1 00:00:00,000 --> 00:00:04,000 So today we're going to continue our focus on DNA which I'm personally 2 00:00:04,000 --> 00:00:09,000 enthusiastic about at least in terms of being such a fascinating molecule. 3 00:00:09,000 --> 00:00:14,000 And I told you the story last time of how we actually came to 4 00:00:14,000 --> 00:00:19,000 understand that DNA was the genetic material. And I still see comments 5 00:00:19,000 --> 00:00:24,000 that, oh, God, all this stuff is not relevant to 6 00:00:24,000 --> 00:00:29,000 the exam. We're trying to construct the exams in ways that test whether 7 00:00:29,000 --> 00:00:34,000 you got the concepts and not just whether you memorized every term 8 00:00:34,000 --> 00:00:39,000 that you ran into in the textbook. So I'm hoping that you will see some 9 00:00:39,000 --> 00:00:43,000 greater purpose in why I'm trying to talk about some of this. 10 00:00:43,000 --> 00:00:47,000 And also I'm sure some of you will forget the details of transformation, 11 00:00:47,000 --> 00:00:51,000 of DNA replication we're going to go into as we sort of burrow into it 12 00:00:51,000 --> 00:00:55,000 over the next lecture or so, but what I am hoping you may 13 00:00:55,000 --> 00:00:59,000 remember ten years from now, even those who don't go in biology, 14 00:00:59,000 --> 00:01:04,000 is how experiments are done, how real people do them. 15 00:01:04,000 --> 00:01:07,000 And that was partly what I was trying to tell you. 16 00:01:07,000 --> 00:01:11,000 And you guys are pretty good at figuring out the basic principle 17 00:01:11,000 --> 00:01:14,000 that someone had to somehow show that a DNA molecule in one organism 18 00:01:14,000 --> 00:01:18,000 could change some organism to have a new characteristic. 19 00:01:18,000 --> 00:01:21,000 And as I sort of told you with the work from Frederick Griffith. 20 00:01:21,000 --> 00:01:25,000 And then his initial stuff wasn't devoted to that at all. 21 00:01:25,000 --> 00:01:28,000 It was trying to solve a very pressing problem which is dealing 22 00:01:28,000 --> 00:01:32,000 with pneumonia in a pre-antibiotic era. 23 00:01:32,000 --> 00:01:36,000 And then the finding that he got, that this odd result that something 24 00:01:36,000 --> 00:01:41,000 in a heat-killed extract could be transferred to a live bacterium sort 25 00:01:41,000 --> 00:01:45,000 of set things up for Avery and his colleagues after a number of years 26 00:01:45,000 --> 00:01:50,000 of work to make a very powerful argument that DNA was the genetic 27 00:01:50,000 --> 00:01:54,000 material. But, as I said at the end of last lecture, 28 00:01:54,000 --> 00:01:59,000 that paper was published in the 1940s. 29 00:01:59,000 --> 00:02:04,000 And people didn't immediately say oh, wow, DNA is the genetic material. 30 00:02:04,000 --> 00:02:09,000 Often, and we'll see it again with genetics, there's sometimes sort of 31 00:02:09,000 --> 00:02:15,000 the body of science the average person thinks about. 32 00:02:15,000 --> 00:02:20,000 Science needs to reach to a certain state before an idea can take hold, 33 00:02:20,000 --> 00:02:26,000 even if there's evidence supporting it. Part of the problem was that 34 00:02:26,000 --> 00:02:31,000 chemists had isolated DNA. And the way they used to isolate DNA 35 00:02:31,000 --> 00:02:35,000 was really rough on it. Crack the cells open. And what 36 00:02:35,000 --> 00:02:39,000 happened, it would all get broken down into little pieces of DNA. 37 00:02:39,000 --> 00:02:43,000 And people had worked out the basic chemical structure that it was the 38 00:02:43,000 --> 00:02:47,000 deoxyribose and how the things were joined together, 39 00:02:47,000 --> 00:02:51,000 but nobody had ever seen anything more than just these little pieces 40 00:02:51,000 --> 00:02:55,000 of DNA. And there was a widely held conception that it was just an 41 00:02:55,000 --> 00:03:00,000 anonymous tetranucleotide of G, A, T and C. 42 00:03:00,000 --> 00:03:04,000 It wasn't clear why the cells made it, but it didn't look like anything 43 00:03:04,000 --> 00:03:08,000 that could encode information. Whereas, as I said, something like 44 00:03:08,000 --> 00:03:12,000 proteins, those seem to be very different. And so the world wasn't 45 00:03:12,000 --> 00:03:16,000 quite ready for it. Another thing, and this came from 46 00:03:16,000 --> 00:03:20,000 one of the comments here, was someone said they didn't know 47 00:03:20,000 --> 00:03:24,000 bacteria could take up DNA from the environment. And, 48 00:03:24,000 --> 00:03:28,000 in fact, most bacteria can. It happens that streptacoccucci and 49 00:03:28,000 --> 00:03:32,000 some other bacteria at certain phases in their lifetime develop 50 00:03:32,000 --> 00:03:36,000 this capacity to take up DNA from the outside. 51 00:03:36,000 --> 00:03:39,000 Given what I've told you about a membrane and how hard it is to get 52 00:03:39,000 --> 00:03:43,000 things across it, you could imagine it's not trivial 53 00:03:43,000 --> 00:03:46,000 to get a DNA molecule which is huge from one side to the other. 54 00:03:46,000 --> 00:03:50,000 So it doesn't normally happen. And what happens if you go into a 55 00:03:50,000 --> 00:03:53,000 lab and you're cloning something or other, and we'll talk about how to 56 00:03:53,000 --> 00:03:57,000 take a couple pieces of DNA and join them together in a test tube and 57 00:03:57,000 --> 00:04:01,000 then put them back into a bacterium. 58 00:04:01,000 --> 00:04:04,000 If we put it into E. coli that doesn't normally take up 59 00:04:04,000 --> 00:04:08,000 DNA you'll find that it's sort of basically black magic. 60 00:04:08,000 --> 00:04:11,000 You cook them up with some divalent cations at very high concentrations, 61 00:04:11,000 --> 00:04:15,000 you do temperature shifts and various things, 62 00:04:15,000 --> 00:04:18,000 or you give them a big jolt of electricity, and the next thing you 63 00:04:18,000 --> 00:04:22,000 know you get some DNA inside. And it's not a very efficient 64 00:04:22,000 --> 00:04:25,000 process, but all you need is one molecule to get in one bacterium and 65 00:04:25,000 --> 00:04:29,000 then you're in business. So that was another reason that 66 00:04:29,000 --> 00:04:33,000 this wasn't accepted right away. Because this was not a phenomenon 67 00:04:33,000 --> 00:04:38,000 that could easily be repeated with other bacteria. 68 00:04:38,000 --> 00:04:43,000 So it looked like it was something perhaps special to streptococcus. 69 00:04:43,000 --> 00:04:47,000 And what did really change people's understanding, 70 00:04:47,000 --> 00:04:52,000 or at least bring people to the understanding that DNA could 71 00:04:52,000 --> 00:04:57,000 possibly be the genetic material came about from the discovery of the 72 00:04:57,000 --> 00:05:02,000 actual structure of -- How the structure of DNA as a long 73 00:05:02,000 --> 00:05:06,000 molecule with complimentary strands and the double helix, 74 00:05:06,000 --> 00:05:10,000 the little pictures I showed you with the base pairs, 75 00:05:10,000 --> 00:05:14,000 which you know about, and how the two strands which now 76 00:05:14,000 --> 00:05:18,000 I'm going to start emphasizing run in opposite directions. 77 00:05:18,000 --> 00:05:22,000 We'll come back to that in a little bit, but the 5 prime to 3 prime 78 00:05:22,000 --> 00:05:26,000 direction is this way on one end and 5 to 3 on the other. 79 00:05:26,000 --> 00:05:30,000 And just remember back here that there's the 5 prime carbon and 80 00:05:30,000 --> 00:05:34,000 that's the 3 prime carbon. So this is the 5 to 3 prime 81 00:05:34,000 --> 00:05:38,000 direction of the strand. And then it twists up in 82 00:05:38,000 --> 00:05:43,000 3-dimensional space to form this double helix. And you've seen that 83 00:05:43,000 --> 00:05:48,000 movie several times. So once that structure was 84 00:05:48,000 --> 00:05:52,000 discovered then people began to see how these could possibly encode 85 00:05:52,000 --> 00:05:57,000 information. It was clearly not just a tetranucleotide 86 00:05:57,000 --> 00:06:02,000 of G, A, T, Cs. But we didn't move immediately to 87 00:06:02,000 --> 00:06:06,000 that understanding. And today, again sort of trying to 88 00:06:06,000 --> 00:06:10,000 show you how biological experiments are done and how they're done by 89 00:06:10,000 --> 00:06:14,000 real people, I want to just go on and tell you the key things that 90 00:06:14,000 --> 00:06:19,000 happened next. So someone who was very struck by 91 00:06:19,000 --> 00:06:23,000 the results of Avery when they came out was Erwin Chargaff who was at 92 00:06:23,000 --> 00:06:27,000 Columbia. And, in fact, my colleague Boris 93 00:06:27,000 --> 00:06:31,000 Magasanik whose office is next to mine was a post-doc 94 00:06:31,000 --> 00:06:35,000 in Chargaff's lab. So I've got a neighbor of mine who 95 00:06:35,000 --> 00:06:39,000 worked with Chargaff. And Chargaff was very struck by 96 00:06:39,000 --> 00:06:42,000 this result from Avery and his colleagues that you could take DNA 97 00:06:42,000 --> 00:06:46,000 and put it in another organism. And here are a couple of quotes 98 00:06:46,000 --> 00:06:50,000 from his writing. One that I'd liked. 99 00:06:50,000 --> 00:06:53,000 I've sort of had a sense of this in my own research career, 100 00:06:53,000 --> 00:06:57,000 this kind of thing. ìI saw before me in dark contours the beginning of 101 00:06:57,000 --> 00:07:01,000 a grammar of biology.î He didn't really know quite how it 102 00:07:01,000 --> 00:07:05,000 worked but he sort of sensed that someone here where you could get 103 00:07:05,000 --> 00:07:09,000 down to the language that biology was written. So he started some 104 00:07:09,000 --> 00:07:14,000 experiments. And I started with the conviction that if different DNA 105 00:07:14,000 --> 00:07:18,000 species exhibited different biological activities there should 106 00:07:18,000 --> 00:07:23,000 exist chemically demonstrable differences between deoxyribonucleic 107 00:07:23,000 --> 00:07:27,000 acids. So he was able to start just doing some simple chemical 108 00:07:27,000 --> 00:07:31,000 experiments to try and look at DNAs from a whole variety of sources and 109 00:07:31,000 --> 00:07:37,000 see what he could learn. And this was not at the structural 110 00:07:37,000 --> 00:07:44,000 level. This was just at the chemical level. 111 00:07:44,000 --> 00:07:51,000 But one thing he learned was that the base content of DNA, 112 00:07:51,000 --> 00:07:58,000 that's the A, G, C, T part of it varied widely between organisms. 113 00:07:58,000 --> 00:08:07,000 So this was what Chargaff found in 114 00:08:07,000 --> 00:08:11,000 his lab, key findings. And that was important because if 115 00:08:11,000 --> 00:08:15,000 DNA was just a molecule of GATC, just a tetranucleotide that every 116 00:08:15,000 --> 00:08:20,000 organism made then you'd expect to find the same base composition in 117 00:08:20,000 --> 00:08:24,000 all organisms. He didn't, so that finding 118 00:08:24,000 --> 00:08:29,000 essentially buried the monotonous tetranucleotide hypothesis. 119 00:08:29,000 --> 00:08:38,000 Another thing he found was that DNA was the same in different 120 00:08:38,000 --> 00:08:49,000 tissues -- 121 00:08:49,000 --> 00:08:57,000 -- from the same organism -- 122 00:08:57,000 --> 00:09:05,000 -- but the proteins varied. 123 00:09:05,000 --> 00:09:08,000 And that's a characteristic you'd expect of something that was the 124 00:09:08,000 --> 00:09:12,000 genetic information from the cell that all cells have to have sort of 125 00:09:12,000 --> 00:09:16,000 the major blueprint. And if you had, even though 126 00:09:16,000 --> 00:09:19,000 proteins look like an attractive possibility for that because they 127 00:09:19,000 --> 00:09:23,000 had so much variation, this kind of finding wasn't 128 00:09:23,000 --> 00:09:27,000 consistent with it and it supported the idea that DNA was the 129 00:09:27,000 --> 00:09:31,000 genetic material. Well, the other thing he could do 130 00:09:31,000 --> 00:09:35,000 was he could measure the ATG and C content of all these different DNAs. 131 00:09:35,000 --> 00:09:40,000 And he noticed some similarities then. And he extracted out of that 132 00:09:40,000 --> 00:09:44,000 a couple of generalizations. One was that if you looked at the 133 00:09:44,000 --> 00:09:49,000 ratio of the purines, those are the ones with the two 134 00:09:49,000 --> 00:09:54,000 rings, adenosine and guanidine over the pyrimidines, 135 00:09:54,000 --> 00:09:58,000 those are the ones with the single ring which were C and T, 136 00:09:58,000 --> 00:10:03,000 there are about one. Another thing he noticed was that 137 00:10:03,000 --> 00:10:08,000 the ratio of A to T was about one and the ratio of G to C was about 138 00:10:08,000 --> 00:10:14,000 one. Now, that was an important clue but it didn't lead to any 139 00:10:14,000 --> 00:10:19,000 immediate breakthrough, even though maybe now that you know 140 00:10:19,000 --> 00:10:24,000 the structure you can see, gee, if I had been there maybe I 141 00:10:24,000 --> 00:10:30,000 would have been smart enough to jump on that number. 142 00:10:30,000 --> 00:10:35,000 So instead the work that led to the structure of DNA now introduces a 143 00:10:35,000 --> 00:10:40,000 couple of other characters who you've heard of a lot, 144 00:10:40,000 --> 00:10:45,000 Jim Watson and Francis Crick. At the time that Avery made his 145 00:10:45,000 --> 00:10:50,000 discovery reporting DNA was transformation, 146 00:10:50,000 --> 00:10:55,000 and Jim Watson described himself later as a precocious college boy in 147 00:10:55,000 --> 00:11:01,000 Chicago who was consumed by ornithology. 148 00:11:01,000 --> 00:11:06,000 So he was into bird watching. That's what he was excited about at 149 00:11:06,000 --> 00:11:12,000 the time Avery did his experiment. And Francis Crick at that point was 150 00:11:12,000 --> 00:11:17,000 a physicist, and he was in the British Navy designing Navel mines. 151 00:11:17,000 --> 00:11:23,000 So that's where those two players were at the time of Avery's results. 152 00:11:23,000 --> 00:11:29,000 So then both Francis Crick and Jim Watson ended up in Cambridge, 153 00:11:29,000 --> 00:11:34,000 England about 1950. I think Crick got there around 1949 154 00:11:34,000 --> 00:11:39,000 and Jim Watson got there in 1951. Francis Crick was a grad student, 155 00:11:39,000 --> 00:11:44,000 35 years old at the time. I'll show you pictures in a minute. 156 00:11:44,000 --> 00:11:49,000 35 years old at the time and still working on his PhD. 157 00:11:49,000 --> 00:11:54,000 So he was a pretty elderly grad student, if you want to think of it 158 00:11:54,000 --> 00:11:59,000 that way. And Jim Watson was a young hot-shot. 159 00:11:59,000 --> 00:12:03,000 He had done his PhD working with Salvador Luria who was at Indiana 160 00:12:03,000 --> 00:12:07,000 University at the time. Salvador Luria was one of the Nobel 161 00:12:07,000 --> 00:12:11,000 Laureates at MIT. He founded the Cancer Center, 162 00:12:11,000 --> 00:12:15,000 which is still here right across from the main biology building. 163 00:12:15,000 --> 00:12:19,000 And Jim was a very, very bright and brash young guy, 164 00:12:19,000 --> 00:12:23,000 and he had done his PhD with Salva and then he went to Cambridge as 165 00:12:23,000 --> 00:12:27,000 well. And the reason they both went to Cambridge was they were attracted 166 00:12:27,000 --> 00:12:31,000 by the power of x-ray crystallography. 167 00:12:31,000 --> 00:12:34,000 Now, I said a little work about that earlier, that if you take x-rays and 168 00:12:34,000 --> 00:12:37,000 you bounce them off a crystal and then measure the diffraction pattern 169 00:12:37,000 --> 00:12:41,000 you can work backwards by Fourier transforms and whatnot to figure out 170 00:12:41,000 --> 00:12:44,000 what the underlying crystal structure is. For the purposes of 171 00:12:44,000 --> 00:12:48,000 this course the mechanics of how that's done, we don't have to worry 172 00:12:48,000 --> 00:12:51,000 about that right now. You just need to know that you can 173 00:12:51,000 --> 00:12:55,000 work backwards from the diffraction pattern to figure out what the 174 00:12:55,000 --> 00:12:59,000 underlying structure was. And I told you, 175 00:12:59,000 --> 00:13:03,000 when I introduced to proteins, that the first clues that there were 176 00:13:03,000 --> 00:13:07,000 these regions of secondary structure, alpha helices and beta sheets came 177 00:13:07,000 --> 00:13:12,000 because people saw characteristic reflections in these diffraction 178 00:13:12,000 --> 00:13:16,000 patterns of certain proteins. And I also told you the story of 179 00:13:16,000 --> 00:13:21,000 how Linus Pauling had gone to Oxford, had gotten sick and tired of reading 180 00:13:21,000 --> 00:13:25,000 detective novels, started to try and explain the 181 00:13:25,000 --> 00:13:29,000 refractions in a certain class of proteins and came up with a model 182 00:13:29,000 --> 00:13:34,000 for the alpha helix. And so that was the sort of thing 183 00:13:34,000 --> 00:13:38,000 that inspired Watson and Crick. They were both interested in when 184 00:13:38,000 --> 00:13:43,000 one could get the structure of DNA. Now, Cambridge also had a very good 185 00:13:43,000 --> 00:13:48,000 x-ray crystallogram group. And just in passing it's 186 00:13:48,000 --> 00:13:52,000 interesting as to why they didn't come up with the structure of the 187 00:13:52,000 --> 00:13:57,000 alpha helix. There were two things. One was just lack of basic 188 00:13:57,000 --> 00:14:02,000 knowledge. I told you that the peptide bond, 189 00:14:02,000 --> 00:14:06,000 if you remember I emphasized that you cannot rotate it because the 190 00:14:06,000 --> 00:14:10,000 electrons are distributed. Pauling was an outstanding chemist. 191 00:14:10,000 --> 00:14:14,000 He knew that fact. And the folks at Cambridge who were doing that 192 00:14:14,000 --> 00:14:18,000 didn't learn this until later, so their models were far less 193 00:14:18,000 --> 00:14:22,000 constrained because they could have rotation around that bond. 194 00:14:22,000 --> 00:14:26,000 And the other one was just an experimental thing that the size of 195 00:14:26,000 --> 00:14:30,000 the photographic plates they used in the Cambridge lab were too small in 196 00:14:30,000 --> 00:14:34,000 the sense that they missed a key reflection that Pauling knew about 197 00:14:34,000 --> 00:14:39,000 and they didn't know about. So this combination led to them 198 00:14:39,000 --> 00:14:45,000 being scooped by the other group. But nevertheless the group at 199 00:14:45,000 --> 00:14:51,000 Cambridge was absolutely outstanding and at one of the top places in the 200 00:14:51,000 --> 00:14:57,000 world to do. And I showed you a couple of pictures when I was 201 00:14:57,000 --> 00:15:02,000 showing you the transition state. Sort of what you get out of working 202 00:15:02,000 --> 00:15:07,000 backwards from these diffraction patterns is they can measure regions 203 00:15:07,000 --> 00:15:12,000 of electron density, and then you fit atoms or fit 204 00:15:12,000 --> 00:15:16,000 molecules to the patterns that you see. And if it's all working you 205 00:15:16,000 --> 00:15:21,000 can explain why there are bumps here. There's an oxygen here and so on. 206 00:15:21,000 --> 00:15:26,000 There's another one. This is an ATP that's bound actually in a 207 00:15:26,000 --> 00:15:31,000 pocket in a protein. But you can sort of see how 208 00:15:31,000 --> 00:15:36,000 beautifully the patterns of electron density deduced from the x-ray 209 00:15:36,000 --> 00:15:41,000 crystallography will match the chemical structures that we put on 210 00:15:41,000 --> 00:15:47,000 the board. So that was the idea, they were going to work out the 211 00:15:47,000 --> 00:15:52,000 structure of DNA. Now, the thing about Watson and 212 00:15:52,000 --> 00:15:57,000 Crick, who at this point looked like this, they didn't look inordinately 213 00:15:57,000 --> 00:16:02,000 distinguished. In fact, Jim probably looked like, 214 00:16:02,000 --> 00:16:07,000 you've probably seen people who look approximately like that around MIT. 215 00:16:07,000 --> 00:16:12,000 He would have fit in right here and no one would have noticed. 216 00:16:12,000 --> 00:16:18,000 They were not actually x-ray crystallographers. 217 00:16:18,000 --> 00:16:23,000 They were just trying to model other people's data. 218 00:16:23,000 --> 00:16:28,000 And the best DNA crystallography data was a young woman Roselyn 219 00:16:28,000 --> 00:16:33,000 Franklin who was working in London. A very somewhat uneasy alliance with 220 00:16:33,000 --> 00:16:37,000 Maurice Wilkins. And in trying to read the history 221 00:16:37,000 --> 00:16:41,000 it's a bit complicated because, at least some of what I've read, 222 00:16:41,000 --> 00:16:46,000 I think that when Roslyn Franklin arrived at the lab she was told this 223 00:16:46,000 --> 00:16:50,000 DNA structure problem was hers. And Maurice Wilkins in whose lab 224 00:16:50,000 --> 00:16:54,000 she was working was told that he was sort of working for her. 225 00:16:54,000 --> 00:16:59,000 So there was a bunch of confusion in this. 226 00:16:59,000 --> 00:17:03,000 But, in any case, Roslyn Franklin was collecting 227 00:17:03,000 --> 00:17:08,000 crystallographic data. And Watson and Crick located some 228 00:17:08,000 --> 00:17:13,000 distance away in Cambridge were trying to come up with models that 229 00:17:13,000 --> 00:17:18,000 could explain the structure of DNA. And they learned about Roslyn's 230 00:17:18,000 --> 00:17:22,000 data. And it was here data that they used to work out the basis, 231 00:17:22,000 --> 00:17:27,000 her crystallographic data that they used when they put together 232 00:17:27,000 --> 00:17:32,000 their structure. So if it hadn't been for her they 233 00:17:32,000 --> 00:17:36,000 wouldn't have been able to make their discovery. 234 00:17:36,000 --> 00:17:40,000 So part of the reason I'm dwelling on this is I think their discovery 235 00:17:40,000 --> 00:17:44,000 of the structure of DNA was arguably one of the great intellectual 236 00:17:44,000 --> 00:17:48,000 advances of our time. It just opened doors. 237 00:17:48,000 --> 00:17:52,000 The whole field of molecular biology became possible once people 238 00:17:52,000 --> 00:17:56,000 suddenly saw that DNA was complimentary strands. 239 00:17:56,000 --> 00:18:00,000 You could almost immediately see how you could copy genetic 240 00:18:00,000 --> 00:18:03,000 information. It laid the groundwork for what 241 00:18:03,000 --> 00:18:07,000 later turned out to be, you know, recombinant DNA and 242 00:18:07,000 --> 00:18:11,000 everything else. So much of this pivots around this 243 00:18:11,000 --> 00:18:15,000 one discovery. And I think I wouldn't be doing 244 00:18:15,000 --> 00:18:19,000 justice to this finding, which you all have heard about for 245 00:18:19,000 --> 00:18:23,000 years and years, if I had let you walk away from here 246 00:18:23,000 --> 00:18:27,000 thinking this was too young geniuses who sat down in a room with some 247 00:18:27,000 --> 00:18:31,000 crystallographic data and emerged with a structure that sort of 248 00:18:31,000 --> 00:18:35,000 changed the course of the study of biology. 249 00:18:35,000 --> 00:18:38,000 And, as you can see, changes our society and everything 250 00:18:38,000 --> 00:18:42,000 else. There are a couple of accounts of this, 251 00:18:42,000 --> 00:18:45,000 there are numerous accounts. One that I found pretty interesting 252 00:18:45,000 --> 00:18:49,000 is called ìThe Eighth Day of Creation,î if you ever want to read 253 00:18:49,000 --> 00:18:52,000 an interesting book on science. This was Horace Judson's effort to 254 00:18:52,000 --> 00:18:56,000 try and put together a history of this happening. 255 00:18:56,000 --> 00:19:00,000 And with all history he's ultimately -- 256 00:19:00,000 --> 00:19:03,000 You know, there are some judgment calls by the historian, 257 00:19:03,000 --> 00:19:07,000 but this one certainly he tried to be pretty fair-handed and 258 00:19:07,000 --> 00:19:11,000 even-handed and he tried to get at the heart of what was going on. 259 00:19:11,000 --> 00:19:14,000 Watson wrote a book called ìThe Double Helixî. 260 00:19:14,000 --> 00:19:18,000 Jim Watson's a very colorful character, quite brash particularly 261 00:19:18,000 --> 00:19:22,000 when he was younger, and that's reflected in this book. 262 00:19:22,000 --> 00:19:26,000 It's an interesting read. Probably more balanced point of view for sure 263 00:19:26,000 --> 00:19:30,000 in ìThe Eighth Day of Creationî. And there are now a lot of other 264 00:19:30,000 --> 00:19:34,000 books. But what I did, just to try and do this in about a 265 00:19:34,000 --> 00:19:38,000 minute or two, was I took a couple of the key 266 00:19:38,000 --> 00:19:42,000 things that happened during their adventure of trying to work out the 267 00:19:42,000 --> 00:19:47,000 structure of DNA and just kind of ran some of their missteps together, 268 00:19:47,000 --> 00:19:51,000 because even though this was a marvelous discovery it just didn't 269 00:19:51,000 --> 00:19:55,000 happen. So they started out, they were inspired by Linus 270 00:19:55,000 --> 00:20:00,000 Pauling's discovery of the alpha helix. 271 00:20:00,000 --> 00:20:04,000 And I don't know if you can remember the story, but what Pauling decided 272 00:20:04,000 --> 00:20:08,000 to do when he was lying in bed and with a strip of paper trying to work 273 00:20:08,000 --> 00:20:12,000 out the structure that was giving these reflections in the crystal 274 00:20:12,000 --> 00:20:16,000 structure, he said I'm going to start by ignoring the side chains. 275 00:20:16,000 --> 00:20:20,000 So that was a brilliant move in the case of the alpha helix because he 276 00:20:20,000 --> 00:20:24,000 was then able to figure out that that hydrogen bond between the 277 00:20:24,000 --> 00:20:28,000 carbonyl and the amino group, you could see how if you got helix 278 00:20:28,000 --> 00:20:32,000 going it would repeat at exactly the way that would give the reflections 279 00:20:32,000 --> 00:20:36,000 that were observed in the crystallography. 280 00:20:36,000 --> 00:20:40,000 So that was how Watson and Crick sort of did it. 281 00:20:40,000 --> 00:20:44,000 Linus Pauling had shown the way. So they decided they would ignore 282 00:20:44,000 --> 00:20:48,000 the side chains of DNA. So they started out by saying we 283 00:20:48,000 --> 00:20:52,000 won't consider the ATs, the Gs and the Cs. Well, 284 00:20:52,000 --> 00:20:56,000 given what you know about the structure of DNA that was not a 285 00:20:56,000 --> 00:21:01,000 helpful move in trying to work out the structure of DNA. 286 00:21:01,000 --> 00:21:05,000 Another thing, for example, that happened was that 287 00:21:05,000 --> 00:21:09,000 Jim Watson has no lack of self-confidence. 288 00:21:09,000 --> 00:21:13,000 And so it turned out when he went to hear scientific talks he didn't 289 00:21:13,000 --> 00:21:18,000 take notes. And so he went to hear a talk on x-ray crystallography 290 00:21:18,000 --> 00:21:22,000 given by Roslyn Franklin, but he didn't quite remember the 291 00:21:22,000 --> 00:21:26,000 numbers right. He got the facts a little jumbled, 292 00:21:26,000 --> 00:21:30,000 and he and Francis spent a while trying to design models to data that 293 00:21:30,000 --> 00:21:35,000 wasn't the right data. It was just not quite remembered 294 00:21:35,000 --> 00:21:41,000 right, so there was kind of an inefficiency there. 295 00:21:41,000 --> 00:21:46,000 And then Jim had a bias almost to the end that the phosphate backbones 296 00:21:46,000 --> 00:21:51,000 they knew would somehow be on the inside and the bases would be on the 297 00:21:51,000 --> 00:21:57,000 outside of the structure. So if that's your sort of starting 298 00:21:57,000 --> 00:22:02,000 place then it's sort of hard. So Watson, excuse me, 299 00:22:02,000 --> 00:22:07,000 Francis Crick was beginning to suspect that maybe the bases were 300 00:22:07,000 --> 00:22:12,000 important. So he hired a young mathematician. 301 00:22:12,000 --> 00:22:16,000 And he said, ìCan you see if you could work out whether there would 302 00:22:16,000 --> 00:22:21,000 be any chemical attraction between any pairs of bases? 303 00:22:21,000 --> 00:22:26,000 And the young mathematician came back and said that he thought G 304 00:22:26,000 --> 00:22:31,000 might go with C and A with T. And given what happened here you 305 00:22:31,000 --> 00:22:36,000 might have thought that a light bulb would have gone off, 306 00:22:36,000 --> 00:22:41,000 but it didn't. And, in fact, Chargaff visited them and the light 307 00:22:41,000 --> 00:22:46,000 bulb went off for nobody. And, in fact, Chargaff wasn't a 308 00:22:46,000 --> 00:22:51,000 terribly big fan of what Watson and Crick were trying to do. 309 00:22:51,000 --> 00:22:56,000 So the pieces are piling up but still not there. 310 00:22:56,000 --> 00:23:01,000 Then a big experimental advance came from Roslyn Franklin. 311 00:23:01,000 --> 00:23:05,000 And that was she discovered that the DNA that they had been diffracting 312 00:23:05,000 --> 00:23:10,000 was actually a mixture of two forms. So there were actually two 313 00:23:10,000 --> 00:23:15,000 structures in the mix that were contributing to the diffractions. 314 00:23:15,000 --> 00:23:20,000 She was able to separate out the two kinds of DNA, 315 00:23:20,000 --> 00:23:25,000 DNA-A and DNA-B she called it. And so now this gave a much clearer 316 00:23:25,000 --> 00:23:30,000 diffraction pattern, and that's the diffraction pattern 317 00:23:30,000 --> 00:23:35,000 that she saw. And Watson and Crick managed to get 318 00:23:35,000 --> 00:23:41,000 a look at this data. And it's a little complicated how 319 00:23:41,000 --> 00:23:46,000 that happened, but Crick realized almost right away 320 00:23:46,000 --> 00:23:51,000 that there were two strands running in opposite directions. 321 00:23:51,000 --> 00:23:57,000 So he know knew it was 5 to 3 in one direction and 5 to 3 in the 322 00:23:57,000 --> 00:24:02,000 other direction like that. So you might have thought they were 323 00:24:02,000 --> 00:24:07,000 home-free, but no. Jim Watson immediately built a 324 00:24:07,000 --> 00:24:12,000 model that paired like with like, A with A, T with T, G with G. They 325 00:24:12,000 --> 00:24:16,000 wrote it up and they were ready to submit the paper. 326 00:24:16,000 --> 00:24:21,000 And they gave a presentation to their colleagues at the lab in 327 00:24:21,000 --> 00:24:26,000 Cambridge. And they were shot down. And one of the key things was they 328 00:24:26,000 --> 00:24:31,000 learned the chemical fact that most of the textbooks were wrong at that 329 00:24:31,000 --> 00:24:36,000 time in the way that they depicted the structure of guanine. 330 00:24:36,000 --> 00:24:46,000 If you look in your textbook, 331 00:24:46,000 --> 00:24:58,000 excuse me, here. 332 00:24:58,000 --> 00:25:03,000 So if you were to look in a textbook today you'd see guanine like this, 333 00:25:03,000 --> 00:25:09,000 but there is another way you could draw this. 334 00:25:09,000 --> 00:25:23,000 So this you may remember when we 335 00:25:23,000 --> 00:25:30,000 were talking about phosphoenolpyruvate that this is an 336 00:25:30,000 --> 00:25:36,000 enol form and this is a keto form. And this is the way most of the 337 00:25:36,000 --> 00:25:40,000 textbooks were showing guanine at the time. So they were looking at 338 00:25:40,000 --> 00:25:44,000 the structure of guanine in textbooks. And if you were trying 339 00:25:44,000 --> 00:25:48,000 to work out schemes for putting bases together you can see what's 340 00:25:48,000 --> 00:25:52,000 going on up here would be very different. And if we have a 341 00:25:52,000 --> 00:25:56,000 hydrogen here versus if we have an oxygen, if you're trying to say make 342 00:25:56,000 --> 00:26:00,000 hydrogen bonds at that particular position, I think all of you 343 00:26:00,000 --> 00:26:04,000 understand hydrogen bonds well enough to see how that 344 00:26:04,000 --> 00:26:09,000 would throw you off. So once that insight came, 345 00:26:09,000 --> 00:26:13,000 once they learned that then the rest of the structure came pretty fast. 346 00:26:13,000 --> 00:26:18,000 And there's a movie about this. One of the nice things in it was 347 00:26:18,000 --> 00:26:22,000 sort of trying to recreate the experience where I think it was 348 00:26:22,000 --> 00:26:26,000 Watson who was shuffling these base pairs around. And he suddenly 349 00:26:26,000 --> 00:26:31,000 realized that you could set up base pairs with A and T and with G and C, 350 00:26:31,000 --> 00:26:35,000 and when you looked at them you could see they were geometrically 351 00:26:35,000 --> 00:26:40,000 exactly the same shape. You could just take the shape of the 352 00:26:40,000 --> 00:26:44,000 G and C pair and lay it right down on the A and T pair. 353 00:26:44,000 --> 00:26:49,000 And then you could see how you could build either a G-C or an A-T 354 00:26:49,000 --> 00:26:53,000 pair into the repeating structure of this DNA and it would be compatible. 355 00:26:53,000 --> 00:26:57,000 So they built a model and they thought, we can just hit the lights 356 00:26:57,000 --> 00:27:05,000 for a second here maybe. 357 00:27:05,000 --> 00:27:08,000 I just want you to see what that first model looked like. 358 00:27:08,000 --> 00:27:12,000 It looks like something you could hack together in a chemistry lab. 359 00:27:12,000 --> 00:27:16,000 They had the bases cut out of metal. And you can see just, 360 00:27:16,000 --> 00:27:20,000 you know, here the retort sort of stands using chemistry and various 361 00:27:20,000 --> 00:27:24,000 clamps that you would use for clamping a flask or something if 362 00:27:24,000 --> 00:27:28,000 you're doing a chemical lab. That's the stuff that they were 363 00:27:28,000 --> 00:27:32,000 using to put the model together. And they published then a paper in 364 00:27:32,000 --> 00:27:38,000 Nature that told about this result. That's the entire paper reporting 365 00:27:38,000 --> 00:27:43,000 the structure of DNA. And maybe you can see there's a 366 00:27:43,000 --> 00:27:49,000 little hand-drawn double helix right there that captures the elements. 367 00:27:49,000 --> 00:27:54,000 That is the paper, and that was in the journal Nature. 368 00:27:54,000 --> 00:28:00,000 And it had in it, right near the end, one of the coyest sentences in 369 00:28:00,000 --> 00:28:05,000 the scientific literature. They didn't want to go into all the 370 00:28:05,000 --> 00:28:10,000 details that if you had an A paired with G and G paired with a C and you 371 00:28:10,000 --> 00:28:16,000 pulled them apart then you could replicate the molecule by redoing it. 372 00:28:16,000 --> 00:28:21,000 So all they said was, ìIt has not escaped our notice that 373 00:28:21,000 --> 00:28:26,000 the specific pairings we expostulated immediately suggests a 374 00:28:26,000 --> 00:28:32,000 copying mechanism for DNA. So this is a picture of Jim Watson 375 00:28:32,000 --> 00:28:37,000 wearing short pants at Cold Spring Harbor in 1953 reporting 376 00:28:37,000 --> 00:28:42,000 this structure of DNA. Cold Spring Harbor is on Long Island. 377 00:28:42,000 --> 00:28:47,000 It's been one of the Meccas for molecule biology since the 1940s. 378 00:28:47,000 --> 00:28:52,000 They have a famous symposium once a year. The topic changes every year 379 00:28:52,000 --> 00:28:57,000 and rarely repeats. And it was at one of those symposia 380 00:28:57,000 --> 00:29:02,000 -- This was the year that they 381 00:29:02,000 --> 00:29:07,000 discovered the structure of DNA. And there was Watson. So two years 382 00:29:07,000 --> 00:29:12,000 ago they had another meeting, a special meeting just exactly this 383 00:29:12,000 --> 00:29:17,000 time of year. It was in February within a couple of days of right now. 384 00:29:17,000 --> 00:29:22,000 So I gave this lecture and I showed the student in the class that this 385 00:29:22,000 --> 00:29:27,000 year, I said here's a picture of Jim Watson displaying the structure. 386 00:29:27,000 --> 00:29:32,000 They're having a meeting 50 years later in 2003. 387 00:29:32,000 --> 00:29:35,000 And I'm going down there. I'm asked to give a talk. 388 00:29:35,000 --> 00:29:38,000 And I'll come back and I'll tell you what it was like. 389 00:29:38,000 --> 00:29:41,000 So I gave my lecture. I dashed out to the airport. 390 00:29:41,000 --> 00:29:45,000 I hoped on the plane. I went down and I registered. 391 00:29:45,000 --> 00:29:48,000 They gave me, you know, the stuff to get into my room, 392 00:29:48,000 --> 00:29:51,000 a little envelope with the key card and things. And I went up to my 393 00:29:51,000 --> 00:29:55,000 room. And I took out the key card. And what did I find myself looking 394 00:29:55,000 --> 00:29:58,000 at? The same picture I had shown to the class just a couple 395 00:29:58,000 --> 00:30:01,000 of hours earlier. Here's another picture of Jim the 396 00:30:01,000 --> 00:30:05,000 way he looked at the time when he made this amazing discovery. 397 00:30:05,000 --> 00:30:09,000 That's Salvador Luria who I mentioned. I tell you about him in 398 00:30:09,000 --> 00:30:12,000 a subsequent lecture. I was at another meeting a few 399 00:30:12,000 --> 00:30:16,000 years earlier where some of the old-timers were razzing each other, 400 00:30:16,000 --> 00:30:20,000 and someone showed this picture. And then they got up and they gave 401 00:30:20,000 --> 00:30:23,000 it a title. And that was ìPicture of a Man Picking His Own Pocketsî. 402 00:30:23,000 --> 00:30:27,000 So they would tease each other a lot. And I'm hoping maybe you'll get a 403 00:30:27,000 --> 00:30:31,000 chance to hear a little bit more about that soon. 404 00:30:31,000 --> 00:30:35,000 This is what Jim Watson looks like now. I asked to get a picture taken 405 00:30:35,000 --> 00:30:39,000 just so you could see he's still around and is very active and still 406 00:30:39,000 --> 00:30:43,000 very controversial. This doesn't make much of a 407 00:30:43,000 --> 00:30:48,000 difference. Here's a picture of Watson and Crick a little bit later 408 00:30:48,000 --> 00:30:52,000 just sitting out on a porch in Cold Spring Harbor. 409 00:30:52,000 --> 00:30:56,000 It's sort of right on the edge of a bay down there in a very relaxed 410 00:30:56,000 --> 00:31:00,000 kind of atmosphere that still permeates molecule biology 411 00:31:00,000 --> 00:31:04,000 research to this day. Francis Crick just died last July at 412 00:31:04,000 --> 00:31:06,000 the age of 88, so we've just lost the link to one 413 00:31:06,000 --> 00:31:09,000 of the two people who did this amazing experiment. 414 00:31:09,000 --> 00:31:11,000 OK. So I want to then set things up for the details of DNA 415 00:31:11,000 --> 00:31:14,000 replication. So there was a basic principle that came across from this 416 00:31:14,000 --> 00:31:16,000 that you could see how this could work, that DNA was sort of like 417 00:31:16,000 --> 00:31:18,000 having a photograph and a negative. And so the information is actually 418 00:31:18,000 --> 00:31:21,000 in there twice. It's just in different forms. 419 00:31:21,000 --> 00:31:23,000 And when I tell you about DNA repair in another lecture you can 420 00:31:23,000 --> 00:31:26,000 maybe see already how useful that is because if you damage one strand 421 00:31:26,000 --> 00:31:28,000 you're not really out of luck because you've still got the 422 00:31:28,000 --> 00:31:33,000 information in the other strand. And you could probably, 423 00:31:33,000 --> 00:31:41,000 on the basis of that, device a repair strategy if you thought about 424 00:31:41,000 --> 00:31:48,000 it. But more importantly for DNA replication finally gave an insight 425 00:31:48,000 --> 00:31:55,000 to this thing that had been vexing people forever. 426 00:31:55,000 --> 00:32:03,000 If you had to have all this information for making a cell, 427 00:32:03,000 --> 00:32:10,000 and every time a cell divided and you saw how it can happen pretty 428 00:32:10,000 --> 00:32:17,000 quickly with something like a bacterium of yeast, 429 00:32:17,000 --> 00:32:25,000 how could you accurately copy all that DNA, excuse me, 430 00:32:25,000 --> 00:32:29,000 all that genetic information? How is it stored? 431 00:32:29,000 --> 00:32:29,000 How could it be done? And once you saw ah, it's just a matter of separating the strands, and if there's an A there put a T there, if there's a C you put a G and so on, was a huge breakthrough. But that then didn't tell people how DNA replicated or even if this 432 00:32:30,000 --> 00:32:30,000 is the mechanism. You can actually come up with all kinds of models for how you could replicate things based on this principle, including crisscrossing between strands and all sorts of things. The predominant model and perhaps 433 00:32:43,000 --> 00:33:07,000 the simplest one was called semi-conservative. 434 00:33:07,000 --> 00:33:15,000 And it thought of the problem in 435 00:33:15,000 --> 00:33:21,000 this kind of way, that if you had two strands of the 436 00:33:21,000 --> 00:33:26,000 original DNA molecule and then you pulled them apart that one of the 437 00:33:26,000 --> 00:33:32,000 strands here would become one of the strands of the daughter, 438 00:33:32,000 --> 00:33:38,000 and then the new one would be here and the same thing would happen on 439 00:33:38,000 --> 00:33:44,000 the other side. And then if you did it again this 440 00:33:44,000 --> 00:33:50,000 thing would happen again with a new strand. This time the skinny strand 441 00:33:50,000 --> 00:33:56,000 here would be like this, the skinny strand here would be like 442 00:33:56,000 --> 00:34:02,000 this, and then this one again. We'd have one that was nearly 443 00:34:02,000 --> 00:34:08,000 synthesized plus one of the originals. So this model was one of 444 00:34:08,000 --> 00:34:14,000 the simplest because it kept this strand intact throughout the whole 445 00:34:14,000 --> 00:34:19,000 process while some of the other models had them being patched back 446 00:34:19,000 --> 00:34:25,000 together, all based on the idea that A pairs with T and G pairs with C. 447 00:34:25,000 --> 00:34:31,000 But proving that this was the correct model was then another 448 00:34:31,000 --> 00:34:36,000 important advance. And that was done by Frank Stahl and 449 00:34:36,000 --> 00:34:40,000 Matt Meselson. Actually, I think I'll skip this 450 00:34:40,000 --> 00:34:44,000 for right now. Matt is a professor up at Harvard, 451 00:34:44,000 --> 00:34:48,000 just up at Harvard Square not very far from here, 452 00:34:48,000 --> 00:34:52,000 still very active. Frank Stahl is a professor out in 453 00:34:52,000 --> 00:34:56,000 Oregon. He's still active. So one of the differences about 454 00:34:56,000 --> 00:35:01,000 this course is a lot of the things I'm telling you about -- 455 00:35:01,000 --> 00:35:06,000 And this is pretty old stuff right now, right, molecule biology. 456 00:35:06,000 --> 00:35:11,000 The people who did these are still around and very active. 457 00:35:11,000 --> 00:35:16,000 This is most of modern biology is a pretty young scientist, 458 00:35:16,000 --> 00:35:22,000 and many of the major characters are still running around and with us 459 00:35:22,000 --> 00:35:27,000 today. So, anyway, what Matt and Frank were at Caltech. 460 00:35:27,000 --> 00:35:32,000 And they with a bunch of other 461 00:35:32,000 --> 00:35:37,000 students had an apartment. And they were sitting around trying 462 00:35:37,000 --> 00:35:42,000 to work out a way to figure out this model. And they came up with an 463 00:35:42,000 --> 00:35:47,000 idea, and that was to see if you could differentially label what we 464 00:35:47,000 --> 00:35:52,000 might call ìold DNAî and the ìnew DNAî here. And since it's 465 00:35:52,000 --> 00:35:57,000 chemically the same stuff it's a bit of a trick. How do you tell old DNA 466 00:35:57,000 --> 00:36:03,000 from new DNA? So their idea was since nitrogen 467 00:36:03,000 --> 00:36:09,000 comes in two different isotopes, N14 which is the common one and N15 468 00:36:09,000 --> 00:36:16,000 with is one mass heavier, that maybe you could start out with 469 00:36:16,000 --> 00:36:22,000 the DNA, for example, grown in N15. And then when you 470 00:36:22,000 --> 00:36:28,000 started replication switch to N14. And then you'd be able to tell, 471 00:36:28,000 --> 00:36:32,000 if you could separate these molecules on the basis of their 472 00:36:32,000 --> 00:36:36,000 density since the one with the N15 would be heavier than the one with 473 00:36:36,000 --> 00:36:41,000 the N14, then maybe you could work this out. And the story goes, 474 00:36:41,000 --> 00:36:45,000 this has been written, they were sitting arguing about this, 475 00:36:45,000 --> 00:36:50,000 or talking about this idea at the table. And it was a good idea but 476 00:36:50,000 --> 00:36:54,000 there was a problem. And that was how could you separate 477 00:36:54,000 --> 00:36:59,000 the two kinds of DNA based on their density? 478 00:36:59,000 --> 00:37:02,000 So they had a piece of fingernail and they were trying to see whether 479 00:37:02,000 --> 00:37:06,000 they could get it to float by dissolving more and more sugar. 480 00:37:06,000 --> 00:37:09,000 And they figured if they added more and more sugar the water would get 481 00:37:09,000 --> 00:37:13,000 denser and denser so the could float the fingernail. 482 00:37:13,000 --> 00:37:16,000 And they weren't able to do it. But all chemists made a periodic, 483 00:37:16,000 --> 00:37:20,000 probably some places here at MIT, they had a periodic chart right in 484 00:37:20,000 --> 00:37:23,000 their living room. So they went and they looked. 485 00:37:23,000 --> 00:37:27,000 And then they looked at sodium. And they went down the periodic 486 00:37:27,000 --> 00:37:31,000 table and then they saw cesium. And thought maybe, 487 00:37:31,000 --> 00:37:35,000 you know, if you took a solution of cesium chloride and you put it a 488 00:37:35,000 --> 00:37:39,000 centrifuge and you spun really hard then you'd get a gradient of varying 489 00:37:39,000 --> 00:37:43,000 concentrations, of slightly different concentrations 490 00:37:43,000 --> 00:37:47,000 of cesium chloride. And that they could tune that to a 491 00:37:47,000 --> 00:37:51,000 range that would discriminate between the heavy and the lighter 492 00:37:51,000 --> 00:37:55,000 forms of DNA. So the experiment they did is known as the 493 00:37:55,000 --> 00:37:59,000 Meselson-Stahl experiment. But, as I say, these are names that 494 00:37:59,000 --> 00:38:05,000 come from real people. And the idea was pretty simple. 495 00:38:05,000 --> 00:38:13,000 They grew the bacteria for many generations -- 496 00:38:13,000 --> 00:38:22,000 -- in N15 medium. 497 00:38:22,000 --> 00:38:30,000 This is the so-called heavy 498 00:38:30,000 --> 00:38:37,000 or H isotope -- 499 00:38:37,000 --> 00:38:45,000 -- of nitrogen. And then that time equals zero in 500 00:38:45,000 --> 00:38:53,000 their experiment, when they're ready to start the 501 00:38:53,000 --> 00:39:01,000 experiment they switched to medium with N14, which we'll think of as 502 00:39:01,000 --> 00:39:09,000 the light or the L isotope. And then they isolated DNA -- 503 00:39:09,000 --> 00:39:21,000 -- after let's say increasing rounds 504 00:39:21,000 --> 00:39:31,000 of replication that you could tell simply by measuring how much DNA was 505 00:39:31,000 --> 00:39:41,000 in your bacterial culture when the bacteria had doubled their DNA. 506 00:39:41,000 --> 00:39:47,000 And this is the data they got which looks something like this. 507 00:39:47,000 --> 00:39:53,000 In fact, in this case the blackboard representation is pretty 508 00:39:53,000 --> 00:39:59,000 close. So this is cesium chloride. And it has been centrifuged very 509 00:39:59,000 --> 00:40:05,000 hard so that there's a gradient now that's light at the top and a little 510 00:40:05,000 --> 00:40:11,000 heavier at the bottom of the gradient. 511 00:40:11,000 --> 00:40:14,000 There's a little more cesium chloride per mil here then there is 512 00:40:14,000 --> 00:40:18,000 there in the tube. And I'll just give us three little 513 00:40:18,000 --> 00:40:22,000 sort of reference marks here. So what they found when they 514 00:40:22,000 --> 00:40:26,000 started was that all of the DNA was at that position down 515 00:40:26,000 --> 00:40:32,000 at the heavy end. And then this is after one 516 00:40:32,000 --> 00:40:40,000 generation. So the DNA has now doubled. What they found was that 517 00:40:40,000 --> 00:40:49,000 all the DNA was now at this intermediate position. 518 00:40:49,000 --> 00:40:58,000 And after two generations or two DNA 519 00:40:58,000 --> 00:41:04,000 replications they now found that some of the DNA was here, 520 00:41:04,000 --> 00:41:10,000 some of the DNA was there. And if they went to three or more 521 00:41:10,000 --> 00:41:16,000 what they saw was they began to pile stuff up there. 522 00:41:16,000 --> 00:41:21,000 And I think most of you could probably make the connection between 523 00:41:21,000 --> 00:41:27,000 that data and that picture that I've got up there. This is 524 00:41:27,000 --> 00:41:32,000 the heavy-heavy DNA. This is the heavy-light. 525 00:41:32,000 --> 00:41:38,000 So this would be heavy-heavy, heavy-light, light-heavy. After one 526 00:41:38,000 --> 00:41:44,000 round it will all be here. After two we have heavy-light, 527 00:41:44,000 --> 00:41:50,000 but this one is light-light, light-light, light-heavy. 528 00:41:50,000 --> 00:41:56,000 And so now we've got light-light, the heavy-light, no heavy-heavy is 529 00:41:56,000 --> 00:42:01,000 ever going to show up again. And the longer you do this the more 530 00:42:01,000 --> 00:42:06,000 you'll get the light accumulating. A very simple experiment done by 531 00:42:06,000 --> 00:42:11,000 real people but enormously powerful because now it showed that this 532 00:42:11,000 --> 00:42:15,000 basic idea, you have the photograph and negative, you pull them apart 533 00:42:15,000 --> 00:42:20,000 and copy them was right. So at this point you begin to see 534 00:42:20,000 --> 00:42:25,000 why data of Avery's that before people had trouble accepting, 535 00:42:25,000 --> 00:42:30,000 all of a sudden now it was really you needed a CYD and A was 536 00:42:30,000 --> 00:42:34,000 the genetic material. And this is what sort of ushered in 537 00:42:34,000 --> 00:42:38,000 this great burst of molecular biology. So in the next lecture 538 00:42:38,000 --> 00:42:42,000 what we're going to start doing now is once you, this is all great, 539 00:42:42,000 --> 00:42:45,000 but once we start figuring out how to replicate it we're going to have 540 00:42:45,000 --> 00:42:49,000 to get down to enzymes and biochemical steps. 541 00:42:49,000 --> 00:42:52,000 And there are some formidable challenges to replicating DNA, 542 00:42:52,000 --> 00:42:56,000 and it's also awesome. I'll tell you at the beginning of next lecture 543 00:42:56,000 --> 00:43:00,000 how much DNA we have and just how accurate it is. 544 00:43:00,000 --> 00:43:03,000 It always blows me away. I'll see you then. Take care.