1 00:00:00,570 --> 00:00:03,420 [Instructor] Hello and welcome to the third lecture 2 00:00:03,420 --> 00:00:07,380 in module three on mitosis and meiosis. 3 00:00:07,380 --> 00:00:10,530 So this one will probably be a bit longer 4 00:00:10,530 --> 00:00:12,990 than the DNA replication lecture, 5 00:00:12,990 --> 00:00:17,070 but not quite as long as (laughing) the gene expression 6 00:00:17,070 --> 00:00:18,360 and regulation lecture. 7 00:00:18,360 --> 00:00:20,040 So somewhere in between, 8 00:00:20,040 --> 00:00:22,410 but maybe get yourself a nice cup of coffee, sit down, 9 00:00:22,410 --> 00:00:25,443 and, you know, give it a listen. 10 00:00:27,450 --> 00:00:29,820 First I wanna start out with a few important definitions 11 00:00:29,820 --> 00:00:32,280 to make sure we're all on the same page. 12 00:00:32,280 --> 00:00:33,930 When we're talking about somatic cells, 13 00:00:33,930 --> 00:00:35,970 these are all cells in the body that are not egg 14 00:00:35,970 --> 00:00:36,810 or sperm cells. 15 00:00:36,810 --> 00:00:38,970 So literally any other cell in the body, 16 00:00:38,970 --> 00:00:41,010 the vast, vast majority of cells in the body, 17 00:00:41,010 --> 00:00:42,630 are somatic cells. 18 00:00:42,630 --> 00:00:44,850 Gametes, these are egg and sperm cells. 19 00:00:44,850 --> 00:00:47,820 So these are in the reproductive organs only. 20 00:00:47,820 --> 00:00:49,710 So that's the only place that they're located. 21 00:00:49,710 --> 00:00:51,420 And the reason why we're separating these 22 00:00:51,420 --> 00:00:54,210 is because somatic cells undergo mitosis, 23 00:00:54,210 --> 00:00:56,160 while gametes undergo meiosis. 24 00:00:56,160 --> 00:01:01,160 And these are two very different, but related, processes. 25 00:01:01,680 --> 00:01:04,590 And it will be important to understand those differences. 26 00:01:04,590 --> 00:01:07,440 Diploid means containing two complete sets of chromosomes, 27 00:01:07,440 --> 00:01:08,820 one from each parent. 28 00:01:08,820 --> 00:01:12,120 This would be in cells that have homologous chromosomes. 29 00:01:12,120 --> 00:01:13,830 We would say, you know, again, 30 00:01:13,830 --> 00:01:16,680 so that would be two sets of each of the 23 chromosomes. 31 00:01:16,680 --> 00:01:18,510 So they have 46 total. 32 00:01:18,510 --> 00:01:21,090 That would be a diploid cell. 33 00:01:21,090 --> 00:01:22,560 Haploid, on the other hand, 34 00:01:22,560 --> 00:01:24,330 these would be cells having a single set 35 00:01:24,330 --> 00:01:25,530 of unpaired chromosomes. 36 00:01:25,530 --> 00:01:27,480 Gametes are haploid. 37 00:01:27,480 --> 00:01:29,250 These are the only cells really in the body 38 00:01:29,250 --> 00:01:30,660 that are haploid. 39 00:01:30,660 --> 00:01:33,810 And that means they have 23 chromosomes total. 40 00:01:33,810 --> 00:01:35,970 All other cells in the body are diploid. 41 00:01:35,970 --> 00:01:38,730 And that means they have 46 chromosomes total, 42 00:01:38,730 --> 00:01:41,460 two of each of the 23 chromosomes that they have, 43 00:01:41,460 --> 00:01:43,890 two complete sets of each of the chromosomes, 44 00:01:43,890 --> 00:01:45,513 one that came from each parent. 45 00:01:46,920 --> 00:01:49,980 Sister chromatids, if you remember from the previous lecture 46 00:01:49,980 --> 00:01:53,160 on DNA replication, these are two identical copies 47 00:01:53,160 --> 00:01:56,400 of the same chromosome formed by DNA replication. 48 00:01:56,400 --> 00:02:00,030 Sister chromatids are different from homologous chromosomes 49 00:02:00,030 --> 00:02:02,430 because homologous chromosomes would be a pair 50 00:02:02,430 --> 00:02:04,920 of chromosomes, each inherited from one parent. 51 00:02:04,920 --> 00:02:08,580 Remember, sister chromatids are identical to one another, 52 00:02:08,580 --> 00:02:11,130 really just completely identical to one another. 53 00:02:11,130 --> 00:02:12,270 If you look at the sequences, 54 00:02:12,270 --> 00:02:15,510 they're going to match up really 100%. 55 00:02:15,510 --> 00:02:17,280 Homologous chromosomes, on the other hand, 56 00:02:17,280 --> 00:02:20,220 are going to be very, very similar to one another, 57 00:02:20,220 --> 00:02:22,110 but there will be some differences 58 00:02:22,110 --> 00:02:23,340 between one and the other, 59 00:02:23,340 --> 00:02:25,110 because one came from one parent 60 00:02:25,110 --> 00:02:26,580 and one come from another parent. 61 00:02:26,580 --> 00:02:30,090 And each of these have their own slight differences 62 00:02:30,090 --> 00:02:31,203 in the sequence. 63 00:02:32,730 --> 00:02:34,020 All right, let's jump in. 64 00:02:34,020 --> 00:02:35,400 Cell division. 65 00:02:35,400 --> 00:02:37,440 Cells need to divide and proliferate 66 00:02:37,440 --> 00:02:39,930 so we can develop from a single fertilized egg 67 00:02:39,930 --> 00:02:42,060 into a fully formed adult, 68 00:02:42,060 --> 00:02:43,650 so we can heal from injuries, 69 00:02:43,650 --> 00:02:47,010 and also so we can replace cells that have a short lifespan. 70 00:02:47,010 --> 00:02:50,040 So cell division is a really critical process. 71 00:02:50,040 --> 00:02:52,590 All cells in the body that are not egg or sperm cells 72 00:02:52,590 --> 00:02:54,502 are called somatic cells, if you remember 73 00:02:54,502 --> 00:02:56,700 from the previous page. 74 00:02:56,700 --> 00:02:59,550 Egg and sperm cells are called gametes. 75 00:02:59,550 --> 00:03:01,530 Remember from the previous slide, 76 00:03:01,530 --> 00:03:06,530 I said that somatic cells are diploid. 77 00:03:06,840 --> 00:03:08,610 What does diploid mean? 78 00:03:08,610 --> 00:03:11,820 Diploid means they have homologous chromosomes, 79 00:03:11,820 --> 00:03:15,090 which means they have 46 chromosomes, 80 00:03:15,090 --> 00:03:19,200 or two pairs of each of their 23 chromosomes. 81 00:03:19,200 --> 00:03:21,180 While gametes are haploid, 82 00:03:21,180 --> 00:03:23,130 which means they only have a single copy 83 00:03:23,130 --> 00:03:27,960 of each of their chromosomes, or 23 chromosomes in total. 84 00:03:27,960 --> 00:03:29,970 When somatic cells divide to form cells 85 00:03:29,970 --> 00:03:33,330 with both copies of each chromosome, this is called mitosis. 86 00:03:33,330 --> 00:03:37,380 So somatic cells undergo mitosis for cell division. 87 00:03:37,380 --> 00:03:39,390 And that results in two cells. 88 00:03:39,390 --> 00:03:42,390 When gametes divide to form cells with only one copy 89 00:03:42,390 --> 00:03:44,940 of each chromosome, this is called meiosis. 90 00:03:44,940 --> 00:03:47,910 So meiosis only occurs in gametes, 91 00:03:47,910 --> 00:03:49,710 never occurs in somatic cells. 92 00:03:49,710 --> 00:03:50,727 Meiosis occurs in gametes. 93 00:03:50,727 --> 00:03:55,727 And the end product of meiosis is four haploid gametes, 94 00:03:58,350 --> 00:04:01,650 haploid being each, meaning each of those cells 95 00:04:01,650 --> 00:04:04,623 has 23 chromosomes. 96 00:04:06,630 --> 00:04:09,480 Let's jump into mitosis to start. 97 00:04:09,480 --> 00:04:12,570 Cells progress through highly regulated phases of growth 98 00:04:12,570 --> 00:04:15,780 in a continual process called the cell cycle. 99 00:04:15,780 --> 00:04:17,370 There are four regular phases: 100 00:04:17,370 --> 00:04:20,640 G1, S, G2, and M. 101 00:04:20,640 --> 00:04:23,040 There is another phase called G0, 102 00:04:23,040 --> 00:04:25,830 but this phase is basically where a cell can exit 103 00:04:25,830 --> 00:04:29,193 the cell cycle and go into growth arrest or quiescence. 104 00:04:30,570 --> 00:04:32,670 Let's start with G1. 105 00:04:32,670 --> 00:04:36,060 In G1, this is where metabolic activity occurs. 106 00:04:36,060 --> 00:04:39,120 So this is where you're having some protein synthesis, 107 00:04:39,120 --> 00:04:44,120 and the cell is preparing to replicate its DNA. 108 00:04:44,910 --> 00:04:46,350 This is where the cell is growing, 109 00:04:46,350 --> 00:04:48,570 and this is a highly variable stage. 110 00:04:48,570 --> 00:04:50,340 So it can last minutes to years, 111 00:04:50,340 --> 00:04:51,873 depending upon the cell type. 112 00:04:53,670 --> 00:04:56,340 In the next phase that follows, this is S phase, 113 00:04:56,340 --> 00:04:59,760 S standing for synthesis, where DNA is replicated. 114 00:04:59,760 --> 00:05:01,380 This process takes about six hours, 115 00:05:01,380 --> 00:05:02,880 and this was the process we talked about 116 00:05:02,880 --> 00:05:04,680 in the previous lecture. 117 00:05:04,680 --> 00:05:07,500 The next phase is G2, or gap two, 118 00:05:07,500 --> 00:05:09,120 where a cell prepares for mitosis 119 00:05:09,120 --> 00:05:10,650 with lots of protein synthesis. 120 00:05:10,650 --> 00:05:14,820 So it's getting ready to enter its cell division phase. 121 00:05:14,820 --> 00:05:17,610 So it needs to prepare certain proteins, 122 00:05:17,610 --> 00:05:20,790 and it needs to perform certain functions 123 00:05:20,790 --> 00:05:23,490 to be ready for this process. 124 00:05:23,490 --> 00:05:26,400 The actual cell division process is called mitosis, 125 00:05:26,400 --> 00:05:29,490 or M phase, where duplicated chromosomes, 126 00:05:29,490 --> 00:05:32,013 chromosomes that were duplicated in S phase, 127 00:05:32,850 --> 00:05:35,730 duplicated chromosomes, again, what are these called? 128 00:05:35,730 --> 00:05:38,133 These are called sister chromatids. 129 00:05:39,330 --> 00:05:41,580 Are separated, two new daughter cells form 130 00:05:41,580 --> 00:05:43,320 from the original parental cell, 131 00:05:43,320 --> 00:05:44,850 each with 46 chromosomes. 132 00:05:44,850 --> 00:05:47,283 This whole process takes about one to two hours. 133 00:05:48,210 --> 00:05:51,630 Again, as I mentioned, the G0, or gap zero, phase 134 00:05:51,630 --> 00:05:54,274 is the phase where a cell will basically exit the cell cycle 135 00:05:54,274 --> 00:05:55,800 and enter quiescence. 136 00:05:55,800 --> 00:05:57,900 It does not happen with every cell cycle. 137 00:05:57,900 --> 00:05:59,580 It doesn't happen with every cell. 138 00:05:59,580 --> 00:06:02,700 It's just when a cell has essentially reached a point 139 00:06:02,700 --> 00:06:04,361 at which it's stopped dividing. 140 00:06:04,361 --> 00:06:08,160 This would apply to a cell like a neuron in the brain, 141 00:06:08,160 --> 00:06:09,450 where once it is formed, 142 00:06:09,450 --> 00:06:11,850 it's not going to continue to divide. 143 00:06:11,850 --> 00:06:14,970 It's basically done until the cell dies. 144 00:06:14,970 --> 00:06:18,513 And so it enters G0 phase and exits the cell cycle. 145 00:06:20,070 --> 00:06:24,300 Let's drill down more on this M phase, or mitosis phase. 146 00:06:24,300 --> 00:06:27,240 Mitosis itself is composed of five distinct stages 147 00:06:27,240 --> 00:06:30,090 to accurately separate duplicated chromosomes 148 00:06:30,090 --> 00:06:32,160 to newly forming daughter cells. 149 00:06:32,160 --> 00:06:34,410 So I'm throwing a lot of information at you here. 150 00:06:34,410 --> 00:06:35,670 And believe me, it will make sense. 151 00:06:35,670 --> 00:06:38,190 I just want to cover each of these phases first, 152 00:06:38,190 --> 00:06:40,470 and then we'll look at it in a bit more detail. 153 00:06:40,470 --> 00:06:42,900 So I just wanna give you kinda of a high-level perspective 154 00:06:42,900 --> 00:06:45,300 of what's happening, and then we'll go into some more detail 155 00:06:45,300 --> 00:06:46,770 for each one. 156 00:06:46,770 --> 00:06:49,200 So we start with prophase, where essentially, 157 00:06:49,200 --> 00:06:51,000 chromosomes are condensing down, 158 00:06:51,000 --> 00:06:55,560 and you're getting attachment of these spindle 159 00:06:55,560 --> 00:06:57,450 to the centers of each chromosome, 160 00:06:57,450 --> 00:06:58,740 the center being the centromere. 161 00:06:58,740 --> 00:07:01,350 Metaphase is where chromosomes line up in the middle 162 00:07:01,350 --> 00:07:02,894 of the cell. 163 00:07:02,894 --> 00:07:04,770 In anaphase, the sister chromatids, 164 00:07:04,770 --> 00:07:06,900 each side of the X shape of a chromosome, 165 00:07:06,900 --> 00:07:10,050 begins to separate with one copy going to each side. 166 00:07:10,050 --> 00:07:12,900 In telophase, sister chromatids complete moving 167 00:07:12,900 --> 00:07:15,210 to the opposite sides and the nuclear members begins 168 00:07:15,210 --> 00:07:16,770 to form around each one. 169 00:07:16,770 --> 00:07:19,140 In cytokinesis, the cytoplasm divides, 170 00:07:19,140 --> 00:07:21,660 cell membrane pinches off between the two cells, 171 00:07:21,660 --> 00:07:25,713 and you have two cells with 46 chromosomes in each. 172 00:07:27,960 --> 00:07:29,760 In preparation for mitosis, 173 00:07:29,760 --> 00:07:32,460 sister chromatids are held together at the center 174 00:07:32,460 --> 00:07:35,340 of the chromosome, called the centromere. 175 00:07:35,340 --> 00:07:37,290 The chromosome only looks like an X 176 00:07:37,290 --> 00:07:40,350 when it's been duplicated but has not yet divided. 177 00:07:40,350 --> 00:07:43,470 So you probably have seen chromosomes depicted 178 00:07:43,470 --> 00:07:45,030 in this X type of shape. 179 00:07:45,030 --> 00:07:46,770 And they do actually look like that, 180 00:07:46,770 --> 00:07:48,960 but they only look like that in a specific time 181 00:07:48,960 --> 00:07:49,890 in the cell cycle. 182 00:07:49,890 --> 00:07:51,900 So they actually only look like that 183 00:07:51,900 --> 00:07:55,170 when they're undergoing or very much preparing for mitosis. 184 00:07:55,170 --> 00:07:59,010 The reason being, the X shape here are actually 185 00:07:59,010 --> 00:08:00,810 sister chromatids of one another. 186 00:08:00,810 --> 00:08:04,500 And they're held together at a near center point 187 00:08:04,500 --> 00:08:06,993 of the chromosome called the centromere. 188 00:08:08,010 --> 00:08:11,853 So if you look at the X type of drawing for a chromosome, 189 00:08:12,750 --> 00:08:15,300 you have one sister chromatid on one side 190 00:08:15,300 --> 00:08:18,567 and another identical sister chromatid on the other side. 191 00:08:18,567 --> 00:08:22,050 And this occurs after the DNA's been replicated. 192 00:08:22,050 --> 00:08:23,430 And what phase is that? 193 00:08:23,430 --> 00:08:26,310 That was S phase, right, S phase. 194 00:08:26,310 --> 00:08:29,910 After S phase and the cell completes G2 phase, 195 00:08:29,910 --> 00:08:32,910 it enters M phase in mitosis. 196 00:08:32,910 --> 00:08:35,220 And this is the point at which the two sister chromatids 197 00:08:35,220 --> 00:08:36,630 are separated from one another, 198 00:08:36,630 --> 00:08:38,943 each one going to a new cell. 199 00:08:40,200 --> 00:08:42,000 And in the end, at the end of mitosis, 200 00:08:42,000 --> 00:08:43,470 at the end of M phase, 201 00:08:43,470 --> 00:08:46,350 the result is that you have one chromatid 202 00:08:46,350 --> 00:08:49,413 per each chromosome for each of the 46 chromosomes. 203 00:08:50,610 --> 00:08:53,400 So remember, these are duplicated identical copies 204 00:08:53,400 --> 00:08:55,530 and not the maternal and paternal copies 205 00:08:55,530 --> 00:08:59,130 which still exist and are separate from one another. 206 00:08:59,130 --> 00:09:02,670 So we're not talking about homologous chromosomes here. 207 00:09:02,670 --> 00:09:06,000 This is for each of the 46 chromosomes 208 00:09:06,000 --> 00:09:08,640 goes through this particular procedure. 209 00:09:08,640 --> 00:09:10,710 Before replication and after cell division, 210 00:09:10,710 --> 00:09:13,383 each chromosome is made up of one chromatid. 211 00:09:14,970 --> 00:09:17,220 I know, it is confusing, believe me. 212 00:09:17,220 --> 00:09:20,220 I completely, completely understand 213 00:09:20,220 --> 00:09:22,147 that you're probably scratching your head and thinking, 214 00:09:22,147 --> 00:09:24,150 "What, what's going on here?" 215 00:09:24,150 --> 00:09:27,420 So I'm going to try to reinforce this a couple of times 216 00:09:27,420 --> 00:09:30,630 throughout, but just want to make sure it's clear. 217 00:09:30,630 --> 00:09:32,700 Sister chromatids are two identical copies 218 00:09:32,700 --> 00:09:33,690 of the same chromosome. 219 00:09:33,690 --> 00:09:36,060 Homologous chromosomes are pairs of chromosomes 220 00:09:36,060 --> 00:09:38,520 that are similar, one which came from your mother 221 00:09:38,520 --> 00:09:40,050 and one which came from your father. 222 00:09:40,050 --> 00:09:41,763 This is kind of showing it here. 223 00:09:42,870 --> 00:09:44,640 So if we're looking at, remember, 224 00:09:44,640 --> 00:09:46,950 this is the centromere where the two sister chromatids 225 00:09:46,950 --> 00:09:49,863 are being held together for mitosis. 226 00:09:50,730 --> 00:09:52,117 This is one chromatid. 227 00:09:52,117 --> 00:09:54,210 This is its sister chromatid. 228 00:09:54,210 --> 00:09:58,440 And these two would be considered homologous chromosomes. 229 00:09:58,440 --> 00:10:01,770 During S phase, each one of these was duplicated, 230 00:10:01,770 --> 00:10:04,440 and you have sister chromatids now that are identical 231 00:10:04,440 --> 00:10:06,390 to one another and will be separated 232 00:10:06,390 --> 00:10:08,253 during the process of mitosis. 233 00:10:09,930 --> 00:10:11,520 Let's take a quick peek at what this looks like 234 00:10:11,520 --> 00:10:14,130 on a karyotype during mitosis. 235 00:10:14,130 --> 00:10:14,963 It looks a bit different. 236 00:10:14,963 --> 00:10:16,740 So you have two. 237 00:10:16,740 --> 00:10:19,680 You can start to see a little bit the sister chromatids. 238 00:10:19,680 --> 00:10:21,630 These are the homologous chromosomes, 239 00:10:21,630 --> 00:10:25,140 as I mentioned in the previous lecture as well. 240 00:10:25,140 --> 00:10:27,030 These are homologous chromosomes. 241 00:10:27,030 --> 00:10:30,210 And you have a pair of sister chromatids attached 242 00:10:30,210 --> 00:10:33,390 at the centromere, the center of the chromosome, 243 00:10:33,390 --> 00:10:35,943 for each one of those chromosomes. 244 00:10:37,830 --> 00:10:39,780 The karyotype after mitosis, 245 00:10:39,780 --> 00:10:42,660 you've separated each of those sister chromatids 246 00:10:42,660 --> 00:10:43,500 from one another. 247 00:10:43,500 --> 00:10:47,010 And so you have a single chromatid for each chromosome, 248 00:10:47,010 --> 00:10:47,970 and you have pairs. 249 00:10:47,970 --> 00:10:50,370 So now we're looking at homologous chromosomes. 250 00:10:50,370 --> 00:10:52,410 Homologous chromosomes are never, never, 251 00:10:52,410 --> 00:10:54,750 never held together at a centromere. 252 00:10:54,750 --> 00:10:58,383 Only identical sister chromatids are actually held together. 253 00:11:00,990 --> 00:11:05,190 All right, let's break down the different phases of mitosis. 254 00:11:05,190 --> 00:11:07,920 It's not really critical that you memorize 255 00:11:07,920 --> 00:11:10,980 each of these phases and know exactly what happens in each. 256 00:11:10,980 --> 00:11:12,570 I want you to conceptually, 257 00:11:12,570 --> 00:11:15,630 to understand this from a conceptual perspective, 258 00:11:15,630 --> 00:11:16,977 what is actually happening. 259 00:11:16,977 --> 00:11:18,840 And the easiest way, I think, to do that 260 00:11:18,840 --> 00:11:20,910 is to look at it by stage by stage, 261 00:11:20,910 --> 00:11:22,950 what exactly is happening in mitosis, 262 00:11:22,950 --> 00:11:24,180 which is why we're going through each 263 00:11:24,180 --> 00:11:25,773 of these stages individually. 264 00:11:27,390 --> 00:11:29,550 So prophase, the chromosomes condense, 265 00:11:29,550 --> 00:11:30,930 nuclear membrane breaks down, 266 00:11:30,930 --> 00:11:33,380 and spindle fibers form and attach to the centromeres 267 00:11:33,380 --> 00:11:34,770 on each chromosome. 268 00:11:34,770 --> 00:11:36,060 So what are spindle fibers? 269 00:11:36,060 --> 00:11:38,070 Well, those are basically like these long, 270 00:11:38,070 --> 00:11:40,350 you can think of them as long tethers 271 00:11:40,350 --> 00:11:44,010 that will be basically found in the end 272 00:11:44,010 --> 00:11:45,720 on opposite ends of the cell. 273 00:11:45,720 --> 00:11:48,396 So what they're gonna do is going to act sort of 274 00:11:48,396 --> 00:11:52,440 like a pulley, and they're going to pull 275 00:11:52,440 --> 00:11:56,100 these spindle fibers, which act as the tether, 276 00:11:56,100 --> 00:11:59,869 which will be attached at the centromere, at the centromere 277 00:11:59,869 --> 00:12:03,270 of each of these chromosomes 278 00:12:03,270 --> 00:12:07,410 and will pull, actually physically pull one 279 00:12:07,410 --> 00:12:09,590 of the two sister chromatids in each 280 00:12:09,590 --> 00:12:11,163 of the opposite directions. 281 00:12:12,000 --> 00:12:14,160 So in prophase, we're just starting this process. 282 00:12:14,160 --> 00:12:17,100 Basically, the chromosomes condense down, 283 00:12:17,100 --> 00:12:19,860 and you have the formation, 284 00:12:19,860 --> 00:12:22,560 starting to form these spindles. 285 00:12:22,560 --> 00:12:24,142 And the nucleus actually breaks down. 286 00:12:24,142 --> 00:12:26,273 During mitosis, this is the only time 287 00:12:26,273 --> 00:12:30,300 when DNA is actually not encased in the nuclear membrane 288 00:12:30,300 --> 00:12:31,530 or in the nucleus itself, 289 00:12:31,530 --> 00:12:35,520 because the nucleus breaks down in order to allow the DNA 290 00:12:35,520 --> 00:12:37,830 to move to opposite sides of the cell 291 00:12:37,830 --> 00:12:40,170 and form into two new cells. 292 00:12:40,170 --> 00:12:41,313 So that's prophase. 293 00:12:41,313 --> 00:12:43,020 In metaphase, the chromosomes line up in the middle 294 00:12:43,020 --> 00:12:45,780 of the cell, so boom, boom, boom, boom. 295 00:12:45,780 --> 00:12:49,020 In this case, we're actually just depicting four chromosomes 296 00:12:49,020 --> 00:12:52,323 here lining up, each with their sister chromatids, right? 297 00:12:54,270 --> 00:12:57,870 But in a human cell, you would have 46 of these lining up 298 00:12:57,870 --> 00:13:00,660 across the center of the cell. 299 00:13:00,660 --> 00:13:04,500 And the spindle would attach at each one 300 00:13:04,500 --> 00:13:05,856 of these centromeres. 301 00:13:05,856 --> 00:13:08,850 So you can imagine it's actually attaching 302 00:13:08,850 --> 00:13:10,800 to each of the two sister chromatids. 303 00:13:10,800 --> 00:13:13,045 So this one's attaching to the sister chromatid 304 00:13:13,045 --> 00:13:16,110 on one side, and this is attaching 305 00:13:16,110 --> 00:13:17,793 to the other sister chromatid. 306 00:13:18,900 --> 00:13:21,090 During anaphase, this is when 307 00:13:21,090 --> 00:13:22,410 the chromosomes begin to separate. 308 00:13:22,410 --> 00:13:24,690 So these spindles start to shorten, 309 00:13:24,690 --> 00:13:27,540 and they actually yank the sister chromatids 310 00:13:27,540 --> 00:13:29,910 in opposite directions, as you can see here. 311 00:13:29,910 --> 00:13:32,850 It's kind of being pulled into, 312 00:13:32,850 --> 00:13:34,440 this side's being pulled to the left. 313 00:13:34,440 --> 00:13:36,390 This side, these sister chromatids are being pulled 314 00:13:36,390 --> 00:13:37,320 to the right. 315 00:13:37,320 --> 00:13:40,470 During telophase, single chromatids, 316 00:13:40,470 --> 00:13:42,270 so these are the chromosomes, complete moving 317 00:13:42,270 --> 00:13:43,230 to the opposite sides, 318 00:13:43,230 --> 00:13:46,353 and the nuclear membrane begins to form around each one. 319 00:13:48,780 --> 00:13:51,000 And cytokinesis finishes the process, 320 00:13:51,000 --> 00:13:54,960 and that basically means that the cytoplasmic membrane 321 00:13:54,960 --> 00:13:56,340 pinches off between the two. 322 00:13:56,340 --> 00:13:58,920 And you actually have two fully formed cells now. 323 00:13:58,920 --> 00:14:02,550 And the chromosomes can decondense and can start 324 00:14:02,550 --> 00:14:04,200 to do their thing, as we talked about 325 00:14:04,200 --> 00:14:07,890 in the gene expression and regulation lecture. 326 00:14:07,890 --> 00:14:08,880 You can actually start 327 00:14:08,880 --> 00:14:11,610 to now have transcription occurring again. 328 00:14:11,610 --> 00:14:14,460 You can have the chromosomes starting to go back 329 00:14:14,460 --> 00:14:17,520 into more of a euchromatin state with the histones starting 330 00:14:17,520 --> 00:14:18,810 to loosen up a bit. 331 00:14:18,810 --> 00:14:22,293 And the cell can start back to its normal activity. 332 00:14:23,460 --> 00:14:25,140 So by the end of mitosis, 333 00:14:25,140 --> 00:14:28,530 a somatic cell has become two identical daughter cells. 334 00:14:28,530 --> 00:14:32,010 Each cell contains 46 chromosomes, 335 00:14:32,010 --> 00:14:36,060 23 homologous chromosome pairs, and remains diploid. 336 00:14:36,060 --> 00:14:39,420 Cell enters interphase by going into G1 phase 337 00:14:39,420 --> 00:14:41,310 and performs its expected function, 338 00:14:41,310 --> 00:14:44,490 or the cell enters G0 if no further division is required, 339 00:14:44,490 --> 00:14:46,200 and the chromosomes decondense 340 00:14:46,200 --> 00:14:48,363 to allow transcription to restart. 341 00:14:49,740 --> 00:14:52,860 Okay, let's switch gears here and we'll talk about meiosis. 342 00:14:52,860 --> 00:14:55,740 So recall, mitosis occurs in somatic cells, 343 00:14:55,740 --> 00:14:58,200 and somatic cells are virtually all cells in the body 344 00:14:58,200 --> 00:15:00,960 with the exception of egg and sperm cells. 345 00:15:00,960 --> 00:15:03,960 What happens in egg and sperm cells is a bit different. 346 00:15:03,960 --> 00:15:05,880 They undergo a process called meiosis. 347 00:15:05,880 --> 00:15:07,170 So meiosis is the process 348 00:15:07,170 --> 00:15:09,780 by which these egg and sperm cells are formed. 349 00:15:09,780 --> 00:15:12,750 Normal gametes have only one copy of each chromosome, 350 00:15:12,750 --> 00:15:15,990 I.e. 23 chromosomes, and this is called being haploid. 351 00:15:15,990 --> 00:15:19,350 One diploid pre-gamete cell, which has 46 chromosomes, 352 00:15:19,350 --> 00:15:22,380 each have been duplicated, undergoes meiosis 353 00:15:22,380 --> 00:15:25,680 to produce four haploid gamete daughter cells. 354 00:15:25,680 --> 00:15:28,950 It is essential for gametes to have only 23 chromosomes 355 00:15:28,950 --> 00:15:31,200 so when one egg and one sperm fuse, 356 00:15:31,200 --> 00:15:33,570 the fertilized cell will have the right number 357 00:15:33,570 --> 00:15:34,650 of total chromosomes. 358 00:15:34,650 --> 00:15:36,870 This is really, really important. 359 00:15:36,870 --> 00:15:41,010 Right, because otherwise, with each time we reproduced, 360 00:15:41,010 --> 00:15:43,350 each time there's a new generation of humans, 361 00:15:43,350 --> 00:15:45,090 we would have twice as many chromosomes. 362 00:15:45,090 --> 00:15:46,860 We would keep doubling our number of chromosomes, 363 00:15:46,860 --> 00:15:50,283 and it would get out of hand and we wouldn't really survive. 364 00:15:51,180 --> 00:15:53,160 So this is an important process. 365 00:15:53,160 --> 00:15:56,254 There are two rounds of meiosis: meiosis I and meiosis II. 366 00:15:56,254 --> 00:15:57,360 And these always occur in succession, 367 00:15:57,360 --> 00:15:58,890 and they always occur together. 368 00:15:58,890 --> 00:16:00,270 So you always have meiosis I 369 00:16:00,270 --> 00:16:02,610 and it's always followed by meiosis II. 370 00:16:02,610 --> 00:16:04,920 Meiosis I separates homologous chromosomes, 371 00:16:04,920 --> 00:16:07,710 while meiosis II separates sister chromatids. 372 00:16:07,710 --> 00:16:10,520 So meiosis II is a bit more like mitosis 373 00:16:10,520 --> 00:16:12,840 in that it's separating sister chromatids, 374 00:16:12,840 --> 00:16:15,690 but it follows the process of meiosis I, 375 00:16:15,690 --> 00:16:18,633 which actually separates homologous chromosomes. 376 00:16:20,040 --> 00:16:22,590 Let's take a peek at what this actually looks like. 377 00:16:22,590 --> 00:16:25,980 So starting again with DNA replication, 378 00:16:25,980 --> 00:16:28,530 we end up with sister chromatids. 379 00:16:28,530 --> 00:16:32,880 And these cells go into meiosis I. 380 00:16:32,880 --> 00:16:34,533 Meiosis I here. 381 00:16:35,580 --> 00:16:40,080 They each have the same basic phases as you had in mitosis. 382 00:16:40,080 --> 00:16:42,480 They're just denoted as I or II, 383 00:16:42,480 --> 00:16:45,660 depending upon if it's occurring in meiosis I or meiosis II. 384 00:16:45,660 --> 00:16:47,790 So prophase I, again, you don't need 385 00:16:47,790 --> 00:16:49,410 to memorize these phases. 386 00:16:49,410 --> 00:16:51,090 Just kind of understand the concept 387 00:16:51,090 --> 00:16:52,380 of what's happening here. 388 00:16:52,380 --> 00:16:57,380 In prophase I, instead of actually the alignment of, 389 00:16:59,430 --> 00:17:03,300 instead of all of the chromosomes lining up 390 00:17:03,300 --> 00:17:05,700 down the center, all 46 aligning down the center 391 00:17:05,700 --> 00:17:07,680 and separating the sister chromatids, 392 00:17:07,680 --> 00:17:12,060 instead, what happens is the homologous chromosomes line up 393 00:17:12,060 --> 00:17:14,040 next to each other. 394 00:17:14,040 --> 00:17:17,970 So you'll have 23 pairs lined up next to each other. 395 00:17:17,970 --> 00:17:20,910 And then what happens is that each one of these two 396 00:17:20,910 --> 00:17:23,970 gets pulled, each one of these two homologous chromosomes 397 00:17:23,970 --> 00:17:25,770 gets pulled in a different direction 398 00:17:26,820 --> 00:17:30,060 to form two different cells. 399 00:17:30,060 --> 00:17:34,050 And each of these cells now has 23 chromosomes, 400 00:17:34,050 --> 00:17:36,060 each having been duplicated. 401 00:17:36,060 --> 00:17:38,880 So it resolves the duplicated chromosomes 402 00:17:38,880 --> 00:17:42,870 by undergoing another round of meiosis, or meiosis II, 403 00:17:42,870 --> 00:17:45,450 which again is very, very similar to mitosis 404 00:17:45,450 --> 00:17:47,580 in that what happens now is the separation 405 00:17:47,580 --> 00:17:48,990 of sister chromatids. 406 00:17:48,990 --> 00:17:51,090 The difference here being you're starting with, 407 00:17:51,090 --> 00:17:55,200 instead of 46 chromosomes, you're starting with 23. 408 00:17:55,200 --> 00:17:58,140 Each one of these 23 chromosomes aligns in the center again, 409 00:17:58,140 --> 00:18:00,420 just like in mitosis, and gets pulled, 410 00:18:00,420 --> 00:18:04,050 the sister chromatids get separated into opposite cells. 411 00:18:04,050 --> 00:18:07,650 So the end result would be four total cells, 412 00:18:07,650 --> 00:18:09,630 which are each haploid. 413 00:18:09,630 --> 00:18:12,273 So that means they each have 23 chromosomes. 414 00:18:14,790 --> 00:18:17,640 And what's happening in crossing over? 415 00:18:17,640 --> 00:18:19,020 What is crossing over? 416 00:18:19,020 --> 00:18:21,720 Well, crossing over's actually a very important process. 417 00:18:21,720 --> 00:18:24,150 And we'll talk about it in more detail in the next module 418 00:18:24,150 --> 00:18:28,470 when we start talking about what can go wrong in meiosis 419 00:18:28,470 --> 00:18:32,700 and some of the chromosomal anomalies that can result. 420 00:18:32,700 --> 00:18:34,680 But crossing over is a normal process 421 00:18:34,680 --> 00:18:36,450 that occurs in meiosis I. 422 00:18:36,450 --> 00:18:40,680 It only occurs in meiosis I, ever occurs in meiosis II. 423 00:18:40,680 --> 00:18:41,513 Why is that? 424 00:18:41,513 --> 00:18:44,224 Because crossing over is actually the exchange 425 00:18:44,224 --> 00:18:48,990 of genetic material between the two homologous chromosomes. 426 00:18:48,990 --> 00:18:51,690 So if you look at the diagram on the right, 427 00:18:51,690 --> 00:18:53,280 this is basically taking your paternal 428 00:18:53,280 --> 00:18:54,840 and your maternal chromosomes, 429 00:18:54,840 --> 00:18:55,980 which you got from your mother, 430 00:18:55,980 --> 00:18:57,120 which you got from your father, 431 00:18:57,120 --> 00:19:00,270 they start shuffling DNA between each of these two. 432 00:19:00,270 --> 00:19:02,640 In normal crossing over, 433 00:19:02,640 --> 00:19:05,740 it will move the same region of one chromosome over 434 00:19:07,950 --> 00:19:10,410 from, say, the maternal chromosome to the paternal 435 00:19:10,410 --> 00:19:11,243 and vice versa. 436 00:19:11,243 --> 00:19:14,040 So the paternal piece gets stuck on the maternal piece. 437 00:19:14,040 --> 00:19:17,655 And in the end, you end up with some combination 438 00:19:17,655 --> 00:19:20,310 of both maternal and paternal for each of these. 439 00:19:20,310 --> 00:19:22,680 And why do we actually do that? 440 00:19:22,680 --> 00:19:25,950 Like why even bother with this weird process? 441 00:19:25,950 --> 00:19:28,170 Well, it gives us a lot of genetic variation, 442 00:19:28,170 --> 00:19:32,340 which is really critical for survival of our species. 443 00:19:32,340 --> 00:19:35,910 So now, instead of passing on to your offspring 444 00:19:35,910 --> 00:19:37,410 either the chromosome you inherited 445 00:19:37,410 --> 00:19:39,219 from your mother or your father, 446 00:19:39,219 --> 00:19:43,260 you're passing on some combination of both of those 447 00:19:43,260 --> 00:19:45,900 for each of the chromosomes that you contribute 448 00:19:45,900 --> 00:19:47,133 to your offspring. 449 00:19:49,680 --> 00:19:50,970 This is a figure from the book, 450 00:19:50,970 --> 00:19:52,560 and I wanted to comment on this. 451 00:19:52,560 --> 00:19:54,330 There is a part of this figure to the left 452 00:19:54,330 --> 00:19:56,368 that actually reads no crossing over. 453 00:19:56,368 --> 00:19:57,900 That doesn't ever happen. 454 00:19:57,900 --> 00:19:59,160 There's always crossing over. 455 00:19:59,160 --> 00:20:01,770 So just kind of ignore that. 456 00:20:01,770 --> 00:20:03,810 I don't know why they included that in there, 457 00:20:03,810 --> 00:20:04,740 but it could be confusing. 458 00:20:04,740 --> 00:20:06,720 So just know, there will always be crossing over 459 00:20:06,720 --> 00:20:08,220 in meiosis I. 460 00:20:08,220 --> 00:20:10,380 And this is depicting it a bit here, 461 00:20:10,380 --> 00:20:12,270 kind of similar to that last diagram. 462 00:20:12,270 --> 00:20:15,090 If the paternal chromosome is shown with the blue 463 00:20:15,090 --> 00:20:18,120 and the maternal with the pinkish color, 464 00:20:18,120 --> 00:20:20,760 then in the end, you actually can get some switching 465 00:20:20,760 --> 00:20:24,750 of, say, the end parts of this particular chromosome. 466 00:20:24,750 --> 00:20:26,760 The paternal gets swapped with the maternal, 467 00:20:26,760 --> 00:20:29,460 and you end up with some kinda combination of the two. 468 00:20:30,690 --> 00:20:33,900 In meiosis I, the homologous chromosomes separate 469 00:20:33,900 --> 00:20:37,110 into the two different cells. 470 00:20:37,110 --> 00:20:40,560 And then each of these two cells subsequently divides 471 00:20:40,560 --> 00:20:45,270 into two cells, each of which undergoing meiosis II, 472 00:20:45,270 --> 00:20:47,040 which is very similar to mitosis 473 00:20:47,040 --> 00:20:48,690 in that it's really a separation 474 00:20:48,690 --> 00:20:51,990 of duplicated identical sister chromatids. 475 00:20:51,990 --> 00:20:55,290 The end result is from one single cell, 476 00:20:55,290 --> 00:21:00,290 which was diploid, you have four haploid gametes 477 00:21:01,770 --> 00:21:03,903 after the process of meiosis. 478 00:21:04,830 --> 00:21:06,360 So let's summarize here. 479 00:21:06,360 --> 00:21:09,720 Mitosis is the process of cell division in somatic cells. 480 00:21:09,720 --> 00:21:12,374 The end result it two diploid daughter cells. 481 00:21:12,374 --> 00:21:15,690 Meiosis is the process of creating gametes. 482 00:21:15,690 --> 00:21:18,570 The end result is four haploid gametes. 483 00:21:18,570 --> 00:21:21,750 Meiosis has two rounds that always occur in succession: 484 00:21:21,750 --> 00:21:24,990 meiosis I, in which homologous chromosomes are separated, 485 00:21:24,990 --> 00:21:28,650 and meiosis II, in which sister chromatids are separated. 486 00:21:28,650 --> 00:21:30,090 When homologous chromosomes, 487 00:21:30,090 --> 00:21:33,060 I.e. maternal and paternal, align in meiosis I, 488 00:21:33,060 --> 00:21:35,400 they swap pieces to create new combinations 489 00:21:35,400 --> 00:21:37,683 of maternal and paternal sequences. 490 00:21:39,030 --> 00:21:40,110 So what's next? 491 00:21:40,110 --> 00:21:43,320 Now that you've made it through module three, 492 00:21:43,320 --> 00:21:45,390 well, guess what we're gonna talk about next? 493 00:21:45,390 --> 00:21:48,600 We're gonna actually start talking about some disease. 494 00:21:48,600 --> 00:21:49,777 I'm sure you're probably thinking, 495 00:21:49,777 --> 00:21:51,060 "Finally. (laughing) 496 00:21:51,060 --> 00:21:52,710 Finally we're going to talk about something 497 00:21:52,710 --> 00:21:57,270 with, you know, some clinical application directly to it." 498 00:21:57,270 --> 00:22:00,750 But hopefully you can appreciate the rationale 499 00:22:00,750 --> 00:22:03,570 behind why I'm presenting the material in this way. 500 00:22:03,570 --> 00:22:06,060 What I think is important is for you to understand 501 00:22:06,060 --> 00:22:08,640 how the processes normally occur, 502 00:22:08,640 --> 00:22:10,830 so the non-disease processes, 503 00:22:10,830 --> 00:22:14,760 before we start talking about when these processes go wrong 504 00:22:14,760 --> 00:22:15,990 and the resulting disease. 505 00:22:15,990 --> 00:22:17,700 So that way, you can understand how things 506 00:22:17,700 --> 00:22:18,930 are supposed to function 507 00:22:18,930 --> 00:22:20,640 and how things are supposed to work. 508 00:22:20,640 --> 00:22:23,250 And then if something goes wrong in any of those processes, 509 00:22:23,250 --> 00:22:25,080 what are the implications for human health? 510 00:22:25,080 --> 00:22:28,390 And so we'll really go into that throughout the rest 511 00:22:31,524 --> 00:22:32,357 of the course. 512 00:22:32,357 --> 00:22:35,430 So thank you for hanging in there with me through this. 513 00:22:35,430 --> 00:22:38,580 I look forward to any questions and comments 514 00:22:38,580 --> 00:22:42,600 and all of your great discussion that you shared 515 00:22:42,600 --> 00:22:44,550 on the discussion board so far. 516 00:22:44,550 --> 00:22:45,510 So in the next module, 517 00:22:45,510 --> 00:22:47,250 we're going to talk about aneuploidy, 518 00:22:47,250 --> 00:22:52,050 or inheritance of the incorrect number of chromosomes, 519 00:22:52,050 --> 00:22:55,470 chromosomal abnormalities, so we're talking more 520 00:22:55,470 --> 00:22:58,380 sort of the larger-scale abnormalities of chromosomes, 521 00:22:58,380 --> 00:23:00,810 and then also mosaicism. 522 00:23:00,810 --> 00:23:04,110 So those'll be some of the topics in our next module. 523 00:23:04,110 --> 00:23:07,140 And with that, I'd like to say thank you so much, 524 00:23:07,140 --> 00:23:10,740 and I hope you take a moment and just feel appreciative 525 00:23:10,740 --> 00:23:14,430 for yourself, for all of the time and energy you've put in 526 00:23:14,430 --> 00:23:16,860 to learning everything you've learned so far. 527 00:23:16,860 --> 00:23:19,952 Just take a moment of gratitude for yourself 528 00:23:19,952 --> 00:23:22,590 that you, as busy as you are, 529 00:23:22,590 --> 00:23:25,770 have taken the time to prioritize learning this material. 530 00:23:25,770 --> 00:23:27,750 Think about how much you've learned just so far 531 00:23:27,750 --> 00:23:30,510 in this first couple of weeks, how far you've come, 532 00:23:30,510 --> 00:23:33,240 and just know we're going to take this really up 533 00:23:33,240 --> 00:23:35,250 to the next level and really start to find some 534 00:23:35,250 --> 00:23:36,960 of those clinical applications coming up 535 00:23:36,960 --> 00:23:38,760 in the next few weeks. 536 00:23:38,760 --> 00:23:40,590 So with that, I will say goodbye, 537 00:23:40,590 --> 00:23:43,260 and I will speak with you again in the next module. 538 00:23:43,260 --> 00:23:44,313 Thanks very much.