister chromatids and actually[br]pull them apart. 0:14:02.670,0:14:03.530 And it's actually[br]not understood 0:14:03.530,0:14:04.720 exactly how that works. 0:14:04.720,0:14:09.080 It's just been observed that[br]this actually happens. 0:14:09.080,0:14:14.020 Once prophase is done,[br]essentially the cells then 0:14:14.020,0:14:17.190 just make sure that the[br]chromosomes are well aligned. 0:14:17.190,0:14:19.270 I kind of drew them well aligned[br]here, but that just 0:14:19.270,0:14:22.760 kind of formally occurs[br]during metaphase, 0:14:22.760,0:14:24.090 which is the next phase. 0:14:24.090,0:14:26.040 The first one was prophase. 0:14:26.040,0:14:29.280 Now we're in metaphase, and[br]metaphase really is just an 0:14:29.280,0:14:32.680 aligning of the chromosomes, so[br]all of the chromosomes are 0:14:32.680,0:14:35.300 going to be aligned at the[br]center of the cell. 0:14:35.300,0:14:42.430 So I have my magenta one here,[br]I have my magenta one here, 0:14:42.430,0:14:45.660 and I have my other one here,[br]my green one there, and, of 0:14:45.660,0:14:49.960 course, you have your[br]centrosomes, the microspindles 0:14:49.960,0:14:51.570 that are coming off of them. 0:14:51.570,0:14:54.250 Some of them are kinetochore[br]microspindles that are 0:14:54.250,0:14:59.200 actually attaching to the[br]centromeres of the actual 0:14:59.200,0:15:00.470 chromosomes. 0:15:00.470,0:15:01.910 It's very confusing, right? 0:15:01.910,0:15:05.850 The centrosomes are these[br]structures that help direct 0:15:05.850,0:15:08.070 what happens to these[br]microtubules. 0:15:08.070,0:15:11.880 Centrioles are these little[br]structures, these little 0:15:11.880,0:15:14.850 can-shaped structures inside[br]the centrosomes, and the 0:15:14.850,0:15:19.050 centromere are the center[br]points where the two 0:15:19.050,0:15:22.170 chromatids attached to each[br]other within a chromosome. 0:15:22.170,0:15:25.850 So this is one sister chromatid,[br]that's another 0:15:25.850,0:15:28.250 sister chromatid, and they[br]attach at the centromere. 0:15:28.250,0:15:30.180 But this is metaphase. 0:15:30.180,0:15:33.100 It's fairly easy. 0:15:33.100,0:15:36.310 Metaphase, you just have this[br]aligning of the cells, and 0:15:36.310,0:15:38.220 there's actually some theories,[br]how does the cell 0:15:38.220,0:15:39.770 know to progress past[br]this point? 0:15:39.770,0:15:40.740 How does it know[br]that everything 0:15:40.740,0:15:42.240 is aligned and attached? 0:15:42.240,0:15:46.000 And then there are some theories[br]that there's actually 0:15:46.000,0:15:48.970 some signaling mechanism that[br]if one of these kinetochore 0:15:48.970,0:15:52.110 proteins isn't properly attached[br]to one of these 0:15:52.110,0:15:56.130 ropes, that somehow a signal[br]is sent that mitosis should 0:15:56.130,0:15:56.950 not continue. 0:15:56.950,0:15:59.200 So this is a very intricate[br]process. 0:15:59.200,0:16:01.540 You can imagine if you have 46[br]chromosomes and you have all 0:16:01.540,0:16:05.630 of this stuff going on in the[br]cell, and it's not like 0:16:05.630,0:16:08.080 there's some individual[br]pushing stuff, or some 0:16:08.080,0:16:08.800 computer here. 0:16:08.800,0:16:14.360 It's really directed[br]by chemistry and by 0:16:14.360,0:16:15.910 thermodynamic processes. 0:16:15.910,0:16:23.130 But just by the intricacy or[br]the elegance of how these 0:16:23.130,0:16:26.690 things are, it happens[br]spontaneously with all of the 0:16:26.690,0:16:29.780 proper checks and balances, so[br]that most of the time, nothing 0:16:29.780,0:16:32.210 bad happens, which is[br]all quite amazing. 0:16:32.210,0:16:36.270 So after metaphase, now we're[br]ready to pull the stuff apart, 0:16:36.270,0:16:37.520 and that's anaphase. 0:16:37.520,0:16:42.570 0:16:42.570,0:16:45.760 So in anaphase-- let[br]me write that down. 0:16:45.760,0:16:48.250 I've changed the color[br]of my cell. 0:16:48.250,0:16:50.280 These guys get pulled apart. 0:16:50.280,0:16:53.490 And as soon as they get pulled[br]apart-- so let's see, this 0:16:53.490,0:16:54.520 guy's getting pulled. 0:16:54.520,0:16:57.150 Let me do it in green. 0:16:57.150,0:17:00.540 So one of the sister-- nope,[br]that's not green. 0:17:00.540,0:17:03.410 One of the sister chromatids is[br]pulling in that direction. 0:17:03.410,0:17:05.660 One is getting pulled[br]in that direction. 0:17:05.660,0:17:08.520 And then the same is true[br]for the magenta ones. 0:17:08.520,0:17:10.000 Pulled in that direction,[br]and one is getting 0:17:10.000,0:17:11.780 pulled in that direction. 0:17:11.780,0:17:15.630 And, of course, you have your[br]centrosomes here and then 0:17:15.630,0:17:19.040 they're connected to the[br]kinetochores that are right 0:17:19.040,0:17:21.119 there and that's where[br]they're pulling. 0:17:21.119,0:17:23.890 There's also a whole microtubule[br]structure that 0:17:23.890,0:17:25.650 isn't connected to the actual[br]chromosomes, but they're 0:17:25.650,0:17:29.260 helping to actually push apart[br]these two centrosomes so that 0:17:29.260,0:17:32.570 everything is going to opposite[br]sides of the cell. 0:17:32.570,0:17:37.960 And so as soon as these two[br]chromatids are separated, and 0:17:37.960,0:17:40.140 I touched on this a little bit[br]before when we talked about 0:17:40.140,0:17:44.120 the vocabulary of DNA, then as[br]soon as that happens, these 0:17:44.120,0:17:47.030 are each referred to[br]as chromosomes. 0:17:47.030,0:17:49.960 So now you can say that[br]the cell has what it 0:17:49.960,0:17:50.750 used to have here. 0:17:50.750,0:17:51.770 It has two chromosomes. 0:17:51.770,0:17:53.900 It now has four chromosomes. 0:17:53.900,0:17:56.210 Because as soon as a chromatid[br]is no longer connected to its 0:17:56.210,0:17:59.590 sister chromatid, they're then[br]considered sister chromosomes, 0:17:59.590,0:18:01.100 which is just a naming[br]convention. 0:18:01.100,0:18:02.740 I mean, they were there before,[br]they were there after. 0:18:02.740,0:18:04.510 They were just attached[br]before. 0:18:04.510,0:18:06.390 Now they're not attached, so[br]you kind of consider them 0:18:06.390,0:18:08.930 their own individual entity. 0:18:08.930,0:18:10.590 And then we're almost done. 0:18:10.590,0:18:11.960 The last stage is telophase. 0:18:11.960,0:18:16.350 0:18:16.350,0:18:19.900 I'm going to draw the cell a[br]little bit different here 0:18:19.900,0:18:22.650 because something is happening[br]simultaneously with telophase 0:18:22.650,0:18:24.720 most of the time. 0:18:24.720,0:18:26.910 So telophase, and actually I'll 0:18:26.910,0:18:28.520 rotate the cell 90 degrees. 0:18:28.520,0:18:30.800 Let's say that this was[br]one centromere. 0:18:30.800,0:18:32.930 This is the other centromere. 0:18:32.930,0:18:34.640 So at this point,[br]it's essentially 0:18:34.640,0:18:37.670 pulled the DNA to itself. 0:18:37.670,0:18:43.000 So this guy has pulled one copy[br]of that chromosome and 0:18:43.000,0:18:45.130 one copy of this chromosome. 0:18:45.130,0:18:46.730 That guy's done the[br]same up here. 0:18:46.730,0:18:50.370 He's pulled over one copy of[br]each-- oh, I used a different 0:18:50.370,0:18:54.060 color-- one copy of each[br]chromosome to himself. 0:18:54.060,0:18:57.190 Let me draw that right[br]there like that. 0:18:57.190,0:19:01.200 And now the nuclear membranes[br]start forming around each of 0:19:01.200,0:19:01.980 these two ends. 0:19:01.980,0:19:04.660 So now you start having a[br]nuclear membrane form around 0:19:04.660,0:19:06.760 each of these two ends. 0:19:06.760,0:19:09.310 And so by the end of the[br]telophase-- that's what we're 0:19:09.310,0:19:13.880 in, the telophase-- we will[br]have completed mitosis. 0:19:13.880,0:19:17.110 We will have completely[br]replicated our two original 0:19:17.110,0:19:21.300 nucleuses and all of the genetic[br]content inside of it. 0:19:21.300,0:19:24.430 Now, at the same time telophase[br]is happening, you 0:19:24.430,0:19:27.440 also normally have this[br]cytokinesis, where this 0:19:27.440,0:19:31.260 cleavage furrow forms, where[br]essentially-- during 0:19:31.260,0:19:33.570 telophase, these things are[br]getting pushed further and 0:19:33.570,0:19:37.720 further apart by those[br]microtubules so that they're 0:19:37.720,0:19:42.166 already at the ends of the cell,[br]of the cytoplasm of the 0:19:42.166,0:19:45.270 cell, and you can almost view[br]them as pushing on the sides 0:19:45.270,0:19:46.860 to elongate the cell. 0:19:46.860,0:19:49.440 As that is happening, you have[br]this furrow forming, this 0:19:49.440,0:19:51.310 little indentation. 0:19:51.310,0:19:54.520 By the end of telophase in[br]mitosis, you also have this 0:19:54.520,0:19:58.340 process of cytokinesis, where[br]this cleavage furrow forms and 0:19:58.340,0:20:01.900 deepens, deepens, deepens[br]until the cytoplasm is 0:20:01.900,0:20:04.730 actually split into two[br]separate cells. 0:20:04.730,0:20:08.560 So this is cytokinesis, which[br]is formally not a part of 0:20:08.560,0:20:13.450 mitosis, but it normally occurs[br]with the telophase, so 0:20:13.450,0:20:16.950 right at the end of mitosis,[br]you do normally have two 0:20:16.950,0:20:18.770 complete identical cells. 0:20:18.770,0:20:22.800 Once you have each of these[br]two cells, then they, each 0:20:22.800,0:20:25.140 individually, enter their[br]own interphase. 0:20:25.140,0:20:27.840 Or they each individually, if[br]we look at just this one, he 0:20:27.840,0:20:30.780 will then be in his G1 phase. 0:20:30.780,0:20:34.650 At some point, these two things[br]are going to replicate, 0:20:34.650,0:20:36.500 and that's the S phase, and you[br]go to the G2 phase, and 0:20:36.500,0:20:41.280 then this guy will experience[br]mitosis all over again.