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