1 00:00:00,660 --> 00:00:01,493 [Instructor] Hello 2 00:00:01,493 --> 00:00:04,290 and welcome to the second lecture in Module 2, 3 00:00:04,290 --> 00:00:07,830 and this lecture is called DNA Structure. 4 00:00:07,830 --> 00:00:09,570 Okay, are you ready to go? 5 00:00:09,570 --> 00:00:11,070 Are you pumped? You psyched? 6 00:00:11,070 --> 00:00:15,030 Yeah, we're ready to talk about structure, molecules? 7 00:00:15,030 --> 00:00:15,930 Excellent. 8 00:00:15,930 --> 00:00:19,620 Well, what I promise you is that I will try my best 9 00:00:19,620 --> 00:00:22,050 to make this as painless as possible 10 00:00:22,050 --> 00:00:27,050 and to boil it down really to what are the key and critical, 11 00:00:28,560 --> 00:00:31,800 most important points that I'd like you to know 12 00:00:31,800 --> 00:00:33,570 in order to build off of that 13 00:00:33,570 --> 00:00:38,550 for what we will learn and explore and find applications for 14 00:00:38,550 --> 00:00:40,890 later on in this course. 15 00:00:40,890 --> 00:00:42,303 So let's get started. 16 00:00:44,640 --> 00:00:48,120 A few points on some basics of genetics 17 00:00:48,120 --> 00:00:51,090 that I want to make sure is clear before we move ahead 18 00:00:51,090 --> 00:00:54,870 because I think these points are absolutely critical, 19 00:00:54,870 --> 00:00:58,980 but can are very often confused in, 20 00:00:58,980 --> 00:01:02,910 I find if I'm reading say, you know, a newspaper article 21 00:01:02,910 --> 00:01:06,960 or something like that on genetics, that many times 22 00:01:06,960 --> 00:01:10,110 the information on some of these basic aspects of genetics 23 00:01:10,110 --> 00:01:11,790 can be incorrect. 24 00:01:11,790 --> 00:01:13,560 And I think that's very common 25 00:01:13,560 --> 00:01:16,740 that there are some misunderstandings 26 00:01:16,740 --> 00:01:18,090 that are important to address 27 00:01:18,090 --> 00:01:22,893 before we move on into some of the details of genetics. 28 00:01:23,820 --> 00:01:26,610 So just to start with a few definitions. 29 00:01:26,610 --> 00:01:30,270 Genetics is the study of the general mechanisms of heredity 30 00:01:30,270 --> 00:01:32,700 and the variation of inherited traits, 31 00:01:32,700 --> 00:01:35,730 typically one gene at a time. 32 00:01:35,730 --> 00:01:37,380 So what do I mean by this? 33 00:01:37,380 --> 00:01:39,780 Well first of all, let me just step back and say 34 00:01:39,780 --> 00:01:42,360 I'm defining genetics, I'm defining genomics 35 00:01:42,360 --> 00:01:44,430 and this is the title of the course. 36 00:01:44,430 --> 00:01:47,760 But I think it's important to define both of these 37 00:01:47,760 --> 00:01:49,950 because they can be overlapping 38 00:01:49,950 --> 00:01:52,110 and sometimes they're used interchangeably, 39 00:01:52,110 --> 00:01:53,550 but they actually do have 40 00:01:53,550 --> 00:01:56,373 two slightly different definitions. 41 00:01:57,210 --> 00:01:58,740 So when we're talking about genetics, 42 00:01:58,740 --> 00:02:02,554 this is really more just the general mechanisms of heredity, 43 00:02:02,554 --> 00:02:04,653 how are traits passed on, 44 00:02:05,490 --> 00:02:07,350 and it may be looking one gene at a time. 45 00:02:07,350 --> 00:02:08,940 This would be, for example, 46 00:02:08,940 --> 00:02:13,940 looking at the gene that is related to cystic fibrosis. 47 00:02:14,160 --> 00:02:16,200 So if you're just studying that one gene 48 00:02:16,200 --> 00:02:21,200 and looking at the influence it has on a particular trait, 49 00:02:22,560 --> 00:02:24,720 that would be really the study of genetics. 50 00:02:24,720 --> 00:02:29,720 If however you are, say, starting with a disease 51 00:02:30,330 --> 00:02:33,750 that you don't know what the genetic aspect of it is, 52 00:02:33,750 --> 00:02:35,070 you don't understand, 53 00:02:35,070 --> 00:02:37,740 there isn't clear which particular gene or genes 54 00:02:37,740 --> 00:02:38,790 would influence it. 55 00:02:38,790 --> 00:02:42,930 And you're utilizing tools like whole genome sequencing 56 00:02:42,930 --> 00:02:44,490 where you're going to find out 57 00:02:44,490 --> 00:02:46,470 all the information about an individual 58 00:02:46,470 --> 00:02:49,770 and you're going to start to work to better understand 59 00:02:49,770 --> 00:02:53,190 how all genes and their interrelationships 60 00:02:53,190 --> 00:02:57,510 are going to influence this particular trait 61 00:02:57,510 --> 00:02:58,650 that you're interested in. 62 00:02:58,650 --> 00:03:00,660 All right, so the other term 63 00:03:00,660 --> 00:03:04,080 that's very important to understand is gene. 64 00:03:04,080 --> 00:03:07,410 Gene is the unit of inheritance 65 00:03:07,410 --> 00:03:11,100 and we'll talk more about what that actually means later. 66 00:03:11,100 --> 00:03:14,610 But basically one gene provides the instructions 67 00:03:14,610 --> 00:03:17,940 for making one particular type of protein. 68 00:03:17,940 --> 00:03:21,990 So one gene equals one protein for the most part. 69 00:03:21,990 --> 00:03:24,390 Again, here comes my standard disclaimer, 70 00:03:24,390 --> 00:03:26,940 but there are certainly exceptions to that 71 00:03:26,940 --> 00:03:31,410 because we do actually have more different types of proteins 72 00:03:31,410 --> 00:03:34,110 than we have different types of genes. 73 00:03:34,110 --> 00:03:36,000 And there are lots of reasons for that 74 00:03:36,000 --> 00:03:38,310 and that gets to become, 75 00:03:38,310 --> 00:03:40,050 that starts to become quite complicated 76 00:03:40,050 --> 00:03:44,250 to describe and understand, so we'll just think about it 77 00:03:44,250 --> 00:03:46,470 more from the majority rule here. 78 00:03:46,470 --> 00:03:47,820 The majority rule is 79 00:03:47,820 --> 00:03:51,330 that one gene provides instructions for making one protein 80 00:03:51,330 --> 00:03:53,370 and that's how a gene is defined. 81 00:03:53,370 --> 00:03:56,640 It's defined as a segment long DNA 82 00:03:56,640 --> 00:03:59,700 that encodes or provides the instructions 83 00:03:59,700 --> 00:04:01,713 for one specific protein, 84 00:04:03,210 --> 00:04:06,300 and proteins perform functions within cells, tissues, organs 85 00:04:06,300 --> 00:04:07,290 in the whole system. 86 00:04:07,290 --> 00:04:09,570 And I know you're familiar with proteins. 87 00:04:09,570 --> 00:04:10,530 I've provided one here. 88 00:04:10,530 --> 00:04:14,070 Insulin is an example of a protein but there are, 89 00:04:14,070 --> 00:04:16,200 I mean tens of thousands more than that 90 00:04:16,200 --> 00:04:19,290 of different proteins in the human body. 91 00:04:19,290 --> 00:04:21,510 And I'm sure you're familiar with, you know, 92 00:04:21,510 --> 00:04:24,330 maybe monitoring certain levels of, 93 00:04:24,330 --> 00:04:26,220 monitoring levels of certain proteins 94 00:04:26,220 --> 00:04:29,340 which may indicate how well a patient is doing 95 00:04:29,340 --> 00:04:32,460 or the state of a particular disease or condition 96 00:04:32,460 --> 00:04:34,020 that they're experiencing. 97 00:04:34,020 --> 00:04:35,610 But you can think of proteins 98 00:04:35,610 --> 00:04:39,450 as really like the workhorse of the cell. 99 00:04:39,450 --> 00:04:43,200 Proteins go out and do virtually all functions 100 00:04:43,200 --> 00:04:45,810 within the cell and as a result, 101 00:04:45,810 --> 00:04:48,990 virtually all functions within organs and tissues 102 00:04:48,990 --> 00:04:51,090 and the body as a whole. 103 00:04:51,090 --> 00:04:54,240 They work together, they get modified, they, you know, 104 00:04:54,240 --> 00:04:57,150 they have different forms and variations, 105 00:04:57,150 --> 00:04:59,550 but the instructions for making that protein 106 00:04:59,550 --> 00:05:02,670 are provided by gene. 107 00:05:02,670 --> 00:05:06,240 When we talk about a genome, so the genome is the entirety 108 00:05:06,240 --> 00:05:08,280 of an organism's genetic material 109 00:05:08,280 --> 00:05:11,310 including genes and parts which are not genes. 110 00:05:11,310 --> 00:05:14,250 So your genome is not just made up of genes, 111 00:05:14,250 --> 00:05:17,970 it's not as though you stretch out all of your DNA 112 00:05:17,970 --> 00:05:19,237 and you look at it and you say, 113 00:05:19,237 --> 00:05:20,970 "Okay, well you know it's just one gene 114 00:05:20,970 --> 00:05:22,920 right next to another, right next to another." 115 00:05:22,920 --> 00:05:24,540 It is not like that at all. 116 00:05:24,540 --> 00:05:26,076 In fact, it's quite the opposite. 117 00:05:26,076 --> 00:05:30,030 The vast majority of your genome, 118 00:05:30,030 --> 00:05:33,690 it's made up of other stretches of DNA, 119 00:05:33,690 --> 00:05:36,120 which have very important functions. 120 00:05:36,120 --> 00:05:40,020 And we'll talk about that more in the next module 121 00:05:40,020 --> 00:05:42,300 when we talk about gene expression. 122 00:05:42,300 --> 00:05:45,150 So when we're talking about genes, genomes, DNA, 123 00:05:45,150 --> 00:05:47,010 are these terms interchangeable? 124 00:05:47,010 --> 00:05:48,780 Well hopefully now you understand 125 00:05:48,780 --> 00:05:52,500 that a gene and genome are not interchangeable, 126 00:05:52,500 --> 00:05:54,210 that they actually mean quite different things. 127 00:05:54,210 --> 00:05:55,920 That your genome includes 128 00:05:55,920 --> 00:05:58,080 the entire set of all of your genes 129 00:05:58,080 --> 00:06:01,923 plus extra sequences that are non-genes. 130 00:06:02,850 --> 00:06:06,750 That one gene generally encodes the instructions 131 00:06:06,750 --> 00:06:08,403 for making one protein. 132 00:06:09,450 --> 00:06:12,780 And so where does DNA fit into all of this? 133 00:06:12,780 --> 00:06:15,420 DNA or deoxyribonucleic acid 134 00:06:15,420 --> 00:06:18,450 is the molecule that stores all the genetic information, 135 00:06:18,450 --> 00:06:20,310 including your genes and non-genes. 136 00:06:20,310 --> 00:06:24,150 So your genome is made up of DNA. 137 00:06:24,150 --> 00:06:26,700 So DNA is the name of the molecule 138 00:06:26,700 --> 00:06:29,580 and along your DNA strand 139 00:06:29,580 --> 00:06:32,670 there will be certain sections of that DNA strand 140 00:06:32,670 --> 00:06:34,680 which encode for a single protein, 141 00:06:34,680 --> 00:06:37,383 and that section is called a gene. 142 00:06:38,370 --> 00:06:41,130 If you take all of the DNA that you have 143 00:06:41,130 --> 00:06:43,260 and say it's one of your cells, 144 00:06:43,260 --> 00:06:46,020 that would be your entire genome 145 00:06:46,020 --> 00:06:49,410 because it's going to be all of the information needed 146 00:06:49,410 --> 00:06:50,970 to make you. 147 00:06:50,970 --> 00:06:53,610 Let's talk some about the structure of DNA. 148 00:06:53,610 --> 00:06:56,310 If we look at it from more of a simplified way 149 00:06:56,310 --> 00:06:59,610 and taking out all of the chemical structure 150 00:06:59,610 --> 00:07:01,410 that goes into it, 151 00:07:01,410 --> 00:07:06,090 we can kind of break it down here, as you see here, 152 00:07:06,090 --> 00:07:09,540 to a unit called a nucleotide. 153 00:07:09,540 --> 00:07:13,380 DNA is made up of long strand of nucleotides, 154 00:07:13,380 --> 00:07:15,720 so repeating strand of nucleotides. 155 00:07:15,720 --> 00:07:17,970 So what is a nucleotide? 156 00:07:17,970 --> 00:07:20,550 A nucleotide has several components to it. 157 00:07:20,550 --> 00:07:24,630 It has a phosphate group shown here as the orange circle, 158 00:07:24,630 --> 00:07:28,503 deoxyribose sugar, which is five-carbon ring sugar, 159 00:07:30,210 --> 00:07:31,980 and it has a base, 160 00:07:31,980 --> 00:07:34,740 and we'll talk more about what the base is, 161 00:07:34,740 --> 00:07:37,980 but there really are four different kinds of bases 162 00:07:37,980 --> 00:07:38,853 it could be, 163 00:07:39,930 --> 00:07:43,080 that it could be that could fill this spot here. 164 00:07:43,080 --> 00:07:48,080 Those would be adenine, thymine, guanine, and cytosine 165 00:07:48,600 --> 00:07:52,770 or A, T, G, and C for short. 166 00:07:52,770 --> 00:07:57,420 And it'll become very important soon 167 00:07:57,420 --> 00:08:00,210 that you start calling them A, T's, G's, and C's 168 00:08:00,210 --> 00:08:03,000 instead of adenine, thymine, guanine, and cytosine. 169 00:08:03,000 --> 00:08:04,710 It just helps to speed things along, 170 00:08:04,710 --> 00:08:08,040 especially if you're looking at a long stretch of DNA, 171 00:08:08,040 --> 00:08:12,393 it becomes important to have a shorthand notation for that. 172 00:08:14,220 --> 00:08:16,350 But in every nucleotide 173 00:08:16,350 --> 00:08:20,790 this phosphate and deoxyribose sugar is present. 174 00:08:20,790 --> 00:08:23,400 The only thing that changes is this base, 175 00:08:23,400 --> 00:08:28,140 whether it's an A, a T, a G, or a C. 176 00:08:28,140 --> 00:08:31,740 Each base is connected, so here this is the base. 177 00:08:31,740 --> 00:08:36,480 Each base is connected to a five-carbon sugar here 178 00:08:36,480 --> 00:08:40,593 called 2-deoxyribose, and a phosphate group. 179 00:08:41,490 --> 00:08:44,400 This unit in its entirety, the phosphate group, 180 00:08:44,400 --> 00:08:48,540 the deoxyribose sugar and the base is a nucleotide. 181 00:08:48,540 --> 00:08:50,460 Now if we zoom out a little bit more 182 00:08:50,460 --> 00:08:53,610 and we see how these nucleotides fit together. 183 00:08:53,610 --> 00:08:56,130 Here this is showing a little bit more 184 00:08:56,130 --> 00:08:57,600 of the chemical structure of it, 185 00:08:57,600 --> 00:08:59,280 but this is still your phosphate group. 186 00:08:59,280 --> 00:09:02,430 So here you have your phosphorus and your oxygen groups 187 00:09:02,430 --> 00:09:03,270 coming off of that. 188 00:09:03,270 --> 00:09:05,340 You don't need to know the details of this, 189 00:09:05,340 --> 00:09:07,410 just know this is a phosphate group. 190 00:09:07,410 --> 00:09:09,720 Phosphate group here again, 191 00:09:09,720 --> 00:09:12,450 showing a little bit more of the structure. 192 00:09:12,450 --> 00:09:17,450 But you have your phosphate group, your deoxyribose sugar, 193 00:09:19,620 --> 00:09:23,373 and this is the base, in this case this is the A or adenine. 194 00:09:24,720 --> 00:09:26,430 Well what you can see is happening here 195 00:09:26,430 --> 00:09:27,263 is that it starts to form 196 00:09:27,263 --> 00:09:30,390 a long chain of connected nucleotides. 197 00:09:30,390 --> 00:09:32,370 And it basically, 198 00:09:32,370 --> 00:09:35,110 every single time you have the phosphate group 199 00:09:36,780 --> 00:09:41,780 attaching or bonding with the 2-deoxyribose sugar 200 00:09:43,380 --> 00:09:45,093 of the previous nucleotide. 201 00:09:47,220 --> 00:09:48,900 And here you can see this, 202 00:09:48,900 --> 00:09:51,240 you can keep going and going and going forever 203 00:09:51,240 --> 00:09:53,130 if you wanted to. 204 00:09:53,130 --> 00:09:56,520 But basically you can start to understand 205 00:09:56,520 --> 00:10:00,900 how you can begin to form really long chains of nucleotides 206 00:10:00,900 --> 00:10:02,910 attached, again, at this 207 00:10:02,910 --> 00:10:05,790 what's called the sugar phosphate backbone. 208 00:10:05,790 --> 00:10:08,400 The reason why it's called the sugar phosphate backbone 209 00:10:08,400 --> 00:10:12,570 is because you have this sugar group, the deoxyribose sugar, 210 00:10:12,570 --> 00:10:14,340 and the phosphate group, right? 211 00:10:14,340 --> 00:10:16,530 And the phosphate attaches to the sugar group 212 00:10:16,530 --> 00:10:18,900 and so on and so on and so on, 213 00:10:18,900 --> 00:10:21,870 so this is considered the backbone of it 214 00:10:21,870 --> 00:10:23,010 and you can kind of see why, 215 00:10:23,010 --> 00:10:26,913 it sort of looks like a backbone going all the way through. 216 00:10:27,870 --> 00:10:28,860 This is the backbone, 217 00:10:28,860 --> 00:10:31,200 these are what will be considered the bases again, 218 00:10:31,200 --> 00:10:35,100 so adenine, guanine, thymine, adenine, cytosine here. 219 00:10:35,100 --> 00:10:40,100 So it can form really any combination of bases in any order 220 00:10:40,400 --> 00:10:45,030 in this, along one strand of nucleotides 221 00:10:45,030 --> 00:10:46,650 or one strand of DNA. 222 00:10:46,650 --> 00:10:49,800 So these nucleotides will form long chains 223 00:10:49,800 --> 00:10:54,060 connected together on one strand by strong bonding 224 00:10:54,060 --> 00:10:56,400 called phosphodiesterase bonding. 225 00:10:56,400 --> 00:10:58,050 You don't really need to know that 226 00:10:58,050 --> 00:11:00,660 but I would be remiss if I didn't at least mention it. 227 00:11:00,660 --> 00:11:03,720 So you've heard the term phosphodiesterase bonding, 228 00:11:03,720 --> 00:11:06,750 so just know that that's the bonding that occurs 229 00:11:06,750 --> 00:11:09,900 between the phosphate group of one nucleotide 230 00:11:09,900 --> 00:11:14,760 and the deoxyribose sugar group of another nucleotide, 231 00:11:14,760 --> 00:11:18,870 which gives a very strong connection between these. 232 00:11:18,870 --> 00:11:21,990 So these will remain in their chain 233 00:11:21,990 --> 00:11:24,060 connected to one another. 234 00:11:24,060 --> 00:11:28,260 Note that the base is not actually involved 235 00:11:28,260 --> 00:11:32,040 in forming the backbone. 236 00:11:32,040 --> 00:11:35,880 So the backbone here is separate from the base 237 00:11:35,880 --> 00:11:36,713 and that's important, 238 00:11:36,713 --> 00:11:39,930 and we'll talk about that in a little while 239 00:11:39,930 --> 00:11:41,220 but it's important to know that 240 00:11:41,220 --> 00:11:43,320 because the base is going to be involved 241 00:11:43,320 --> 00:11:45,750 in some other activity. 242 00:11:45,750 --> 00:11:48,060 If we zoom out even further here, 243 00:11:48,060 --> 00:11:49,680 so what we were looking at there 244 00:11:49,680 --> 00:11:53,010 was the structure of a single strand of DNA. 245 00:11:53,010 --> 00:11:56,700 But DNA in its stable form is actually two strands 246 00:11:56,700 --> 00:11:59,340 and these are called complementary strands 247 00:11:59,340 --> 00:12:00,570 and they're bound together 248 00:12:00,570 --> 00:12:05,250 by hydrogen bonding between bases on opposite strands. 249 00:12:05,250 --> 00:12:08,310 And the two strands wind around each other 250 00:12:08,310 --> 00:12:11,220 in a structure called a double helix. 251 00:12:11,220 --> 00:12:13,680 So if you think of a spiral staircase 252 00:12:13,680 --> 00:12:17,250 with sugar phosphate backbone as the sides of the staircase 253 00:12:17,250 --> 00:12:20,490 and the bases bonded together as the steps, 254 00:12:20,490 --> 00:12:21,720 you kind of get a sense 255 00:12:21,720 --> 00:12:24,030 of what that structure actually looks like. 256 00:12:24,030 --> 00:12:25,440 If we zoomed in here, 257 00:12:25,440 --> 00:12:29,550 the blue line would look like that sugar phosphate backbone 258 00:12:29,550 --> 00:12:31,830 that we were just looking at on the previous slide, 259 00:12:31,830 --> 00:12:33,810 and there would be another one over here. 260 00:12:33,810 --> 00:12:38,220 Each of these rungs of the staircase, so to speak, 261 00:12:38,220 --> 00:12:39,870 would be the base. 262 00:12:39,870 --> 00:12:43,200 So here this is green in this particular image, 263 00:12:43,200 --> 00:12:48,200 so this would be thymine and adenine represented here, 264 00:12:48,450 --> 00:12:53,250 this would be a guanine here and a cytosine here. 265 00:12:53,250 --> 00:12:55,740 So they're actually bonding to one another 266 00:12:55,740 --> 00:12:59,460 across the strands, so each of these is connected 267 00:12:59,460 --> 00:13:03,300 to its sugar phosphate backbone on this one strand. 268 00:13:03,300 --> 00:13:05,100 Same thing on this other strand, 269 00:13:05,100 --> 00:13:06,240 they're kind of facing each other. 270 00:13:06,240 --> 00:13:09,240 The two, the base sides of each of the two strands 271 00:13:09,240 --> 00:13:10,470 are facing one another. 272 00:13:10,470 --> 00:13:12,900 So you can kind of think of the two strands 273 00:13:12,900 --> 00:13:15,210 as facing in towards one another 274 00:13:15,210 --> 00:13:16,500 and they're bonding together 275 00:13:16,500 --> 00:13:20,220 through the interactions between opposite bases. 276 00:13:20,220 --> 00:13:22,170 So let me show you what I mean by that. 277 00:13:23,130 --> 00:13:26,280 This is a process called base pairing. 278 00:13:26,280 --> 00:13:30,870 Base pairing is the bonding of two base groups 279 00:13:30,870 --> 00:13:31,770 to one another. 280 00:13:31,770 --> 00:13:35,370 This does not involve the sugar phosphate backbone. 281 00:13:35,370 --> 00:13:36,915 The sugar phosphate backbone 282 00:13:36,915 --> 00:13:41,915 provides the structure for a single strand of DNA. 283 00:13:41,940 --> 00:13:44,970 Base pairing occurs between two strands of DNA 284 00:13:44,970 --> 00:13:48,003 when it's forming that double helix structure. 285 00:13:49,710 --> 00:13:51,060 Now this is important. 286 00:13:51,060 --> 00:13:55,590 Only adenine or A can bond with thymine or T. 287 00:13:55,590 --> 00:13:59,310 So only A and T can bond together. 288 00:13:59,310 --> 00:14:01,440 So A and T go together, 289 00:14:01,440 --> 00:14:05,130 and only guanine or G combined with cytosine, C. 290 00:14:05,130 --> 00:14:07,050 So G and C always go together, 291 00:14:07,050 --> 00:14:09,480 they always bond to each other. 292 00:14:09,480 --> 00:14:11,460 And let's look for a second, 293 00:14:11,460 --> 00:14:13,440 let's look at why that actually is. 294 00:14:13,440 --> 00:14:16,890 If we look at the chemical structure for these bases 295 00:14:16,890 --> 00:14:19,170 instead of just writing them as A's, T's, C's, and G's, 296 00:14:19,170 --> 00:14:21,480 which we will from here on out, I promise. 297 00:14:21,480 --> 00:14:22,740 Just so that you've seen this 298 00:14:22,740 --> 00:14:26,583 and you have an understanding of why that's the case. 299 00:14:27,810 --> 00:14:29,820 If we look at the alignment 300 00:14:29,820 --> 00:14:32,700 of these two molecules to one another, 301 00:14:32,700 --> 00:14:34,650 in the case of guanine and cytosine, 302 00:14:34,650 --> 00:14:39,650 both of them have three spaces on the cytosine side. 303 00:14:40,020 --> 00:14:42,300 One, two, three, right? 304 00:14:42,300 --> 00:14:45,420 And on the guanine side, one, two, three, 305 00:14:45,420 --> 00:14:48,123 have three spaces for hydrogen bonding. 306 00:14:49,350 --> 00:14:53,223 So they will bond to one another and only to one another. 307 00:14:54,210 --> 00:14:55,620 On adenine and thymine 308 00:14:55,620 --> 00:15:00,180 they each have two specific spots for hydrogen bonding 309 00:15:00,180 --> 00:15:03,300 and so they will only bond to each other. 310 00:15:03,300 --> 00:15:06,600 Adenine and thymine only bonds to each other. 311 00:15:06,600 --> 00:15:07,920 Again, this is across 312 00:15:07,920 --> 00:15:10,863 those two complementary strands of DNA. 313 00:15:12,090 --> 00:15:14,910 And given that guanine and cytosine 314 00:15:14,910 --> 00:15:18,690 have three hydrogen bonds between each other, 315 00:15:18,690 --> 00:15:21,330 and adenine and thymine only have two, 316 00:15:21,330 --> 00:15:24,660 you might be able to guess that guanine and cytosine bonding 317 00:15:24,660 --> 00:15:26,430 is actually a bit stronger 318 00:15:26,430 --> 00:15:29,313 than bonding between adenine and thymine. 319 00:15:30,420 --> 00:15:33,150 Let's take a look at how this all comes together. 320 00:15:33,150 --> 00:15:38,150 Once more, only adenine or A can bond with thymine or T, 321 00:15:38,190 --> 00:15:42,450 and only guanine or G can bind with cytosine or C 322 00:15:42,450 --> 00:15:44,673 between complementary strands. 323 00:15:45,660 --> 00:15:48,960 Remember the sugar phosphate backbone shown here 324 00:15:48,960 --> 00:15:53,880 is not at all involved in bonding between bases. 325 00:15:53,880 --> 00:15:54,810 What we're talking about 326 00:15:54,810 --> 00:15:56,250 when we're talking about base pairing 327 00:15:56,250 --> 00:15:59,310 is between two complimentary strands. 328 00:15:59,310 --> 00:16:03,930 So this strand here and this strand here 329 00:16:03,930 --> 00:16:06,120 are bonding to one another, 330 00:16:06,120 --> 00:16:08,760 and the way that they're attaching to each other 331 00:16:08,760 --> 00:16:12,450 is through hydrogen bonds between the bases. 332 00:16:12,450 --> 00:16:13,650 Just between the bases, 333 00:16:13,650 --> 00:16:16,230 has nothing to do with the sugar phosphate backbone. 334 00:16:16,230 --> 00:16:19,800 So what this means is that on a single strand of DNA, 335 00:16:19,800 --> 00:16:24,780 you can really, you can have any sequence of bases. 336 00:16:24,780 --> 00:16:26,250 And when I say sequence of bases, 337 00:16:26,250 --> 00:16:30,731 I mean the order in which you see A's G's, C's, and T's, 338 00:16:30,731 --> 00:16:34,560 that can occur in any conceivable order. 339 00:16:34,560 --> 00:16:37,830 Once you have the sequence for one strand, however, 340 00:16:37,830 --> 00:16:42,210 the sequence for the complementary strand is completely set. 341 00:16:42,210 --> 00:16:43,650 Why do I say that? 342 00:16:43,650 --> 00:16:46,500 Well, since we know only A combines to T 343 00:16:46,500 --> 00:16:48,300 and only G combine to C, 344 00:16:48,300 --> 00:16:51,840 if let's say this is our first strand here that we have, 345 00:16:51,840 --> 00:16:55,590 that was created A, G, T, A, C, G. 346 00:16:55,590 --> 00:16:58,200 That would mean the complimentary strand 347 00:16:58,200 --> 00:17:03,200 absolutely must read as T because T binds to A, 348 00:17:03,420 --> 00:17:08,293 C because C binds to G, A, T, G, C. 349 00:17:09,150 --> 00:17:13,320 So this is determined, it has to read as such. 350 00:17:13,320 --> 00:17:15,840 If this is the sequence of one of the strands, 351 00:17:15,840 --> 00:17:19,593 then you know the sequence of the complementary strand. 352 00:17:20,850 --> 00:17:25,500 Just as a quick review, this is a nucleotide here, 353 00:17:26,790 --> 00:17:29,943 phosphate, sugar, base. 354 00:17:30,960 --> 00:17:35,010 This is the sugar phosphate backbone of one strand. 355 00:17:35,010 --> 00:17:38,210 This is the sugar phosphate backbone of another strand, 356 00:17:38,210 --> 00:17:41,610 of the complementary strand. 357 00:17:41,610 --> 00:17:46,080 These two strands together are complementary to one another 358 00:17:46,080 --> 00:17:50,370 and together the two strands bonded together 359 00:17:50,370 --> 00:17:53,130 makes up one DNA molecule. 360 00:17:53,130 --> 00:17:55,230 There is bonding between the bases 361 00:17:55,230 --> 00:17:59,190 separate from the sugar phosphate backbone. 362 00:17:59,190 --> 00:18:04,190 These are through hydrogen bonds and again, T to A, G to C. 363 00:18:04,650 --> 00:18:06,930 Always, always, always remember that, 364 00:18:06,930 --> 00:18:09,180 C and G together, T and A together. 365 00:18:09,180 --> 00:18:11,340 Let that soak in for a minute. 366 00:18:11,340 --> 00:18:14,250 Feel good that you have this, you know this, 367 00:18:14,250 --> 00:18:18,870 you are right on track, you are a chemistry rockstar. 368 00:18:18,870 --> 00:18:21,420 Let's zoom out a little bit more here. 369 00:18:21,420 --> 00:18:23,100 So now we understand, 370 00:18:23,100 --> 00:18:27,000 you have this down pat what the structure of DNA. 371 00:18:27,000 --> 00:18:29,580 This is an artist's rendition, 372 00:18:29,580 --> 00:18:31,020 which I think is kind of a cool picture. 373 00:18:31,020 --> 00:18:36,020 So this is showing sugar phosphate backbone right here, 374 00:18:36,930 --> 00:18:39,840 there's another one, so these are two strands. 375 00:18:39,840 --> 00:18:41,910 The rungs of the ladder here 376 00:18:41,910 --> 00:18:44,310 are the bases binding to one another, 377 00:18:44,310 --> 00:18:46,560 so there's a base on each of these strands 378 00:18:46,560 --> 00:18:48,210 and they're binding to each other. 379 00:18:48,210 --> 00:18:51,270 And then it gets curled up into that double helix structure 380 00:18:51,270 --> 00:18:53,070 that I was talking about. 381 00:18:53,070 --> 00:18:54,720 So this is basically, 382 00:18:54,720 --> 00:18:57,090 if you stretch out DNA, it looks like this, 383 00:18:57,090 --> 00:18:59,250 and then as it becomes more and more compacted, 384 00:18:59,250 --> 00:19:01,773 you start to see these other structures form. 385 00:19:03,240 --> 00:19:05,340 So there's 6 billion base pairs of DNA 386 00:19:05,340 --> 00:19:07,020 within each cell's nucleus. 387 00:19:07,020 --> 00:19:08,910 3 billion that you inherited from mom, 388 00:19:08,910 --> 00:19:10,953 3 billion that you inherited from dad. 389 00:19:12,270 --> 00:19:15,180 Stretched out the DNA in a single nucleus 390 00:19:15,180 --> 00:19:17,130 is two meters long, 391 00:19:17,130 --> 00:19:22,130 but an average nucleus is only six micrometers in diameter. 392 00:19:23,970 --> 00:19:26,580 Okay? So that's a big difference. 393 00:19:26,580 --> 00:19:28,890 Two meters. I'm not, I'm not, that's not a mistake. 394 00:19:28,890 --> 00:19:31,920 It is actually two meters if you stretch it out. 395 00:19:31,920 --> 00:19:34,770 So if you take, let's, if you stretch, 396 00:19:34,770 --> 00:19:36,210 pull it all the way out and you stretch it 397 00:19:36,210 --> 00:19:38,050 like into this kind of a structure 398 00:19:39,180 --> 00:19:43,680 and you lay it end-to-end, it's going to be two meters long. 399 00:19:43,680 --> 00:19:45,630 That's a really big 400 00:19:45,630 --> 00:19:48,843 and it has to fit into a really, really tiny space, 401 00:19:49,950 --> 00:19:52,110 six micrometers in diameter. 402 00:19:52,110 --> 00:19:54,210 We're talking microscopic space, 403 00:19:54,210 --> 00:19:56,970 from two meters down to microscopic space. 404 00:19:56,970 --> 00:19:59,850 So how does the cell go about doing this? 405 00:19:59,850 --> 00:20:03,690 Well, DNA becomes tightly compacted to fit into the nucleus 406 00:20:03,690 --> 00:20:06,360 by wrapping around proteins called histones, 407 00:20:06,360 --> 00:20:08,280 like string around a spool, 408 00:20:08,280 --> 00:20:11,760 and supercoiled on itself to form a chromosome. 409 00:20:11,760 --> 00:20:13,890 So histones, diagrammed here 410 00:20:13,890 --> 00:20:18,813 as these kind of pinkish purpley colored round things. 411 00:20:19,800 --> 00:20:24,800 DNA just starts to wrap itself around, wrap itself around, 412 00:20:25,050 --> 00:20:28,770 come around, wraps itself around again and again and again, 413 00:20:28,770 --> 00:20:32,110 just like wrapping string around a spool 414 00:20:33,060 --> 00:20:35,070 which helps to keep it all together. 415 00:20:35,070 --> 00:20:36,150 Because if you imagine, 416 00:20:36,150 --> 00:20:40,020 let's say you have a small drawer for your sewing materials. 417 00:20:40,020 --> 00:20:41,760 And I don't actually sew, 418 00:20:41,760 --> 00:20:44,400 so I apologize for calling it sewing materials. 419 00:20:44,400 --> 00:20:46,770 I don't know, there's probably a better term for it, 420 00:20:46,770 --> 00:20:48,903 but I apologize, I don't know what it is. 421 00:20:50,190 --> 00:20:55,190 But if in your sewing area, if you have a drawer let's say, 422 00:20:57,810 --> 00:20:59,370 like you just have like a small drawer 423 00:20:59,370 --> 00:21:03,210 for all of your sewing stuff, but you have, you know, 424 00:21:03,210 --> 00:21:07,260 20 feet of string that you just bought, 425 00:21:07,260 --> 00:21:10,230 the way that you fit it into that tiny drawer 426 00:21:10,230 --> 00:21:14,820 is it's wrapped around a spool many, many, many times 427 00:21:14,820 --> 00:21:16,270 and then stuck in the drawer. 428 00:21:17,970 --> 00:21:20,700 Well it's not that different for DNA, if DNA was like string 429 00:21:20,700 --> 00:21:23,070 and you just stretched it out like the 20 feet of string, 430 00:21:23,070 --> 00:21:24,840 it would stretch out two meters 431 00:21:24,840 --> 00:21:26,850 and it has to fit into a really small drawer 432 00:21:26,850 --> 00:21:28,680 called the cell nucleus. 433 00:21:28,680 --> 00:21:29,513 The way it does that 434 00:21:29,513 --> 00:21:31,860 is it wraps itself around these histones. 435 00:21:31,860 --> 00:21:32,730 And then beyond that 436 00:21:32,730 --> 00:21:35,070 it starts to wrap what it's wrapped around the histones 437 00:21:35,070 --> 00:21:38,820 around itself even more, and what's called supercoiling. 438 00:21:38,820 --> 00:21:41,580 Supercoiling you can kind of think of, 439 00:21:41,580 --> 00:21:43,890 not to use too many metaphors here, 440 00:21:43,890 --> 00:21:47,100 but you can kind of think of it like a phone cord, 441 00:21:47,100 --> 00:21:49,203 if you can remember what those look like. 442 00:21:50,344 --> 00:21:52,980 For those who are on, you know, 443 00:21:52,980 --> 00:21:57,570 who just use cell phones these days or cordless phones, 444 00:21:57,570 --> 00:21:58,620 maybe you can think back 445 00:21:58,620 --> 00:22:01,920 and remember what it was like to have a corded phone. 446 00:22:01,920 --> 00:22:06,843 Remember that they have that sort of natural coil to them. 447 00:22:07,890 --> 00:22:09,090 But inevitably, 448 00:22:09,090 --> 00:22:11,610 what at least has always happened in my experience 449 00:22:11,610 --> 00:22:13,980 is that they end up getting coiled around each other 450 00:22:13,980 --> 00:22:15,150 and around and around and around. 451 00:22:15,150 --> 00:22:20,040 And so you have this big like coil blob of phone cable 452 00:22:20,040 --> 00:22:22,020 and that's supercoiling. 453 00:22:22,020 --> 00:22:23,460 So if you stretch it out, 454 00:22:23,460 --> 00:22:26,820 you still have more coiling that's natural to it 455 00:22:26,820 --> 00:22:30,660 but it starts to kind of compact in into a smaller space 456 00:22:30,660 --> 00:22:33,120 by wrapping around itself more and more. 457 00:22:33,120 --> 00:22:35,460 This is kind of similar to what happens with DNA. 458 00:22:35,460 --> 00:22:36,293 So it's wrapped, 459 00:22:36,293 --> 00:22:38,820 it has its natural kind of helical structure 460 00:22:38,820 --> 00:22:40,800 and then starts to wrap around these histones, 461 00:22:40,800 --> 00:22:43,200 and then the histones start to wrap around each other 462 00:22:43,200 --> 00:22:45,360 and it compacts and compacts and compacts 463 00:22:45,360 --> 00:22:49,920 until you get to something that is of far more condensed 464 00:22:49,920 --> 00:22:51,600 called a chromosome. 465 00:22:51,600 --> 00:22:53,250 So if you take a chromosome 466 00:22:53,250 --> 00:22:55,350 and you pull it and you stretch it out, 467 00:22:55,350 --> 00:22:59,850 you would see it contains one long molecule of DNA 468 00:22:59,850 --> 00:23:02,370 that's actually wrapped around lots of these histones 469 00:23:02,370 --> 00:23:04,530 and then packed and packed and packed around itself 470 00:23:04,530 --> 00:23:07,740 until you get this kind of puffy looking structure 471 00:23:07,740 --> 00:23:12,600 that is far shorter than the full length of DNA. 472 00:23:12,600 --> 00:23:16,260 If we go back to our schematic of a cell, 473 00:23:16,260 --> 00:23:18,180 remember all of this is taking place, 474 00:23:18,180 --> 00:23:21,210 all of this is present in the nucleus. 475 00:23:21,210 --> 00:23:22,650 So this is that little, 476 00:23:22,650 --> 00:23:25,380 that lovely hairball I mentioned before. 477 00:23:25,380 --> 00:23:28,260 If you look at it more closely, what you start to see 478 00:23:28,260 --> 00:23:31,050 is exactly what we were just talking about. 479 00:23:31,050 --> 00:23:35,340 So if we start out from the big structure, these would be, 480 00:23:35,340 --> 00:23:38,460 if you look at each one of these strands that you see here, 481 00:23:38,460 --> 00:23:40,560 this would, each of these would be that chromosome. 482 00:23:40,560 --> 00:23:44,913 So it is, even though it all looks kind of messy in here, 483 00:23:45,924 --> 00:23:49,200 these are highly compacted DNA strands. 484 00:23:49,200 --> 00:23:51,600 So if you zoom in a little bit more on that, 485 00:23:51,600 --> 00:23:53,460 and let's take a peek at that. 486 00:23:53,460 --> 00:23:56,100 So if we look at each one of these as a chromosome, 487 00:23:56,100 --> 00:23:58,710 and we'll talk more about the structure of chromosomes 488 00:23:58,710 --> 00:24:01,230 in some subsequent modules, 489 00:24:01,230 --> 00:24:04,770 but if we zoom in on the chromosome and we start to pull, 490 00:24:04,770 --> 00:24:07,290 like pulling a thread out of a sweater, 491 00:24:07,290 --> 00:24:08,730 you start to pull and pull and pull. 492 00:24:08,730 --> 00:24:12,990 What you start to see is that within each chromosome 493 00:24:12,990 --> 00:24:16,440 is a single molecule of DNA. 494 00:24:16,440 --> 00:24:18,630 So that would mean a single DNA double helix 495 00:24:18,630 --> 00:24:20,550 that's just been wound and wound and wound and wound 496 00:24:20,550 --> 00:24:24,930 and really kind of smooshed together, compacted in 497 00:24:24,930 --> 00:24:27,930 to form this puffy looking structure of a chromosome. 498 00:24:27,930 --> 00:24:32,190 But if you stretch that out like this and you zoom in a bit, 499 00:24:32,190 --> 00:24:33,360 you start to see that, yep, 500 00:24:33,360 --> 00:24:36,690 that's a single strand of DNA wrapped around these histones, 501 00:24:36,690 --> 00:24:38,850 which kind of act like spools 502 00:24:38,850 --> 00:24:41,340 with the DNA being more like the thread. 503 00:24:41,340 --> 00:24:44,460 And if you zoom in more and more and more on each of these, 504 00:24:44,460 --> 00:24:46,080 if you pull it apart from the histones 505 00:24:46,080 --> 00:24:48,000 and you stretch it out some more, 506 00:24:48,000 --> 00:24:50,163 you see that double helix structure. 507 00:24:51,030 --> 00:24:55,470 And if we zoom in more, let's say we take this stretch here 508 00:24:55,470 --> 00:24:59,190 and we look at what the base sequence is against sequence, 509 00:24:59,190 --> 00:25:01,740 is the order of the bases 510 00:25:01,740 --> 00:25:05,853 that are found in a particular strand of DNA. 511 00:25:07,410 --> 00:25:10,350 Given that we always know what the complementary strand is 512 00:25:10,350 --> 00:25:13,590 based upon the sequence of one of the strands, 513 00:25:13,590 --> 00:25:14,940 we only ever really look 514 00:25:14,940 --> 00:25:18,030 at the sequence of one of the strands of DNA 515 00:25:18,030 --> 00:25:20,670 because the other one is going to be the 516 00:25:20,670 --> 00:25:24,183 basically mirror image of that sequence. 517 00:25:25,470 --> 00:25:27,390 So if we zoom in here and, you know, 518 00:25:27,390 --> 00:25:29,610 we know that it has a sugar phosphate backbone, 519 00:25:29,610 --> 00:25:32,520 which is identical throughout the whole strand of DNA. 520 00:25:32,520 --> 00:25:35,490 The part of it that's different is the base or the bases. 521 00:25:35,490 --> 00:25:37,770 And so we look at this, let's say we zoomed in here, 522 00:25:37,770 --> 00:25:40,800 we see dah, dah, dah, dah, these are the different bases 523 00:25:40,800 --> 00:25:43,380 that are in that particular order. 524 00:25:43,380 --> 00:25:45,540 It's really important that they're in this order 525 00:25:45,540 --> 00:25:47,390 and we'll talk about that more later. 526 00:25:48,360 --> 00:25:51,540 Let's say this segment here, we would consider to be a gene. 527 00:25:51,540 --> 00:25:54,900 So this segment and, you know, there's a, 528 00:25:54,900 --> 00:25:59,850 you know, a gene would be significantly longer than. 529 00:25:59,850 --> 00:26:02,460 An average gene would be significantly longer than this 530 00:26:02,460 --> 00:26:04,380 but this is just to give you a demonstration 531 00:26:04,380 --> 00:26:06,180 of what we're talking about. 532 00:26:06,180 --> 00:26:09,360 So a gene exists along a single molecule of DNA, 533 00:26:09,360 --> 00:26:13,173 so which is to say it is also on a chromosome. 534 00:26:14,490 --> 00:26:19,490 This chromosome is made up of a single molecule of DNA 535 00:26:19,650 --> 00:26:22,140 plus wrapped around histone proteins. 536 00:26:22,140 --> 00:26:24,930 And what you would see along any chromosome 537 00:26:24,930 --> 00:26:27,780 is you're going to have multiple, 538 00:26:27,780 --> 00:26:30,600 hundreds or even thousands of genes 539 00:26:30,600 --> 00:26:32,910 spaced out in different areas 540 00:26:32,910 --> 00:26:34,920 and stretches along this chromosome. 541 00:26:34,920 --> 00:26:37,860 But you'll also have some regions that are not genes 542 00:26:37,860 --> 00:26:39,480 and that's what I was talking about before 543 00:26:39,480 --> 00:26:41,010 when I was saying your genome includes 544 00:26:41,010 --> 00:26:43,290 regions that are genes and regions that are not. 545 00:26:43,290 --> 00:26:44,940 Because if I stretch out this DNA, 546 00:26:44,940 --> 00:26:46,200 this part's gonna be a gene 547 00:26:46,200 --> 00:26:47,820 and let's say the next section here 548 00:26:47,820 --> 00:26:51,450 doesn't actually give instructions for a specific protein 549 00:26:51,450 --> 00:26:53,160 so it would not be considered a gene. 550 00:26:53,160 --> 00:26:54,720 But then let's say down here somewhere, 551 00:26:54,720 --> 00:26:56,280 oh, there's another gene that pops up 552 00:26:56,280 --> 00:26:58,380 maybe down here somewhere, there's another one that pops up, 553 00:26:58,380 --> 00:27:00,120 maybe there's a long stretch 554 00:27:00,120 --> 00:27:03,183 and then you get some more down in this area, for example. 555 00:27:04,200 --> 00:27:06,390 So chromosomes contain many genes, 556 00:27:06,390 --> 00:27:09,660 these genes are always in the same position 557 00:27:09,660 --> 00:27:12,780 on its particular chromosome. 558 00:27:12,780 --> 00:27:14,040 And that becomes important 559 00:27:14,040 --> 00:27:17,550 when we start thinking about chromosomal abnormalities 560 00:27:17,550 --> 00:27:20,730 and conditions and syndromes associated with those, 561 00:27:20,730 --> 00:27:25,443 which we will get into in detail in another module. 562 00:27:28,020 --> 00:27:31,020 Let's take a quick peek at all of the chromosomes together. 563 00:27:31,020 --> 00:27:35,100 So DNA is divided into 24 different chromosomes. 564 00:27:35,100 --> 00:27:38,310 That does not mean we all have 24 different chromosomes. 565 00:27:38,310 --> 00:27:40,560 That means there are 24 different chromosomes 566 00:27:40,560 --> 00:27:42,240 in the human genome. 567 00:27:42,240 --> 00:27:46,680 And by this I mean there are 22 what are called autosomes, 568 00:27:46,680 --> 00:27:48,810 so these are chromosomes 1 through 22, 569 00:27:48,810 --> 00:27:51,060 which you can see here on a karyotype, 570 00:27:51,060 --> 00:27:53,550 which you've probably seen something like this before. 571 00:27:53,550 --> 00:27:57,270 Karyotypes are microscopic imaging 572 00:27:57,270 --> 00:28:01,950 of the full set of chromosomes that an individual has. 573 00:28:01,950 --> 00:28:05,367 So we have chromosomes that are just labeled 1 through 22 574 00:28:05,367 --> 00:28:08,850 and those, that's their name, not very, 575 00:28:08,850 --> 00:28:10,890 not terribly creative labeling, 576 00:28:10,890 --> 00:28:15,690 but that's what they're called, chromosomes 1 through 22. 577 00:28:15,690 --> 00:28:20,340 And you can see chromosomes with lower numbers 578 00:28:20,340 --> 00:28:23,673 are actually longer, bigger chromosomes, 579 00:28:25,620 --> 00:28:27,870 whereas the higher the number, 580 00:28:27,870 --> 00:28:30,453 you get shorter and shorter chromosomes. 581 00:28:31,320 --> 00:28:33,060 In this case, this is a male 582 00:28:33,060 --> 00:28:35,640 because this individual has one X chromosome 583 00:28:35,640 --> 00:28:36,930 and one Y chromosome. 584 00:28:36,930 --> 00:28:39,120 Coming back here to our different types of chromosomes, 585 00:28:39,120 --> 00:28:41,100 we have 22 autosomes, 586 00:28:41,100 --> 00:28:43,710 chromosomes 1 through 22 are called autosomes 587 00:28:43,710 --> 00:28:47,130 because they're not involved in sex determination. 588 00:28:47,130 --> 00:28:50,580 And then there are two different kinds of sex chromosomes, 589 00:28:50,580 --> 00:28:52,890 an X chromosome and a Y chromosome. 590 00:28:52,890 --> 00:28:54,303 X and Y. 591 00:28:55,920 --> 00:28:57,150 So it's really, 592 00:28:57,150 --> 00:29:01,650 you'll hear it said that we have 23 chromosomes, 593 00:29:01,650 --> 00:29:06,650 which we do in a way in that we have 1 through 22 594 00:29:06,810 --> 00:29:09,030 and then we have sex chromosomes, 595 00:29:09,030 --> 00:29:11,010 which could be considered 23 I guess. 596 00:29:11,010 --> 00:29:15,210 But we really, really have 24 different kinds of chromosomes 597 00:29:15,210 --> 00:29:18,000 because you can have an X and a Y 598 00:29:18,000 --> 00:29:20,313 and then 1 through 22 of the autosomes. 599 00:29:22,410 --> 00:29:25,470 Each chromosome has one double stranded DNA molecule 600 00:29:25,470 --> 00:29:27,360 wrapped around histones and compacted 601 00:29:27,360 --> 00:29:29,610 as we looked at before. 602 00:29:29,610 --> 00:29:32,100 All cells except egg and sperm 603 00:29:32,100 --> 00:29:34,170 have two copies of each autosome 604 00:29:34,170 --> 00:29:36,540 and either two X chromosomes for female 605 00:29:36,540 --> 00:29:41,540 or one X and one Y chromosome for a total of 46 chromosomes. 606 00:29:42,690 --> 00:29:43,733 Okay, so I'm throwing a lot of numbers at you. 607 00:29:43,733 --> 00:29:48,690 24, 23, 46. What am I talking about? 608 00:29:48,690 --> 00:29:50,880 Let's make sure we boil this down. 609 00:29:50,880 --> 00:29:55,320 24 different kinds of chromosomes are present, 610 00:29:55,320 --> 00:29:58,260 1 through 22, as you can see here 611 00:29:58,260 --> 00:30:01,863 and then X and Y are two different kinds of chromosomes. 612 00:30:03,720 --> 00:30:07,800 But you have 46 in the end. Where do you get 46 from? 613 00:30:07,800 --> 00:30:11,160 Well, as you can see here in this karyotype 614 00:30:11,160 --> 00:30:14,910 there is one, two. 615 00:30:14,910 --> 00:30:17,880 You have a pair of chromosomes 616 00:30:17,880 --> 00:30:21,690 for each one of these designated chromosomes. 617 00:30:21,690 --> 00:30:25,020 So for example, chromosome 8, you have two. 618 00:30:25,020 --> 00:30:28,560 Why do you have two? One came from mom, one came from dad. 619 00:30:28,560 --> 00:30:32,640 So if we just do some simple multiplication, we get to 46 620 00:30:32,640 --> 00:30:35,850 because two times 22, 621 00:30:35,850 --> 00:30:39,810 which would be the number of autosomes we have, equals 44. 622 00:30:39,810 --> 00:30:42,570 And then you have one, 623 00:30:42,570 --> 00:30:45,330 either if you're a male you have one X and one Y, 624 00:30:45,330 --> 00:30:48,210 which makes 45, 46 chromosomes total. 625 00:30:48,210 --> 00:30:50,960 Or if you're a female, you would have two X chromosomes 626 00:30:52,140 --> 00:30:54,540 so then you would also be at 44 autosomes 627 00:30:54,540 --> 00:30:58,353 plus 45, one X chromosome, 46, second X chromosome. 628 00:30:59,880 --> 00:31:02,700 So make sure you think about that for a minute 629 00:31:02,700 --> 00:31:04,410 because it's actually kind of important 630 00:31:04,410 --> 00:31:07,260 to understand the number of chromosomes. 631 00:31:07,260 --> 00:31:12,260 And this will be a bigger part of our next module 632 00:31:12,330 --> 00:31:14,040 so if you're not totally getting it now, 633 00:31:14,040 --> 00:31:14,920 don't worry about it. 634 00:31:14,920 --> 00:31:19,080 It'll come back up quite a bit in the next module. 635 00:31:19,080 --> 00:31:22,800 Okay, well now I want to just give you 636 00:31:22,800 --> 00:31:27,800 a little bit of strange but true information about genetics. 637 00:31:28,020 --> 00:31:31,320 If all of the DNA in your 50 trillion cells 638 00:31:31,320 --> 00:31:32,670 was placed end-to-end 639 00:31:32,670 --> 00:31:34,590 and stretched out from its condensed form 640 00:31:34,590 --> 00:31:36,210 in a straight line, 641 00:31:36,210 --> 00:31:40,683 it would reach from the Earth to the Sun over 30 times. 642 00:31:41,790 --> 00:31:43,800 Yes, that's correct. 643 00:31:43,800 --> 00:31:46,810 If all of the DNA in your body 644 00:31:47,880 --> 00:31:52,170 was stretched out to its longest possible length 645 00:31:52,170 --> 00:31:54,443 and placed end-to-end in a straight line, 646 00:31:54,443 --> 00:31:56,880 it'd be like taking all the string 647 00:31:56,880 --> 00:32:00,420 that you have in those sewing drawers 648 00:32:00,420 --> 00:32:03,240 and you place them end-to-end-to-end-to-end-to-end, 649 00:32:03,240 --> 00:32:05,250 and you stretch it all the way out, 650 00:32:05,250 --> 00:32:09,570 it would be long enough, because you have 50 trillion cells 651 00:32:09,570 --> 00:32:10,530 and each one of those cells 652 00:32:10,530 --> 00:32:12,750 has two meters worth of DNA in it. 653 00:32:12,750 --> 00:32:14,790 If you put it end-to-end and you stretch it out, 654 00:32:14,790 --> 00:32:17,580 it would go to from here to the Sun and back, 655 00:32:17,580 --> 00:32:21,030 which is a really long way, 92 million miles, 656 00:32:21,030 --> 00:32:23,250 it would go over 30 times. 657 00:32:23,250 --> 00:32:25,830 That's a lot. That's a lot, right? 658 00:32:25,830 --> 00:32:28,863 This little kid's like, "What?" Exactly. 659 00:32:29,760 --> 00:32:31,410 If you were to start reciting the order 660 00:32:31,410 --> 00:32:34,650 of the A's, T's, C's, and G's in your DNA tomorrow morning 661 00:32:34,650 --> 00:32:38,700 at a rate of 100 each minute, which is pretty fast, 662 00:32:38,700 --> 00:32:43,080 57 years would pass before you reached the end. 663 00:32:43,080 --> 00:32:45,660 This provided that you did not stop to eat, drink, sleep, 664 00:32:45,660 --> 00:32:47,610 use the bathroom, et cetera. 665 00:32:47,610 --> 00:32:51,000 Remember that from each of your parents, 666 00:32:51,000 --> 00:32:53,760 you received 3 billion with a B, 667 00:32:53,760 --> 00:32:58,760 3 billion bases worth of DNA, so that's a lot. 668 00:32:58,890 --> 00:33:01,860 You could fit 1,000 nuclei in a cell 669 00:33:01,860 --> 00:33:04,833 across the period at the end of this sentence here. 670 00:33:06,120 --> 00:33:08,460 That's how small a cell nucleus is. 671 00:33:08,460 --> 00:33:09,630 And even smaller than that, 672 00:33:09,630 --> 00:33:12,120 you could fit 1 million threads of DNA 673 00:33:12,120 --> 00:33:14,220 across the period at the end of this sentence. 674 00:33:14,220 --> 00:33:16,890 And by that I mean this is the width of DNA. 675 00:33:16,890 --> 00:33:19,830 Not the length of DNA, but the width of DNA, 676 00:33:19,830 --> 00:33:20,700 you could fit it, 677 00:33:20,700 --> 00:33:24,150 line it up sort of side-by-side-by-side-by-side 678 00:33:24,150 --> 00:33:26,520 across the period at the end of the sentence 679 00:33:26,520 --> 00:33:28,230 and you could fit a million of them. 680 00:33:28,230 --> 00:33:29,460 So while they're really, really long, 681 00:33:29,460 --> 00:33:31,803 they're also really super, super thin. 682 00:33:33,630 --> 00:33:34,770 All right, 683 00:33:34,770 --> 00:33:37,470 well hopefully your mind was blown a little bit there, 684 00:33:38,310 --> 00:33:40,050 but you can start to understand 685 00:33:40,050 --> 00:33:43,533 how this is all fitting together and working. 686 00:33:44,580 --> 00:33:48,240 Let's look at a summary of some of what we've learned here. 687 00:33:48,240 --> 00:33:50,250 Deoxyribonucleic acid or DNA 688 00:33:50,250 --> 00:33:53,130 is the molecule used to store genetic information 689 00:33:53,130 --> 00:33:54,540 and a single strand of it 690 00:33:54,540 --> 00:33:57,480 is made up of nucleotides joined together in a chain 691 00:33:57,480 --> 00:33:59,940 through their sugar phosphate backbone. 692 00:33:59,940 --> 00:34:02,490 DNA is made up of two complementary strands 693 00:34:02,490 --> 00:34:04,440 which bind together through hydrogen bonds 694 00:34:04,440 --> 00:34:07,641 between adenine and thymine, so A and T go together, 695 00:34:07,641 --> 00:34:10,560 and guanine and cytosine, G and C go together. 696 00:34:10,560 --> 00:34:12,840 DNA has a natural double helical shape 697 00:34:12,840 --> 00:34:15,900 and further compacts around histones to form chromosomes. 698 00:34:15,900 --> 00:34:20,760 So a chromosome is made up of a long stretch of DNA 699 00:34:20,760 --> 00:34:23,340 that's wrapped and wound around histones 700 00:34:23,340 --> 00:34:25,170 and compacted down together. 701 00:34:25,170 --> 00:34:28,233 And within that there are segments called genes. 702 00:34:29,190 --> 00:34:31,920 There are 24 different chromosomes, 22 autosomes, 703 00:34:31,920 --> 00:34:35,730 and two sex chromosomes and each cell has 46 chromosomes. 704 00:34:35,730 --> 00:34:40,200 Again, two times the 22 autosomes, so 44 autosomes, 705 00:34:40,200 --> 00:34:42,810 and either two X chromosomes if you're female 706 00:34:42,810 --> 00:34:44,730 or one X and one Y chromosome. 707 00:34:44,730 --> 00:34:48,243 So 44 plus two equals 46. 708 00:34:49,260 --> 00:34:52,710 All right, so what's in the next module? You ready to go? 709 00:34:52,710 --> 00:34:55,020 Ready to take on the next module, which is really, 710 00:34:55,020 --> 00:34:58,230 so we learned a lot about the structure of DNA, 711 00:34:58,230 --> 00:35:01,140 but what does it do exactly? 712 00:35:01,140 --> 00:35:04,620 Why do we care about DNA? 713 00:35:04,620 --> 00:35:08,730 So we'll talk about that next in the next lecture. 714 00:35:08,730 --> 00:35:10,350 Thank you so much for your patience 715 00:35:10,350 --> 00:35:13,530 and congratulations making it through this lecture. 716 00:35:13,530 --> 00:35:15,930 I look forward to speaking with you in the next.