WEBVTT 00:00:06.370 --> 00:00:12.480 ERIC LANDER: And so the issue became how does DNA 00:00:12.480 --> 00:00:13.875 replication work. 00:00:17.370 --> 00:00:18.650 And so I'm about to go into it. 00:00:18.650 --> 00:00:25.130 Now, I'm going to note we're going to be starting this DNA 00:00:25.130 --> 00:00:35.790 goes to RNA, goes to protein, and DNA goes to itself. 00:00:35.790 --> 00:00:38.100 DNA is replicated. 00:00:38.100 --> 00:00:39.450 It makes RNA. 00:00:39.450 --> 00:00:41.710 The RNA is used to make protein. 00:00:41.710 --> 00:00:42.780 This will be what we'll be talking 00:00:42.780 --> 00:00:44.700 about today and tomorrow. 00:00:44.700 --> 00:00:47.640 So the first step of that is, how does DNA give 00:00:47.640 --> 00:00:50.370 rise to more DNA? 00:00:50.370 --> 00:00:53.330 Well, how do you find an enzyme? 00:00:53.330 --> 00:00:54.580 How do you do biochemistry? 00:00:56.950 --> 00:00:57.540 What do you do? 00:00:57.540 --> 00:00:58.890 AUDIENCE: Assays. 00:00:58.890 --> 00:00:59.350 ERIC LANDER: Assay. 00:00:59.350 --> 00:01:00.730 So you've got to grind up the cell. 00:01:00.730 --> 00:01:02.730 I got to choose a cell in which I'm likely to find an 00:01:02.730 --> 00:01:05.830 enzyme, grind it up, break it up into different fractions, 00:01:05.830 --> 00:01:07.200 and test each fraction. 00:01:07.200 --> 00:01:08.930 That's all biochemists do, right? 00:01:08.930 --> 00:01:13.950 So what cell might have the enzyme we're looking for? 00:01:13.950 --> 00:01:15.675 What cells might be able to copy DNA? 00:01:18.642 --> 00:01:20.400 How about all cells? 00:01:20.400 --> 00:01:21.570 So let's use a simple cell. 00:01:21.570 --> 00:01:24.180 What's a simple cell? 00:01:24.180 --> 00:01:25.330 Let's use bacteria. 00:01:25.330 --> 00:01:27.000 So we'll take some bacteria, we'll grow it up, 00:01:27.000 --> 00:01:27.920 we'll grind it up. 00:01:27.920 --> 00:01:30.130 We'll fractionate it into different fractions, and we'll 00:01:30.130 --> 00:01:33.810 see if one of those fractions has the ability to copy DNA. 00:01:33.810 --> 00:01:35.150 If we're going to run an assay, we have 00:01:35.150 --> 00:01:36.880 to give it a substrate. 00:01:36.880 --> 00:01:40.610 What substrate would you like to give it? 00:01:40.610 --> 00:01:41.760 What do you think it needs? 00:01:41.760 --> 00:01:43.010 AUDIENCE: [INAUDIBLE]. 00:01:45.450 --> 00:01:47.380 ERIC LANDER: It better have some free nucleotides 00:01:47.380 --> 00:01:48.980 otherwise, how are we going to make DNA. 00:01:48.980 --> 00:01:51.010 What else? 00:01:51.010 --> 00:01:53.490 Are you going to ask it to make DNA all by itself? 00:01:53.490 --> 00:01:56.110 We want something that can copy one of the strands of a 00:01:56.110 --> 00:01:57.310 double helix. 00:01:57.310 --> 00:01:58.140 So what should we give it? 00:01:58.140 --> 00:02:00.240 AUDIENCE: [INAUDIBLE]. 00:02:00.240 --> 00:02:01.150 ERIC LANDER: Sorry? 00:02:01.150 --> 00:02:02.020 AUDIENCE: Half a helix. 00:02:02.020 --> 00:02:02.850 ERIC LANDER: Half a helix. 00:02:02.850 --> 00:02:06.480 A strand of DNA, the strand to be used as a template. 00:02:06.480 --> 00:02:07.965 So let's give it a template strand. 00:02:11.880 --> 00:02:14.860 So we'll take a template strand of DNA. 00:02:14.860 --> 00:02:16.390 There's my template of DNA. 00:02:19.600 --> 00:02:22.810 Let's actually give it a little 00:02:22.810 --> 00:02:24.980 sequence actually, here. 00:02:24.980 --> 00:02:34.330 Let's say A phosphate, T phosphate, G phosphate, C 00:02:34.330 --> 00:02:41.480 phosphate, A phosphate, T phosphate, T phosphate, A 00:02:41.480 --> 00:02:46.460 phosphate, G phosphate, G phosphate. 00:02:46.460 --> 00:02:48.810 I'm going to not write the phosphates too much longer, 00:02:48.810 --> 00:02:51.760 guys, but anyway C phosphate, C phosphate, T 00:02:51.760 --> 00:02:55.060 phosphate, like that. 00:02:55.060 --> 00:02:57.410 Pretty soon, in fact, almost immediately, I'm going to 00:02:57.410 --> 00:02:59.820 start dropping the phosphates in here. 00:02:59.820 --> 00:03:01.930 But that's the way it goes. 00:03:01.930 --> 00:03:04.610 All right. 00:03:04.610 --> 00:03:05.860 That's a template. 00:03:08.640 --> 00:03:14.845 We need floating around in the solution some trinucleotides. 00:03:25.960 --> 00:03:33.230 We have some nucleotides floating around. 00:03:36.160 --> 00:03:39.330 And now will this enzyme work? 00:03:39.330 --> 00:03:41.540 We would try different fractions and see if it's able 00:03:41.540 --> 00:03:45.940 to just install the right letters in the right place. 00:03:45.940 --> 00:03:50.620 Now, it turned out it needed one more thing, and the person 00:03:50.620 --> 00:03:53.370 who discovered this, Arthur Kornberg, thought of it. 00:03:53.370 --> 00:03:54.760 It needed a head start. 00:03:54.760 --> 00:03:56.010 It needed a primer. 00:03:59.450 --> 00:04:05.690 So the primer goes let's say, phosphate T, phosphate A, 00:04:05.690 --> 00:04:16.670 phosphate C, phosphate G, phosphate T, phosphate A, 00:04:16.670 --> 00:04:18.399 let's say like that. 00:04:18.399 --> 00:04:23.950 So this is the five prime end of DNA. 00:04:23.950 --> 00:04:26.490 Remember the phosphate is hanging off the five prime 00:04:26.490 --> 00:04:27.940 carbon, right? 00:04:27.940 --> 00:04:29.400 What's look at the other end. 00:04:29.400 --> 00:04:34.640 The other end ends in the hydroxyl on the three prime 00:04:34.640 --> 00:04:37.790 end of the ribose. 00:04:37.790 --> 00:04:42.010 Since this is anti-parallel, this strand is going five 00:04:42.010 --> 00:04:48.090 prime phosphate to three prime hydroxyl. 00:04:48.090 --> 00:04:50.540 You're going to need to know five prime and three prime. 00:04:50.540 --> 00:04:52.470 So I'm doing this so you get used to five 00:04:52.470 --> 00:04:53.880 prime and three prime. 00:04:53.880 --> 00:04:54.580 There you go. 00:04:54.580 --> 00:04:57.460 If you're handed a primer to get a head start, and you're 00:04:57.460 --> 00:05:00.460 handed a template, and you hand it some nucleotides, you 00:05:00.460 --> 00:05:03.800 then assay different fractions exactly as you suggested and 00:05:03.800 --> 00:05:09.300 we see is one of them capable of extending this strand by 00:05:09.300 --> 00:05:12.640 putting in an A, putting in a T, putting in a C, putting in 00:05:12.640 --> 00:05:17.320 a C, putting in a C, putting in a G. That's the assay. 00:05:17.320 --> 00:05:29.450 And Arthur Kornberg discovered an enzyme that could do this. 00:05:29.450 --> 00:05:31.000 And the biochemists went nuts. 00:05:31.000 --> 00:05:31.940 They thought, wow. 00:05:31.940 --> 00:05:33.520 This is so cool. 00:05:33.520 --> 00:05:36.770 Kornberg is able to discover an enzyme that 00:05:36.770 --> 00:05:39.220 can accomplish this. 00:05:39.220 --> 00:05:43.470 The enzyme polymerizes DNA. 00:05:43.470 --> 00:05:46.754 Coincidentally, what is the enzyme called? 00:05:46.754 --> 00:05:47.660 AUDIENCE: DNA polymerase. 00:05:47.660 --> 00:05:49.320 ERIC LANDER: DNA polymerase. 00:05:49.320 --> 00:05:51.130 Accidentally, has a nice name. 00:05:51.130 --> 00:05:52.310 Good. 00:05:52.310 --> 00:05:54.050 DNA polymerase. 00:05:58.590 --> 00:06:01.190 Excellent. 00:06:01.190 --> 00:06:03.640 Now, notice what it does. 00:06:03.640 --> 00:06:08.130 It takes this triphosphate, puts it in here, and it joins 00:06:08.130 --> 00:06:10.140 it into a sugar phosphate chain. 00:06:10.140 --> 00:06:12.205 Where does it get the energy for that synthesis? 00:06:14.780 --> 00:06:17.600 Hydrolysis of the triphosphates, right? 00:06:17.600 --> 00:06:19.500 It's the hydrolysis of the triphosphate. 00:06:19.500 --> 00:06:21.800 That's the energy. 00:06:21.800 --> 00:06:26.030 What direction is the synthesis proceeding? 00:06:26.030 --> 00:06:31.120 Starts here at the five prime end, and it moves adding on to 00:06:31.120 --> 00:06:33.400 the three prime end. 00:06:33.400 --> 00:06:40.880 So it's five prime to three prime direction. 00:06:40.880 --> 00:06:42.540 That's the direction it moves. 00:06:42.540 --> 00:06:44.720 It adds to the three prime end. 00:06:44.720 --> 00:06:46.370 It adds to the free nucleotides to 00:06:46.370 --> 00:06:47.620 the three prime end. 00:06:50.350 --> 00:06:52.025 Why not do it the other way? 00:06:52.025 --> 00:06:55.140 AUDIENCE: [INAUDIBLE]. 00:06:55.140 --> 00:06:56.170 ERIC LANDER: Sorry? 00:06:56.170 --> 00:06:57.370 AUDIENCE: Phosphates. 00:06:57.370 --> 00:06:58.286 ERIC LANDER: Can't hear you. 00:06:58.286 --> 00:06:58.700 Shout loud. 00:06:58.700 --> 00:06:59.586 AUDIENCE: Phosphates. 00:06:59.586 --> 00:07:02.000 ERIC LANDER: Phosphates, yes. 00:07:02.000 --> 00:07:05.075 You see, suppose we were going the other way. 00:07:08.850 --> 00:07:11.540 Suppose the primer was this way. 00:07:11.540 --> 00:07:15.600 Where would as we added each base, the triphosphate would 00:07:15.600 --> 00:07:18.590 be on the strands, right? 00:07:18.590 --> 00:07:24.150 And we'd be adding to the three prime end here. 00:07:24.150 --> 00:07:28.320 That means the energy supplied by the triphosphate would be 00:07:28.320 --> 00:07:33.240 on the growing strands rather than in the free nucleotides. 00:07:33.240 --> 00:07:37.160 Why would it be a terrible idea to put your energy source 00:07:37.160 --> 00:07:39.729 on the growing strand? 00:07:39.729 --> 00:07:42.094 MIKE: [INAUDIBLE]. 00:07:42.094 --> 00:07:44.280 ERIC LANDER: Well Mike, you know, those triphosphate bonds 00:07:44.280 --> 00:07:45.040 are pretty unstable. 00:07:45.040 --> 00:07:47.320 They hydrolyzed by themselves at some frequency. 00:07:47.320 --> 00:07:49.550 If you're a free nucleotide and the triphosphate 00:07:49.550 --> 00:07:52.010 hydrolyzes, big deal. 00:07:52.010 --> 00:07:55.900 That free nucleotide floating around loses its triphosphate. 00:07:55.900 --> 00:07:57.960 But what if I'm the growing strand, and I lose my 00:07:57.960 --> 00:07:59.115 triphosphate? 00:07:59.115 --> 00:07:59.720 AUDIENCE: [LAUGHS] 00:07:59.720 --> 00:08:00.480 ERIC LANDER: Exactly. 00:08:00.480 --> 00:08:01.540 AUDIENCE: There goes your chain. 00:08:01.540 --> 00:08:03.470 ERIC LANDER: There goes my chain. 00:08:03.470 --> 00:08:05.530 So you know, life's not stupid. 00:08:05.530 --> 00:08:06.930 It doesn't do it that way. 00:08:06.930 --> 00:08:07.870 It does it this way. 00:08:07.870 --> 00:08:10.050 No one has ever found a polymerase that goes this way. 00:08:10.050 --> 00:08:13.230 They find them all going that way for just that reason. 00:08:13.230 --> 00:08:14.660 Exactly. 00:08:14.660 --> 00:08:16.100 Bingo. 00:08:16.100 --> 00:08:19.210 That was why life evolved it that way, because you want 00:08:19.210 --> 00:08:24.190 your triphosphates, those hydrolyzable triphosphates to 00:08:24.190 --> 00:08:27.320 be floating around freely rather than investing. 00:08:27.320 --> 00:08:28.270 Now just think about that. 00:08:28.270 --> 00:08:29.140 It's a kind of cool thing. 00:08:29.140 --> 00:08:29.670 It doesn't matter. 00:08:29.670 --> 00:08:30.910 Your book doesn't talk about it. 00:08:30.910 --> 00:08:33.080 But to me, it helps me remember which way it's going 00:08:33.080 --> 00:08:35.370 and how it is, and it's kind of interesting. 00:08:35.370 --> 00:08:36.110 Any way. 00:08:36.110 --> 00:08:36.710 All right. 00:08:36.710 --> 00:08:42.440 So Kornberg wins the Nobel Prize for this. 00:08:42.440 --> 00:08:42.870 Good stuff. 00:08:42.870 --> 00:08:47.920 It's very deserved, but you know, there's some questions. 00:08:47.920 --> 00:08:52.340 Where does the primer come from in life? 00:08:52.340 --> 00:08:57.270 See, Kornberg gave this test tube a primer. 00:08:57.270 --> 00:09:00.520 But suppose I'm replicating some DNA. 00:09:00.520 --> 00:09:06.770 So let's suppose I have a double strand of DNA, and I'm 00:09:06.770 --> 00:09:13.460 just going to open it up here, five prime to three prime, 00:09:13.460 --> 00:09:18.280 five prime to three prime. 00:09:18.280 --> 00:09:20.769 I need to get like a primer here. 00:09:25.080 --> 00:09:28.830 Then the primer can be extended by polymerase. 00:09:28.830 --> 00:09:32.560 Well, where's the primer come from? 00:09:32.560 --> 00:09:35.850 It turns out there is an enzyme specially devoted to 00:09:35.850 --> 00:09:37.440 making those primers. 00:09:37.440 --> 00:09:41.080 Kornberg didn't know it, but there's an enzyme. 00:09:41.080 --> 00:09:46.420 And by coincidence, it is called primase. 00:09:46.420 --> 00:09:48.240 Exactly. 00:09:48.240 --> 00:09:50.290 Primase makes the primer. 00:09:50.290 --> 00:09:56.025 So you need a primer here, and the primer is made by primase. 00:10:00.960 --> 00:10:07.810 Once primase makes a primer, polymerase can chug along and 00:10:07.810 --> 00:10:10.340 do it just fine. 00:10:10.340 --> 00:10:11.860 Let's check out the other strand. 00:10:14.370 --> 00:10:18.390 Primer here, polymerase chugs along. 00:10:21.790 --> 00:10:25.210 But now as this double helix opens up, what 00:10:25.210 --> 00:10:26.460 happens over here? 00:10:31.380 --> 00:10:34.150 The synthesis going this way. 00:10:34.150 --> 00:10:35.670 So what do I have to do here? 00:10:35.670 --> 00:10:37.318 AUDIENCE: [INAUDIBLE]. 00:10:37.318 --> 00:10:39.232 ERIC LANDER: Another primer. 00:10:39.232 --> 00:10:40.482 Need another primer. 00:10:42.810 --> 00:10:44.510 Then as it opens up more, what do I need? 00:10:44.510 --> 00:10:45.530 AUDIENCE: Another primer. 00:10:45.530 --> 00:10:46.780 ERIC LANDER: Another primer. 00:10:49.410 --> 00:10:53.100 So the two strands are experiencing very different 00:10:53.100 --> 00:10:54.450 kind of replication. 00:10:54.450 --> 00:10:57.420 In one place, one primer in the five prime to three prime 00:10:57.420 --> 00:10:59.840 direction is enough to keep going. 00:10:59.840 --> 00:11:02.830 In the other strand, as it keeps opening up, you gotta 00:11:02.830 --> 00:11:04.510 keep making primers. 00:11:04.510 --> 00:11:06.550 You have all these little fragments there. 00:11:09.910 --> 00:11:16.520 Now, those little fragments were discovered by Okazaki, 00:11:16.520 --> 00:11:20.960 and they are called Okazaki fragments. 00:11:20.960 --> 00:11:23.260 Again, I just mention these things. 00:11:23.260 --> 00:11:25.570 They are known to molecular biologists. 00:11:25.570 --> 00:11:28.900 But these little guys are Okazaki fragments, and they 00:11:28.900 --> 00:11:31.480 tell you that you're on the right track here. 00:11:31.480 --> 00:11:33.280 This is indeed how it's working. 00:11:33.280 --> 00:11:36.010 You can see those little fragments there. 00:11:36.010 --> 00:11:40.260 But now, what's the problem with the Okazaki fragments? 00:11:40.260 --> 00:11:41.970 They're not connected, right? 00:11:41.970 --> 00:11:44.400 The primase makes a primer. 00:11:44.400 --> 00:11:50.010 The polymerase copies the DNA, it bumps into the next primer, 00:11:50.010 --> 00:11:52.690 but you've got to connect them. 00:11:52.690 --> 00:11:54.420 So that's a problem. 00:11:54.420 --> 00:11:57.250 That's a real problem. 00:11:57.250 --> 00:11:58.500 I'll redraw that here. 00:12:04.140 --> 00:12:05.160 Here was my primer. 00:12:05.160 --> 00:12:06.710 I got a new primer over here. 00:12:09.450 --> 00:12:12.680 I got a new primer over here. 00:12:12.680 --> 00:12:13.380 Right there. 00:12:13.380 --> 00:12:14.120 Right there. 00:12:14.120 --> 00:12:17.340 They're not contiguous connected. 00:12:17.340 --> 00:12:20.820 The word we use for connecting two pieces of DNA, which is a 00:12:20.820 --> 00:12:23.700 standard English word not used that often is to ligate two 00:12:23.700 --> 00:12:25.060 things together. 00:12:25.060 --> 00:12:27.960 Ligature, for example, in music. 00:12:27.960 --> 00:12:31.460 You ligate things together. 00:12:31.460 --> 00:12:33.650 How do you think the cell deals with ligating these 00:12:33.650 --> 00:12:36.230 things together? 00:12:36.230 --> 00:12:37.315 An enzyme called-- 00:12:37.315 --> 00:12:38.110 AUDIENCE: Ligase. 00:12:38.110 --> 00:12:40.390 ERIC LANDER: Exactly. 00:12:40.390 --> 00:12:44.340 So ligase does the ligation. 00:12:44.340 --> 00:12:48.070 Ligase ligates. 00:12:48.070 --> 00:12:51.950 It is so lucky that these words turn out to have 00:12:51.950 --> 00:12:53.780 accidentally made sense. 00:12:53.780 --> 00:12:56.400 It's really cool. 00:12:56.400 --> 00:12:59.950 So ligase ligates. 00:12:59.950 --> 00:13:05.000 Now, I'll tell you a factoid, but don't worry 00:13:05.000 --> 00:13:05.820 about it too much. 00:13:05.820 --> 00:13:10.360 Primase actually doesn't make DNA. 00:13:10.360 --> 00:13:11.540 We haven't gotten there yet, but it turns out 00:13:11.540 --> 00:13:14.350 primase makes RNA. 00:13:14.350 --> 00:13:16.450 Turns out to be easier to start an RNA 00:13:16.450 --> 00:13:18.740 than a DNA from scratch. 00:13:18.740 --> 00:13:20.710 Cell doesn't like to start DNA from scratch. 00:13:20.710 --> 00:13:22.650 It likes to start RNA from scratch as we'll get to a 00:13:22.650 --> 00:13:23.870 moment with transcription. 00:13:23.870 --> 00:13:26.010 So as a factoid, I'll mention to you that those little 00:13:26.010 --> 00:13:29.340 primers are actually RNA primers, and what happens is 00:13:29.340 --> 00:13:32.800 they get extended into DNA, and they bump into and kind of 00:13:32.800 --> 00:13:36.070 displace the previous RNA, so it's slightly more complicated 00:13:36.070 --> 00:13:36.890 than I told you. 00:13:36.890 --> 00:13:38.270 You're welcome to forget that. 00:13:38.270 --> 00:13:40.290 If you would like to believe that primase is actually 00:13:40.290 --> 00:13:43.550 making little segments of DNA, it'll be just fine. 00:13:43.550 --> 00:13:45.460 But in fact, it doesn't actually. 00:13:45.460 --> 00:13:47.990 It's making little segments of RNA so there's a whole other 00:13:47.990 --> 00:13:50.070 machinery that has to deal with that. 00:13:50.070 --> 00:13:52.810 But the basic concept five prime to three prime, little 00:13:52.810 --> 00:13:56.765 primers, getting extended, getting ligated, that's how 00:13:56.765 --> 00:13:57.620 you make your DNA. 00:13:57.620 --> 00:13:58.980 And you can check it out, and it works. 00:13:58.980 --> 00:14:00.230 All right. 00:14:09.500 --> 00:14:17.770 Well, it turns out to even be a little more complicated. 00:14:17.770 --> 00:14:20.920 That was how we got the synthesis going, but we also 00:14:20.920 --> 00:14:23.030 have a little bit of a topological problem. 00:14:31.090 --> 00:14:35.120 This again, says a lot about how people do science. 00:14:35.120 --> 00:14:37.300 You gotta just like not worry about certain things. 00:14:37.300 --> 00:14:40.390 If Kornberg had said, oh my goodness. 00:14:40.390 --> 00:14:43.860 I can't give my test tube a primer, because I don't know 00:14:43.860 --> 00:14:46.550 how the cell would make a primer, he wouldn't have made 00:14:46.550 --> 00:14:47.120 any progress. 00:14:47.120 --> 00:14:49.023 So he throws in the primer and says, the cell 00:14:49.023 --> 00:14:50.070 will figure it out. 00:14:50.070 --> 00:14:54.400 I'm just giving it a primer, and I'll see what happens. 00:14:54.400 --> 00:14:56.820 Now, there's another problem, this topological problem that 00:14:56.820 --> 00:14:58.300 also can make your head hurt. 00:14:58.300 --> 00:15:01.050 Let me try to explain what the topological problem is. 00:15:01.050 --> 00:15:08.735 Suppose I have DNA like that. 00:15:08.735 --> 00:15:10.580 Make that a little prettier. 00:15:10.580 --> 00:15:12.110 So I have some DNA like that. 00:15:21.570 --> 00:15:24.360 And maybe it goes around for a very long distance like a 00:15:24.360 --> 00:15:25.800 circle or something like that. 00:15:25.800 --> 00:15:30.070 I now want to copy that DNA. 00:15:30.070 --> 00:15:33.390 So I have one strand, and I'm copying it. 00:15:36.730 --> 00:15:43.650 I have this other strand, and I'm copying it. 00:15:43.650 --> 00:15:47.760 And remember, these two strands are wrapped around, 00:15:47.760 --> 00:15:51.560 and around, and around, and around each other. 00:15:51.560 --> 00:15:53.170 One is going like this. 00:15:53.170 --> 00:15:55.050 One is going like that, and there's some wrapped around. 00:15:55.050 --> 00:15:59.430 And as I tug them apart to make a new strand, to 00:15:59.430 --> 00:16:05.560 synthesize a new strand, those two new double helices are so 00:16:05.560 --> 00:16:10.360 totally intertwined with each other. 00:16:10.360 --> 00:16:14.720 Every turn that there was in the double helix is now a 00:16:14.720 --> 00:16:18.780 twist and turn connecting the two, sort of entangling the 00:16:18.780 --> 00:16:20.380 two helices. 00:16:20.380 --> 00:16:23.440 So I have the two new double helices 00:16:23.440 --> 00:16:26.130 entangled with each other. 00:16:26.130 --> 00:16:27.380 Why is that going to be a problem? 00:16:31.160 --> 00:16:34.070 I'm going to send these to two daughter cells. 00:16:34.070 --> 00:16:37.060 These are the two genomes for the two daughter cells. 00:16:37.060 --> 00:16:39.990 In fact in particular, if this thing was a circle, the two 00:16:39.990 --> 00:16:44.360 new circles will be totally wrapped around each other with 00:16:44.360 --> 00:16:46.650 a gazillion wraps. 00:16:46.650 --> 00:16:50.130 No way they're going to two daughter cells. 00:16:50.130 --> 00:16:51.560 Now, here is where mathematicians are very 00:16:51.560 --> 00:16:55.120 useful, because it is a theorem that if I take two 00:16:55.120 --> 00:16:59.020 circles wrapped around each other like that, there is no 00:16:59.020 --> 00:17:01.750 topological deformation possible that 00:17:01.750 --> 00:17:02.590 can separate them. 00:17:02.590 --> 00:17:04.359 It's like these puzzles, you get some strings wrapped 00:17:04.359 --> 00:17:06.240 around each other separate them. 00:17:06.240 --> 00:17:08.960 It's a theorem that two circles wrapped around each 00:17:08.960 --> 00:17:13.839 other like that cannot be separated unless, 00:17:13.839 --> 00:17:14.520 of course, you cheat. 00:17:14.520 --> 00:17:15.352 What's cheating? 00:17:15.352 --> 00:17:16.500 AUDIENCE: You cut it. 00:17:16.500 --> 00:17:17.319 ERIC LANDER: You cut it, obviously. 00:17:17.319 --> 00:17:18.980 If you cut it, then you can separate it. 00:17:18.980 --> 00:17:20.670 But otherwise, it's mathematically impossible to 00:17:20.670 --> 00:17:22.430 separate them. 00:17:22.430 --> 00:17:25.349 So this could concern people. 00:17:25.349 --> 00:17:27.450 How could a cell do this? 00:17:27.450 --> 00:17:28.867 So what does the cell do? 00:17:28.867 --> 00:17:29.781 AUDIENCE: It cuts it. 00:17:29.781 --> 00:17:30.820 ERIC LANDER: It cuts it. 00:17:30.820 --> 00:17:32.020 It's got no choice, right? 00:17:32.020 --> 00:17:32.990 It's a theorem, right? 00:17:32.990 --> 00:17:35.050 Even cells can't violate theorems. 00:17:35.050 --> 00:17:38.310 So it cuts it. 00:17:38.310 --> 00:17:40.440 The only way to get these things apart is to cut it. 00:17:40.440 --> 00:17:43.570 Now, what it does, is it takes those double helices. 00:17:43.570 --> 00:17:46.030 I'll represent the double helix as a thicker 00:17:46.030 --> 00:17:46.620 kind of thing now. 00:17:46.620 --> 00:17:49.480 That was my double helix, this other double helix 00:17:49.480 --> 00:17:52.200 wrapped around it. 00:17:52.200 --> 00:17:55.150 It's got to cut it. 00:17:55.150 --> 00:18:00.840 Now, when I take two DNAs that are wrapped around each other 00:18:00.840 --> 00:18:02.720 or two DNAs that are separate, have I done any 00:18:02.720 --> 00:18:04.920 chemistry on them? 00:18:04.920 --> 00:18:05.160 I'm sorry. 00:18:05.160 --> 00:18:07.040 Are they chemically different? 00:18:07.040 --> 00:18:09.440 They're chemically the same molecules. 00:18:09.440 --> 00:18:13.110 But they're topologically different. 00:18:13.110 --> 00:18:15.150 Topologically means wrapped around. 00:18:15.150 --> 00:18:17.160 In one case, they were topologically entangled. 00:18:17.160 --> 00:18:19.510 In the other case, they're topologically separated from 00:18:19.510 --> 00:18:20.280 each other. 00:18:20.280 --> 00:18:23.030 So they're still the same chemical bonds, the same 00:18:23.030 --> 00:18:29.260 molecules, but when I separate these two double helices now, 00:18:29.260 --> 00:18:32.220 the difference between these is that they are what are 00:18:32.220 --> 00:18:35.860 called topoisomers. 00:18:35.860 --> 00:18:38.620 They are isomers because they're exactly the same 00:18:38.620 --> 00:18:39.920 chemical formula. 00:18:39.920 --> 00:18:41.940 But they're topoisomers because they 00:18:41.940 --> 00:18:43.070 have different topology. 00:18:43.070 --> 00:18:46.060 They're not wrapped around each other anymore. 00:18:46.060 --> 00:18:48.900 So it turns out there is an enzyme that just gets in there 00:18:48.900 --> 00:18:51.040 and makes a double stranded cut in one of the double 00:18:51.040 --> 00:18:54.920 helices, grabs the two ends, passes it around the other 00:18:54.920 --> 00:18:59.380 side, and ligates them back together, and keeps doing that 00:18:59.380 --> 00:19:01.410 until they're disentangled. 00:19:01.410 --> 00:19:02.960 Pretty clever. 00:19:02.960 --> 00:19:06.445 Cut, paste, cut, paste till it can separate those two double 00:19:06.445 --> 00:19:08.520 helices from each other. 00:19:08.520 --> 00:19:13.200 Remarkably, this enzyme is called topoisomerase. 00:19:18.980 --> 00:19:26.750 This job is done by topoisomerase, actually, by 00:19:26.750 --> 00:19:28.750 topoisomerase II. 00:19:28.750 --> 00:19:30.820 There's a couple of different topoisomerases, and it's 00:19:30.820 --> 00:19:34.740 topoisomerase II that does this particular job, cuts and 00:19:34.740 --> 00:19:37.680 seals up that double-stranded break. 00:19:37.680 --> 00:19:40.080 All right. 00:19:40.080 --> 00:19:42.250 It is amazing how this works. 00:19:42.250 --> 00:19:47.080 Let's take another problem in how we do DNA replication. 00:19:47.080 --> 00:19:53.900 So let's deal with fidelity. 00:19:53.900 --> 00:19:57.305 The fidelity, accuracy of replication. 00:20:05.100 --> 00:20:07.810 I have my strand. 00:20:07.810 --> 00:20:09.730 Which direction do we go? 00:20:09.730 --> 00:20:13.900 We go, for this template, five prime to three prime. 00:20:13.900 --> 00:20:16.550 This way goes five prime to three prime, the opposite 00:20:16.550 --> 00:20:17.890 direction there. 00:20:17.890 --> 00:20:21.040 I now add on. 00:20:21.040 --> 00:20:22.790 If this is a T, what do I add in? 00:20:22.790 --> 00:20:23.996 AUDIENCE: [INAUDIBLE]. 00:20:23.996 --> 00:20:31.000 ERIC LANDER: If it's a GCGTAAT, et cetera. 00:20:31.000 --> 00:20:32.740 Why does the right base go in? 00:20:37.540 --> 00:20:38.864 Why does the right base go in? 00:20:38.864 --> 00:20:39.318 Yeah? 00:20:39.318 --> 00:20:40.680 AUDIENCE: Hydrogen bonding. 00:20:40.680 --> 00:20:41.030 ERIC LANDER: Hydrogen bonding. 00:20:41.030 --> 00:20:42.880 It's got that these hydrogen bonds. 00:20:42.880 --> 00:20:44.440 AT makes two hydrogen bonds. 00:20:44.440 --> 00:20:46.610 GC makes three hydrogen bonds. 00:20:46.610 --> 00:20:50.500 The wrong base could never go in. 00:20:50.500 --> 00:20:50.800 Sorry. 00:20:50.800 --> 00:20:54.160 In biochemistry, do you ever say never? 00:20:54.160 --> 00:20:56.400 No, we say K equilibrium. 00:20:56.400 --> 00:20:59.940 We say how much more unfavored is it for the 00:20:59.940 --> 00:21:02.350 wrong base to go in? 00:21:02.350 --> 00:21:05.500 It's not impossible, it's just disfavored, because it's 00:21:05.500 --> 00:21:08.260 energetically less good. 00:21:08.260 --> 00:21:10.940 How much energetically less good is it? 00:21:10.940 --> 00:21:14.660 What is the delta G for putting in the wrong base? 00:21:14.660 --> 00:21:15.910 It's not infinity. 00:21:18.470 --> 00:21:24.660 It turns out that there is an equilibrium constant for 00:21:24.660 --> 00:21:33.260 putting in the wrong base, and that is K equilibrium is about 00:21:33.260 --> 00:21:37.280 10 to the third for the right base, 10 to the minus third 00:21:37.280 --> 00:21:39.590 for the wrong base. 00:21:39.590 --> 00:21:40.290 Thank goodness. 00:21:40.290 --> 00:21:43.930 So only one time in 1,000 does it put in the wrong base. 00:21:43.930 --> 00:21:45.130 That's what that has to mean, right? 00:21:45.130 --> 00:21:48.870 If it's 1,000 times less favored energetically, it 00:21:48.870 --> 00:21:55.410 means you only make a mistake one letter in 1,000. 00:21:55.410 --> 00:21:57.700 How do you feel about that for your own genomes? 00:21:57.700 --> 00:21:59.440 Is that a level of quality control you 00:21:59.440 --> 00:22:00.500 are satisfied with? 00:22:00.500 --> 00:22:01.226 AUDIENCE: No. 00:22:01.226 --> 00:22:02.180 ERIC LANDER: No. 00:22:02.180 --> 00:22:04.980 How big is a typical gene? 00:22:04.980 --> 00:22:08.220 Typical gene is, in terms of its protein coding 00:22:08.220 --> 00:22:10.708 information, you guys already know about DNA goes to RNA 00:22:10.708 --> 00:22:11.090 goes to protein. 00:22:11.090 --> 00:22:13.870 It's about 2,000 bases of protein coding information. 00:22:13.870 --> 00:22:18.870 That guarantees two mistakes per cell division. 00:22:18.870 --> 00:22:20.310 Not good. 00:22:20.310 --> 00:22:22.320 Two mistakes per cell division. 00:22:22.320 --> 00:22:23.110 That's not OK. 00:22:23.110 --> 00:22:26.340 That's two mistakes per cell division. 00:22:26.340 --> 00:22:31.030 That would be two errors per cell division, and you have a 00:22:31.030 --> 00:22:35.440 lot of cell divisions, you're in a lot of trouble. 00:22:35.440 --> 00:22:38.200 So it turns out something more is needed. 00:22:38.200 --> 00:22:41.030 Quality control is needed. 00:22:41.030 --> 00:22:49.770 So later, it was discovered that the enzyme DNA 00:22:49.770 --> 00:23:01.350 polymerase, which has a five prime to three prime 00:23:01.350 --> 00:23:08.200 polymerization activity also does a second thing. 00:23:12.680 --> 00:23:16.590 That same enzyme, DNA polymerase, is also a three 00:23:16.590 --> 00:23:19.590 prime to five prime exonuclease. 00:23:22.360 --> 00:23:25.120 What do you think an exonuclease is? 00:23:25.120 --> 00:23:26.000 AUDIENCE: [INAUDIBLE]. 00:23:26.000 --> 00:23:28.320 ERIC LANDER: Take stuff out. 00:23:28.320 --> 00:23:31.740 So it adds bases in the forward direction, but it also 00:23:31.740 --> 00:23:34.910 goes backwards and takes bases out. 00:23:34.910 --> 00:23:36.710 Isn't that dumb? 00:23:36.710 --> 00:23:38.220 I thought we were trying to synthesize, but we're also 00:23:38.220 --> 00:23:39.470 unsynthesizing. 00:23:41.410 --> 00:23:44.598 With some probability, it goes backwards and takes out bases. 00:23:48.590 --> 00:23:53.290 Turns out that the probability of taking out a base backwards 00:23:53.290 --> 00:23:54.860 is higher if it's the wrong base. 00:23:58.840 --> 00:24:02.540 It's proofreading as it goes as I hope you are. 00:24:02.540 --> 00:24:05.440 It's proofreading. 00:24:05.440 --> 00:24:09.160 It goes backwards and takes bases out more often. 00:24:09.160 --> 00:24:12.400 Sometimes it takes out the right bases, but it is 00:24:12.400 --> 00:24:14.170 proofreading its work. 00:24:20.230 --> 00:24:23.080 And more often when it's the wrong base, it goes backwards, 00:24:23.080 --> 00:24:25.920 and so you get the benefit of a K equilibrium from the 00:24:25.920 --> 00:24:26.940 original base. 00:24:26.940 --> 00:24:29.420 And then there's a separate K equilibrium for the 00:24:29.420 --> 00:24:30.985 proofreading, and that helps you. 00:24:30.985 --> 00:24:35.260 And when you combine the proofreading with the original 00:24:35.260 --> 00:24:42.520 accuracy, now, we're down to something like 10 to the minus 00:24:42.520 --> 00:24:48.070 five or 10 to the minus six errors per 00:24:48.070 --> 00:24:49.760 base, per cell division. 00:24:53.510 --> 00:24:55.935 It's only making on the order of one error per million. 00:25:00.000 --> 00:25:03.080 Now are we satisfied? 00:25:03.080 --> 00:25:03.730 No. 00:25:03.730 --> 00:25:05.370 You guys pretty hard nosed. 00:25:05.370 --> 00:25:09.760 Not good enough, because you have 50 cell divisions to make 00:25:09.760 --> 00:25:11.340 more and some cells go through many, many, 00:25:11.340 --> 00:25:12.880 many more cell divisions. 00:25:12.880 --> 00:25:14.400 Not acceptable. 00:25:14.400 --> 00:25:15.650 But it's a start. 00:25:20.430 --> 00:25:22.940 So proofreading helps. 00:25:22.940 --> 00:25:26.850 So we have the fidelity of replication. 00:25:26.850 --> 00:25:31.120 Replication makes an error at a rate of 10 00:25:31.120 --> 00:25:32.650 to the minus third. 00:25:32.650 --> 00:25:39.100 Proofreading brings you down to 10 to the minus six, and 00:25:39.100 --> 00:25:41.130 there's another process. 00:25:41.130 --> 00:25:44.400 There are a set of enzymes that go around and feel the 00:25:44.400 --> 00:25:48.340 DNA double helix after it's finished, and if you put in 00:25:48.340 --> 00:25:53.620 the wrong base, the width of the helix is not right. 00:25:53.620 --> 00:25:55.550 The shape is wrong. 00:25:55.550 --> 00:25:57.900 It feels for mismatches. 00:25:57.900 --> 00:26:00.060 So there is a mismatch repair system. 00:26:04.980 --> 00:26:10.900 Mismatch repair comes along, and if there was an error 00:26:10.900 --> 00:26:15.120 right here, the helix bulges out too much let's say. 00:26:15.120 --> 00:26:22.400 Mismatch repair cuts, removes some DNA, and gives the cell 00:26:22.400 --> 00:26:25.510 another chance to do it again. 00:26:25.510 --> 00:26:30.350 Mismatch repair gets you down to something in the 00:26:30.350 --> 00:26:33.020 neighborhood of 10 to the minus eighth, 10 00:26:33.020 --> 00:26:35.840 to the minus ninth. 00:26:35.840 --> 00:26:37.200 Let's say for the sake of argument, 10 00:26:37.200 --> 00:26:38.390 to the minus ninth. 00:26:38.390 --> 00:26:41.610 You're genome is about three times 10 to the ninth. 00:26:41.610 --> 00:26:45.100 Now making that's one or two errors per genome, 00:26:45.100 --> 00:26:46.350 that's not so bad. 00:26:49.540 --> 00:26:51.240 Why do we care? 00:26:51.240 --> 00:26:52.890 Why am I bothering you with this? 00:26:52.890 --> 00:26:55.810 Who cares between 10 to minus sixth, 10 to the minus ninth? 00:26:55.810 --> 00:26:57.060 Big deal. 00:26:59.630 --> 00:27:07.450 Well, a few percent of you in this class are heterozygous 00:27:07.450 --> 00:27:11.590 for a mutation in the mismatch repair enzymes. 00:27:11.590 --> 00:27:12.730 Don't worry. 00:27:12.730 --> 00:27:16.000 Your cells have the other copy that's good. 00:27:16.000 --> 00:27:20.930 But suppose one of your cells were to lose, by mutation, the 00:27:20.930 --> 00:27:23.060 good copy of the mismatch repair enzyme? 00:27:23.060 --> 00:27:26.250 And now that cell in your body had no copies of mismatch 00:27:26.250 --> 00:27:27.500 repair enzyme. 00:27:31.780 --> 00:27:33.250 What do you think is going to happen to your DNA 00:27:33.250 --> 00:27:35.930 replication? 00:27:35.930 --> 00:27:37.660 Instead of being one in a billion, it would be one in a 00:27:37.660 --> 00:27:40.220 million accuracy. 00:27:40.220 --> 00:27:41.460 Turns out you have an extremely high 00:27:41.460 --> 00:27:44.780 risk of colon cancer. 00:27:44.780 --> 00:27:47.660 There are hereditary colon cancer syndromes that are due 00:27:47.660 --> 00:27:50.800 to inherited defects in the mismatch repair system. 00:27:50.800 --> 00:27:53.380 It is not at all trivial. 00:27:53.380 --> 00:27:55.900 Hereditary polyposis coli is due to a 00:27:55.900 --> 00:27:57.340 defect in this enzyme. 00:28:02.770 --> 00:28:03.720 It matters. 00:28:03.720 --> 00:28:06.010 You've got to get it down to that level because otherwise, 00:28:06.010 --> 00:28:09.780 you're getting mutations that cause cancer, that is, when 00:28:09.780 --> 00:28:12.430 you lose both copies, if you lost both copies. 00:28:12.430 --> 00:28:14.890 Most of your cells would be fine, but if you'd lose the 00:28:14.890 --> 00:28:18.100 other good copy, by chance, that cell can 00:28:18.100 --> 00:28:21.490 go on to cause cancer. 00:28:21.490 --> 00:28:23.460 So this stuff actually matters. 00:28:23.460 --> 00:28:29.120 Finally, finally, speed. 00:28:29.120 --> 00:28:31.900 Kind of fun to talk about speed. 00:28:31.900 --> 00:28:35.291 How fast does polymerase work? 00:28:35.291 --> 00:28:43.720 It turns out that polymerase is able to polymerize 2,000 00:28:43.720 --> 00:28:50.180 nucleotides per second. 00:28:50.180 --> 00:28:52.070 That's very impressive to me. 00:28:52.070 --> 00:28:54.910 It zips along at 2,000 nucleotides per second, 00:28:54.910 --> 00:28:59.050 installing the right base, getting it right only 99.9% of 00:28:59.050 --> 00:29:03.260 the time, proofreading as it goes, and gets the whole thing 00:29:03.260 --> 00:29:06.580 done 2,000 letters in a second. 00:29:06.580 --> 00:29:08.580 That is impressive engineering. 00:29:08.580 --> 00:29:12.110 That is really impressive engineering. 00:29:12.110 --> 00:29:17.170 So that's kind of how DNA replication works well, except 00:29:17.170 --> 00:29:18.420 for one thing. 00:29:29.540 --> 00:29:30.790 Kornberg was a biochemist. 00:29:33.290 --> 00:29:35.300 Biochemists purify things in test tubes. 00:29:41.180 --> 00:29:49.650 He discovered an enzyme, Kornberg's polymerase. 00:29:54.880 --> 00:29:59.420 How do we know it's the enzyme the cell actually 00:29:59.420 --> 00:30:03.070 uses to copy its DNA? 00:30:03.070 --> 00:30:04.600 See, I'm a geneticist. 00:30:04.600 --> 00:30:07.300 I look at Kornberg and I say, nice job. 00:30:07.300 --> 00:30:12.030 You showed me an enzyme that in a test tube is capable of 00:30:12.030 --> 00:30:14.250 polymerizing DNA. 00:30:14.250 --> 00:30:17.340 How do I know that's the enzyme that's actually doing 00:30:17.340 --> 00:30:19.110 it from the cell copies its whole genome? 00:30:22.210 --> 00:30:25.858 What does a geneticist want to see? 00:30:25.858 --> 00:30:27.350 AUDIENCE: A mutant. 00:30:27.350 --> 00:30:28.230 ERIC LANDER: A mutant. 00:30:28.230 --> 00:30:32.000 Show me a mutant then I'll believe. 00:30:32.000 --> 00:30:39.330 So someone went along and took E. colis one at a time because 00:30:39.330 --> 00:30:40.400 what else could they do. 00:30:40.400 --> 00:30:43.670 And for every single E. coli they grew up from a plate, 00:30:43.670 --> 00:30:45.722 they purified Kornberg's enzyme. 00:30:45.722 --> 00:30:48.680 And you know what they found? 00:30:48.680 --> 00:30:55.710 They found a mutant E. coli that lacked Kornberg's enzyme, 00:30:55.710 --> 00:30:57.900 and it could replicate its DNA just fine. 00:31:01.950 --> 00:31:03.200 What does that tell us? 00:31:07.440 --> 00:31:11.370 Kornberg actually had the wrong enzyme. 00:31:11.370 --> 00:31:13.270 He still deserves a Nobel Prize for it because he got an 00:31:13.270 --> 00:31:15.340 enzyme that could copy DNA. 00:31:15.340 --> 00:31:18.790 It's actually not the main enzyme that does the job. 00:31:18.790 --> 00:31:22.270 Because we can make a mutant that lacks that enzyme and it 00:31:22.270 --> 00:31:27.220 can still copy the DNA, it can't be the main enzyme. 00:31:27.220 --> 00:31:31.320 Turns out what Kornberg found was a minor polymerase that 00:31:31.320 --> 00:31:34.320 was used in those mismatch repair situations that would 00:31:34.320 --> 00:31:37.190 come along and do the tidying and clean up. 00:31:37.190 --> 00:31:39.810 The main enzyme turned out to be another enzyme, a more 00:31:39.810 --> 00:31:41.390 complicated enzyme. 00:31:41.390 --> 00:31:45.450 So my point about biochemistry and genetics both having to 00:31:45.450 --> 00:31:48.900 talk to each other, you only really know something when you 00:31:48.900 --> 00:31:51.940 have it from a biochemical point of view and the genetic 00:31:51.940 --> 00:31:53.000 point of view. 00:31:53.000 --> 00:31:54.500 The two have to go together. 00:31:54.500 --> 00:31:56.110 Kornberg's enzyme is a great enzyme, 00:31:56.110 --> 00:31:57.470 it's a fantastic enzyme. 00:31:57.470 --> 00:32:00.410 It just happens not to be the main enzyme, and you can only 00:32:00.410 --> 00:32:02.560 know that by genetics. 00:32:02.560 --> 00:32:05.360 Of course, you can only purify it by biochemistry. 00:32:05.360 --> 00:32:06.050 All right. 00:32:06.050 --> 00:32:09.910 So that's DNA replication. 00:32:09.910 --> 00:32:12.640 Any questions about DNA replication before I go on? 00:32:12.640 --> 00:32:13.060 Yes? 00:32:13.060 --> 00:32:15.210 AUDIENCE: [INAUDIBLE]. 00:32:15.210 --> 00:32:17.450 ERIC LANDER: Polymerase III or polymerase II, depending on 00:32:17.450 --> 00:32:18.200 the organism. 00:32:18.200 --> 00:32:19.330 They're all called polymerases. 00:32:19.330 --> 00:32:20.650 They're all DNA polymerases. 00:32:20.650 --> 00:32:22.470 They just get different names and numbers. 00:32:22.470 --> 00:32:25.150 Turns out most cells have multiple polymerases and 00:32:25.150 --> 00:32:27.780 Kornberg found the kind of simpler polymerase. 00:32:27.780 --> 00:32:29.980 The main replication polymerase also called 00:32:29.980 --> 00:32:32.190 polymerase but with a different number, is a 00:32:32.190 --> 00:32:33.410 different more complicated enzyme. 00:32:33.410 --> 00:32:33.800 Yes? 00:32:33.800 --> 00:32:39.620 AUDIENCE: How does the enzyme know which one is the right..? 00:32:39.620 --> 00:32:41.520 ERIC LANDER: how does it know which one is right? 00:32:41.520 --> 00:32:44.747 AUDIENCE: [INAUDIBLE]. 00:32:44.747 --> 00:32:47.080 ERIC LANDER: Because 50% of the time you get it wrong. 00:32:47.080 --> 00:32:48.530 Do you know what bacteria do? 00:32:48.530 --> 00:32:49.420 What a great question. 00:32:49.420 --> 00:32:50.670 How would it know which one to get right? 00:32:53.500 --> 00:32:56.400 Know what bacteria do? 00:32:56.400 --> 00:32:57.780 They're very tricky. 00:32:57.780 --> 00:33:00.210 They mark their DNA, don't worry about this. 00:33:00.210 --> 00:33:02.680 They mark their DNA with methyl groups. 00:33:02.680 --> 00:33:04.870 There is an enzyme that comes along and put methyl groups at 00:33:04.870 --> 00:33:11.640 certain positions, but that enzyme is kind of slow. 00:33:11.640 --> 00:33:15.100 So I have a methyl-marked DNA double helix. 00:33:15.100 --> 00:33:18.580 When I replicate it, the new strand is made, and what does 00:33:18.580 --> 00:33:19.610 the new strand lack? 00:33:19.610 --> 00:33:21.000 AUDIENCE: Little methyl groups. 00:33:21.000 --> 00:33:22.910 ERIC LANDER: Little methyl groups. 00:33:22.910 --> 00:33:25.110 It'll get them eventually because that slow enzyme will 00:33:25.110 --> 00:33:29.050 come along and put them on, but mismatch repair is fast. 00:33:29.050 --> 00:33:31.940 So what is mismatch repair looking for? 00:33:31.940 --> 00:33:34.480 The little methyl groups that are kind of breadcrumbs that 00:33:34.480 --> 00:33:38.330 say, this was the old strand, and this guy is the new stand. 00:33:38.330 --> 00:33:39.370 It's thought of everything. 00:33:39.370 --> 00:33:41.020 It's really smart. 00:33:41.020 --> 00:33:42.270 Very, very smart.