1 00:00:00,500 --> 00:00:03,479 [CREAKING] 2 00:00:16,420 --> 00:00:19,100 BARBARA IMPERIALI: So what we are going to talk about today, 3 00:00:19,100 --> 00:00:23,390 and I'm going to do something a little annoying, as usual, 4 00:00:23,390 --> 00:00:26,190 I want to take you somewhere here. 5 00:00:26,190 --> 00:00:29,090 Yeah, sure, back to carbon, that's fine. 6 00:00:29,090 --> 00:00:31,300 But what I want to show you is the sizes 7 00:00:31,300 --> 00:00:34,930 of the molecular players in translation 8 00:00:34,930 --> 00:00:39,220 because we are going here now from the smallest carbon atom. 9 00:00:39,220 --> 00:00:42,400 And we spent the first four lectures on amino acids, 10 00:00:42,400 --> 00:00:45,100 nucleotides, sugars, phospholipids. 11 00:00:45,100 --> 00:00:47,568 But we're entering new territory here, 12 00:00:47,568 --> 00:00:49,360 and I'm just going to do this in such a way 13 00:00:49,360 --> 00:00:52,850 that it comes onto the screens. 14 00:00:52,850 --> 00:00:55,420 What you can see here is, in order 15 00:00:55,420 --> 00:00:59,300 to make proteins like hemoglobin and antibodies-- 16 00:00:59,300 --> 00:01:02,260 these are made up of amino acids, the really tiny things 17 00:01:02,260 --> 00:01:04,269 that are out of view right now-- 18 00:01:04,269 --> 00:01:08,740 we need large entities that are made up of either nucleic acids 19 00:01:08,740 --> 00:01:12,140 alone or nucleic acids plus proteins. 20 00:01:12,140 --> 00:01:14,140 And so the things that will feature today 21 00:01:14,140 --> 00:01:17,480 are the transfer RNAs and the ribosome. 22 00:01:17,480 --> 00:01:20,980 And I want you to look at the size of this molecular machine. 23 00:01:20,980 --> 00:01:23,320 This is we're getting pretty large now. 24 00:01:23,320 --> 00:01:25,930 We're needing to use large machines 25 00:01:25,930 --> 00:01:29,950 to make the smaller catalysts that are essential in the cell. 26 00:01:29,950 --> 00:01:34,000 And, just so sort of maybe size doesn't seem so important, 27 00:01:34,000 --> 00:01:37,000 but let's just go a little bit further here. 28 00:01:37,000 --> 00:01:45,090 And what to me is intriguing is that the size of the ribosome 29 00:01:45,090 --> 00:01:48,450 is pretty similar to the size of the rhinovirus, 30 00:01:48,450 --> 00:01:51,030 a little smaller than the hepatitis virus, 31 00:01:51,030 --> 00:01:54,090 but quite a bit smaller than some of these other viruses. 32 00:01:54,090 --> 00:01:57,960 So the ribosome organelle is a large entity in the cell. 33 00:01:57,960 --> 00:02:03,930 And, when you do look at electron micrographs of cells, 34 00:02:03,930 --> 00:02:07,380 you can see these dark dots, which represent ribosomes. 35 00:02:07,380 --> 00:02:10,770 They're big enough to see, whereas the proteins themselves 36 00:02:10,770 --> 00:02:12,210 are not big enough to see. 37 00:02:12,210 --> 00:02:15,450 So that's what you're destined for today. 38 00:02:19,170 --> 00:02:26,720 All right, OK, so I've started to place on this board 39 00:02:26,720 --> 00:02:30,200 some of the molecular players, the messenger RNA, the transfer 40 00:02:30,200 --> 00:02:33,290 RNA, and the ribosomes, which are made up 41 00:02:33,290 --> 00:02:35,840 of ribosomal RNA plus protein. 42 00:02:35,840 --> 00:02:39,680 And I want to just remind you about the structure 43 00:02:39,680 --> 00:02:46,880 of the mature messenger RNA just for a minute 44 00:02:46,880 --> 00:02:48,680 because, in the last class, we talked 45 00:02:48,680 --> 00:02:53,750 about a lot of manipulations of that pre-messenger RNA, 46 00:02:53,750 --> 00:02:56,000 the fresh thing out of transcription. 47 00:02:56,000 --> 00:02:57,710 And now I just want to remind you 48 00:02:57,710 --> 00:03:03,230 that the messenger is single-stranded RNA, obviously. 49 00:03:03,230 --> 00:03:08,690 It has a 5 prime cap, which has got this funky 5 prime, 50 00:03:08,690 --> 00:03:13,100 5 prime bridge that's resistant to exonucleases. 51 00:03:13,100 --> 00:03:17,510 Somewhere in that sequence is a start codon. 52 00:03:17,510 --> 00:03:23,660 It's something that says this is the bit I want to translate. 53 00:03:23,660 --> 00:03:29,360 Often, there's a lot of stuff here that you don't translate. 54 00:03:29,360 --> 00:03:36,440 It's part of what's known as the ribosome binding site. 55 00:03:36,440 --> 00:03:38,750 And there are many features in this part 56 00:03:38,750 --> 00:03:42,410 of the sequence that are very important for translation. 57 00:03:42,410 --> 00:03:45,590 They contribute to the efficiency of translation. 58 00:03:45,590 --> 00:03:48,080 Generically, we'd call it the ribosome binding site, 59 00:03:48,080 --> 00:03:51,350 but there are funny things called Shine-Dalgarno sequences 60 00:03:51,350 --> 00:03:52,040 and stuff. 61 00:03:52,040 --> 00:03:53,910 Don't worry about any of that. 62 00:03:53,910 --> 00:03:58,190 I just want you to appreciate that, the mature transcript, 63 00:03:58,190 --> 00:04:00,620 you don't translate the whole thing. 64 00:04:00,620 --> 00:04:03,230 A lot of this stuff is structural, functional 65 00:04:03,230 --> 00:04:05,210 for other reasons that contribute 66 00:04:05,210 --> 00:04:07,160 to the success of translation. 67 00:04:07,160 --> 00:04:09,560 Once you see one of these, the ribosome 68 00:04:09,560 --> 00:04:14,060 mows its way through and reads the nucleic acids 69 00:04:14,060 --> 00:04:15,110 in the message. 70 00:04:15,110 --> 00:04:18,740 So the message is being conveyed over to the new machinery. 71 00:04:18,740 --> 00:04:21,640 And then, when you hit one of these three codons, 72 00:04:21,640 --> 00:04:24,200 and we'll discuss these properly when we get to them, 73 00:04:24,200 --> 00:04:27,230 it's time to stop and finish translating. 74 00:04:27,230 --> 00:04:32,000 The other end the message has a poly-adenine tail. 75 00:04:32,000 --> 00:04:35,330 Remember that, once again, is structural to protect 76 00:04:35,330 --> 00:04:37,280 the ends of that transcript. 77 00:04:37,280 --> 00:04:41,720 Even if some of the hundreds of adenine nucleotides 78 00:04:41,720 --> 00:04:44,810 are nibbled off, you don't get into the part 79 00:04:44,810 --> 00:04:48,050 of the gene that's critical to be translated. 80 00:04:48,050 --> 00:04:51,260 At a certain stage, though, you might get in. 81 00:04:51,260 --> 00:04:53,960 Exonucleases might chew up enough. 82 00:04:53,960 --> 00:04:56,930 And they may end up chewing up your transcript, 83 00:04:56,930 --> 00:05:00,140 but that probably suggests that the messenger has been around 84 00:05:00,140 --> 00:05:04,580 too long, and it's time for it to retire to a better life, OK? 85 00:05:04,580 --> 00:05:06,840 So remember the poly-A tail. 86 00:05:06,840 --> 00:05:09,800 And this, once again, plays other functional roles 87 00:05:09,800 --> 00:05:12,770 with respect to being recognized as a transcript 88 00:05:12,770 --> 00:05:14,910 and being helped to get out of the nucleus. 89 00:05:14,910 --> 00:05:18,980 So this was really what we talked about last week. 90 00:05:18,980 --> 00:05:20,480 There's one more feature in here. 91 00:05:20,480 --> 00:05:22,460 I'm just going to remind you that 92 00:05:22,460 --> 00:05:26,030 the final mature transcript has also 93 00:05:26,030 --> 00:05:31,160 been through splicing with removal of introns 94 00:05:31,160 --> 00:05:34,520 and the pasting together of exons, which is a wonderful way 95 00:05:34,520 --> 00:05:37,970 to diversify transcripts of translation 96 00:05:37,970 --> 00:05:42,590 and give you much more proteins than one gene can encode, OK? 97 00:05:42,590 --> 00:05:44,090 And we talked about that last time. 98 00:05:44,090 --> 00:05:44,850 Great. 99 00:05:44,850 --> 00:05:48,650 So the first thing we have to think about here 100 00:05:48,650 --> 00:05:54,830 is how do we go from four bases to a language that 101 00:05:54,830 --> 00:05:56,850 includes 20 letters, right? 102 00:05:59,370 --> 00:06:01,380 Or it's better really, more precise, 103 00:06:01,380 --> 00:06:03,180 for me to call them nucleotides. 104 00:06:07,990 --> 00:06:12,820 It's more precise because the base just 105 00:06:12,820 --> 00:06:16,660 represents the ring system that's attached to the ribose. 106 00:06:16,660 --> 00:06:19,930 Nucleotide means the whole thing, including a phosphate. 107 00:06:19,930 --> 00:06:25,375 So we go from four bases to encoding 20 amino acids. 108 00:06:29,000 --> 00:06:31,090 Now there are a few organisms-- 109 00:06:31,090 --> 00:06:35,310 and, in fact, we have one spare one as well, selenocysteine. 110 00:06:35,310 --> 00:06:37,780 There are a couple of other amino acids 111 00:06:37,780 --> 00:06:41,860 that might be designated as the so-called 21st, 22nd amino 112 00:06:41,860 --> 00:06:42,670 acid. 113 00:06:42,670 --> 00:06:45,910 They're not found globally in all organisms. 114 00:06:45,910 --> 00:06:49,960 We have selenocysteine in just very, very few proteins. 115 00:06:49,960 --> 00:06:53,530 So it's something beyond the list of 20 that you saw. 116 00:06:53,530 --> 00:06:56,200 There are other organisms, for example, 117 00:06:56,200 --> 00:06:58,090 in archaea, these guys who hang out 118 00:06:58,090 --> 00:07:01,120 in bubbling hot pots in Yellowstone, 119 00:07:01,120 --> 00:07:05,200 for example, that have another amino acid known 120 00:07:05,200 --> 00:07:06,130 as pyrrolysine. 121 00:07:06,130 --> 00:07:08,650 I'm going to mention that a little bit later on, 122 00:07:08,650 --> 00:07:10,900 but the ones you really need to think about 123 00:07:10,900 --> 00:07:15,370 are the ones we're encoding in the global genetic code. 124 00:07:15,370 --> 00:07:19,270 This is the ones that are common to everybody, all right? 125 00:07:19,270 --> 00:07:23,950 So, obviously, when you look at the language of bases, 126 00:07:23,950 --> 00:07:25,690 one base-- 127 00:07:25,690 --> 00:07:28,960 if the language translated directly one base 128 00:07:28,960 --> 00:07:36,890 to one amino acid, we could only encode for amino acids. 129 00:07:36,890 --> 00:07:39,320 So we know it's not one base, one amino acid. 130 00:07:39,320 --> 00:07:42,770 And I know, at this stage, you know that it's three bases, 131 00:07:42,770 --> 00:07:46,190 but let's just go through the math or the original questions 132 00:07:46,190 --> 00:07:48,830 that were sort of circulating. 133 00:07:48,830 --> 00:07:52,160 Like how do we go from this language to the other language? 134 00:07:52,160 --> 00:08:02,120 If two bases encoded each amino acid, 135 00:08:02,120 --> 00:08:05,170 we could only encode eight amino acids. 136 00:08:05,170 --> 00:08:08,700 That's not enough still. 137 00:08:08,700 --> 00:08:12,410 16 amino-- wait a minute. 138 00:08:12,410 --> 00:08:16,760 16 amino acids, sorry, I can never-- 139 00:08:16,760 --> 00:08:20,390 anyway, that's 4 to the 2. 140 00:08:20,390 --> 00:08:22,310 It's pretty good if I can't get 4 to the 2 141 00:08:22,310 --> 00:08:23,510 up at the blackboard. 142 00:08:23,510 --> 00:08:26,780 This would be 4 to the 1 power and then-- 143 00:08:26,780 --> 00:08:28,880 so what that came down to realizing 144 00:08:28,880 --> 00:08:30,560 there were not enough. 145 00:08:30,560 --> 00:08:33,260 That wouldn't be a sufficient language 146 00:08:33,260 --> 00:08:35,760 to encode the 20 amino acids. 147 00:08:35,760 --> 00:08:41,230 So it's finally deduced that three bases 148 00:08:41,230 --> 00:08:45,400 encoded each amino acid. 149 00:08:45,400 --> 00:08:52,370 That would give us 64 possible words in the language that 150 00:08:52,370 --> 00:08:54,310 needs to be translated. 151 00:08:54,310 --> 00:08:55,820 That's a lot more than we need. 152 00:08:55,820 --> 00:09:01,150 We only need 20 for the encoded amino acids, 153 00:09:01,150 --> 00:09:04,630 so 64 possibilities. 154 00:09:04,630 --> 00:09:07,140 But what else do we need in the language? 155 00:09:07,140 --> 00:09:08,575 We need a few more things anyway. 156 00:09:12,420 --> 00:09:14,605 Yeah, up there. 157 00:09:14,605 --> 00:09:15,980 AUDIENCE: Oh, I don't [INAUDIBLE] 158 00:09:15,980 --> 00:09:17,680 BARBARA IMPERIALI: Oh, you weren't. 159 00:09:17,680 --> 00:09:18,695 Up there. 160 00:09:18,695 --> 00:09:20,778 AUDIENCE: You need to know when to start and stop. 161 00:09:20,778 --> 00:09:23,510 BARBARA IMPERIALI: So, exactly, so here this isn't necessarily 162 00:09:23,510 --> 00:09:26,780 one of the ones that uniformly codes amino acid 163 00:09:26,780 --> 00:09:27,660 through the sequence. 164 00:09:27,660 --> 00:09:30,800 We need a precise way to say start. 165 00:09:30,800 --> 00:09:34,190 And, in fact, we need a way to say stop. 166 00:09:34,190 --> 00:09:37,950 And there are multiple three letter words that say stop. 167 00:09:37,950 --> 00:09:41,120 So it turns out that the genetic code, which 168 00:09:41,120 --> 00:09:50,090 forms the basis of this entire concept, 169 00:09:50,090 --> 00:09:54,500 has some features to it where it does have some degeneracy. 170 00:09:54,500 --> 00:09:56,240 But we'll go through the degeneracy, 171 00:09:56,240 --> 00:09:58,040 and we'll take a look at the genetic code 172 00:09:58,040 --> 00:10:02,555 because it will tell us exactly how the three letter-- 173 00:10:02,555 --> 00:10:05,570 the words made of three bases encode everything 174 00:10:05,570 --> 00:10:08,210 we need for translation, all right? 175 00:10:08,210 --> 00:10:11,120 So let's just go back and take a look at this. 176 00:10:11,120 --> 00:10:13,160 So we've looked at the messenger. 177 00:10:13,160 --> 00:10:16,400 I've told you a little bit about the tRNAs and the proteins. 178 00:10:16,400 --> 00:10:18,650 But then we just sort of give you a little bit 179 00:10:18,650 --> 00:10:20,150 of the back history. 180 00:10:20,150 --> 00:10:22,700 And, once the structure of double-stranded DNA 181 00:10:22,700 --> 00:10:24,800 was deduced, really, everyone was 182 00:10:24,800 --> 00:10:28,580 moving on to trying to understand how that converted 183 00:10:28,580 --> 00:10:30,830 to the translation to proteins. 184 00:10:30,830 --> 00:10:33,800 And there were a lot of workers deeply involved in this. 185 00:10:33,800 --> 00:10:37,550 Crick and Brenner realized it was three bases to code 186 00:10:37,550 --> 00:10:42,170 for one amino acid, but Khorana, Nirenberg, and Holley, 187 00:10:42,170 --> 00:10:46,400 Khorana who was part of our faculty for many, many years, 188 00:10:46,400 --> 00:10:49,250 actually defined that genetic code 189 00:10:49,250 --> 00:10:52,100 and got all of the details. 190 00:10:52,100 --> 00:10:54,950 Brenner and Crick started to have the ideas, 191 00:10:54,950 --> 00:10:58,760 but, really, the definition by doing a process known 192 00:10:58,760 --> 00:11:02,720 as cell-free translation where they could very carefully add 193 00:11:02,720 --> 00:11:08,690 components to understand how the code, the genetic code, 194 00:11:08,690 --> 00:11:11,630 was formulated where they put in specific messenger 195 00:11:11,630 --> 00:11:14,330 RNAs and amino acids and transfer 196 00:11:14,330 --> 00:11:17,570 RNAs and actually made proteins from that. 197 00:11:17,570 --> 00:11:20,880 So that's the work that Khorana and others did. 198 00:11:20,880 --> 00:11:23,570 And that was awarded them a Nobel Prize for that work. 199 00:11:26,450 --> 00:11:30,290 And then, later on, things started to get-- you know, 200 00:11:30,290 --> 00:11:33,260 these are decades of work I want to point out to you. 201 00:11:33,260 --> 00:11:34,910 The ribosomes were discovered. 202 00:11:34,910 --> 00:11:38,240 That was a decade later, the sort 203 00:11:38,240 --> 00:11:41,390 of details of the structure, but not the structure itself. 204 00:11:41,390 --> 00:11:44,450 And it was really exciting in the 2000s 205 00:11:44,450 --> 00:11:48,080 when Ramakrishnan, Steitz, and Yonath solved the structure 206 00:11:48,080 --> 00:11:50,150 of the prokaryotic ribosome. 207 00:11:50,150 --> 00:11:53,670 So each of these things has taken a decade to happen, 208 00:11:53,670 --> 00:11:56,300 but they are fundamental, major, important things 209 00:11:56,300 --> 00:12:00,780 that we can act on and move forward to understand more. 210 00:12:00,780 --> 00:12:06,220 All right, so let's move to the transfer RNAs. 211 00:12:06,220 --> 00:12:09,040 And I've flashed up this slide a couple of times, 212 00:12:09,040 --> 00:12:12,940 but I actually now have the movie of the structure 213 00:12:12,940 --> 00:12:14,870 of a transfer RNA. 214 00:12:14,870 --> 00:12:19,600 So the transfer RNA is a linear segment of RNA, 215 00:12:19,600 --> 00:12:25,030 but it's folded up the way ribonucleic acids are 216 00:12:25,030 --> 00:12:29,260 with three, four stretches of double strand coming together 217 00:12:29,260 --> 00:12:31,340 and loops in between them. 218 00:12:31,340 --> 00:12:33,400 And the one end, this is the 3 prime end. 219 00:12:33,400 --> 00:12:35,200 I always try to draw it on the left 220 00:12:35,200 --> 00:12:37,270 because, otherwise, things get confusing. 221 00:12:37,270 --> 00:12:40,090 It's much easier to sort of see where everything ends up 222 00:12:40,090 --> 00:12:43,470 if you put the long arm on the 3 prime side 223 00:12:43,470 --> 00:12:45,303 to the left of your picture. 224 00:12:45,303 --> 00:12:46,720 And what I think is cool, when you 225 00:12:46,720 --> 00:12:50,260 look at the structure of RNA, we think of messenger RNA 226 00:12:50,260 --> 00:12:52,880 as being a sort of rather floppy entity, 227 00:12:52,880 --> 00:12:55,300 but it actually really likes to form 228 00:12:55,300 --> 00:12:57,430 short segments of double helix. 229 00:12:57,430 --> 00:12:59,500 It just doesn't do well with the really long 230 00:12:59,500 --> 00:13:02,980 double-stranded structure the same way that DNA does. 231 00:13:02,980 --> 00:13:05,950 But this folded up structure is very important. 232 00:13:05,950 --> 00:13:09,300 And it was on the observation of these folded structures 233 00:13:09,300 --> 00:13:13,090 that the ribosome hypothesis was formulated. 234 00:13:13,090 --> 00:13:16,540 But the two things that you want to remember about the ribosome 235 00:13:16,540 --> 00:13:18,940 are that the 3 prime hydroxyl group 236 00:13:18,940 --> 00:13:23,620 of the last ribose within this ribosomal sequence 237 00:13:23,620 --> 00:13:26,920 is where the amino acid that's going to be loaded 238 00:13:26,920 --> 00:13:29,230 into your protein is attached. 239 00:13:29,230 --> 00:13:30,740 So that's one point. 240 00:13:30,740 --> 00:13:33,790 And then there's another landmark on this structure. 241 00:13:33,790 --> 00:13:35,410 And that's what's called-- 242 00:13:35,410 --> 00:13:37,760 one of the loops has a special name. 243 00:13:37,760 --> 00:13:40,300 It's called the anticodon loop. 244 00:13:40,300 --> 00:13:43,900 It comprises three nucleotides that 245 00:13:43,900 --> 00:13:48,970 are complementary to the nucleotides in your messenger 246 00:13:48,970 --> 00:13:50,050 sequence. 247 00:13:50,050 --> 00:13:52,900 So this really is a decoder because, at one end, 248 00:13:52,900 --> 00:13:55,150 it's carrying an amino acid, but it's 249 00:13:55,150 --> 00:13:58,600 carrying the amino acid that corresponds 250 00:13:58,600 --> 00:14:02,830 to the code that's in the messenger via that anticodon 251 00:14:02,830 --> 00:14:03,460 loop. 252 00:14:03,460 --> 00:14:06,340 So it's a large structure, but don't mistake it 253 00:14:06,340 --> 00:14:08,110 for being something that's just sort 254 00:14:08,110 --> 00:14:10,300 of amino acid and anticodon. 255 00:14:10,300 --> 00:14:12,520 A lot goes on with the rest of the structure. 256 00:14:12,520 --> 00:14:16,600 It's a very important structure in the mechanisms of protein 257 00:14:16,600 --> 00:14:19,360 translation and synthesis. 258 00:14:19,360 --> 00:14:24,190 OK, so here I've got to sum that up with a couple more 259 00:14:24,190 --> 00:14:27,670 of the ways that you would see the transfer RNAs. 260 00:14:27,670 --> 00:14:29,890 You might see it in this globular form. 261 00:14:29,890 --> 00:14:32,920 And I pointed out the anticodon loop. 262 00:14:32,920 --> 00:14:35,590 The place where the amino acid gets linked 263 00:14:35,590 --> 00:14:38,110 is also called the acceptor stem. 264 00:14:38,110 --> 00:14:41,020 And, up here, I show you that linkage. 265 00:14:41,020 --> 00:14:42,380 And you should-- yes? 266 00:14:42,380 --> 00:14:43,797 AUDIENCE: I was just going to ask, 267 00:14:43,797 --> 00:14:46,885 I see how the anticodons are specialized. 268 00:14:46,885 --> 00:14:50,443 How does the 3 prime end of the tRNA 269 00:14:50,443 --> 00:14:51,610 know which protein is bound? 270 00:14:51,610 --> 00:14:54,160 BARBARA IMPERIALI: Yeah, and, in a moment, not quite yet, 271 00:14:54,160 --> 00:14:56,710 I will show you structures of tRNAs 272 00:14:56,710 --> 00:14:59,530 bound to their synthetases, which 273 00:14:59,530 --> 00:15:01,160 are the enzymes that load them. 274 00:15:01,160 --> 00:15:04,000 So there is specificity throughout that whole thing. 275 00:15:04,000 --> 00:15:06,890 It's not just bystander stuffing. 276 00:15:06,890 --> 00:15:08,450 It's really involved. 277 00:15:08,450 --> 00:15:10,720 And it's a great question, and I hope 278 00:15:10,720 --> 00:15:13,150 you'll get an answer that's reasonably satisfied 279 00:15:13,150 --> 00:15:15,290 from the structure perspective. 280 00:15:15,290 --> 00:15:19,120 And so, if you look up here at the acceptor stem, 281 00:15:19,120 --> 00:15:21,910 the amino acid is joined by an ester bond 282 00:15:21,910 --> 00:15:26,590 to the 3 prime end of the transfer RNA, 283 00:15:26,590 --> 00:15:28,120 but, hopefully, you can see in here. 284 00:15:28,120 --> 00:15:30,040 There's the carboxyl. 285 00:15:30,040 --> 00:15:31,270 There's the amine. 286 00:15:31,270 --> 00:15:34,900 And CHR designates the amino acid 287 00:15:34,900 --> 00:15:37,990 where R would be the side chain of your amino acid. 288 00:15:37,990 --> 00:15:40,660 So that's what that looks like at that end of the transfer 289 00:15:40,660 --> 00:15:41,590 RNA. 290 00:15:41,590 --> 00:15:44,560 And, coming down to the anticodon loop, 291 00:15:44,560 --> 00:15:49,420 you're going to read the messenger 5 prime to 3 prime. 292 00:15:49,420 --> 00:15:52,390 And the anticodon loop, when you draw 293 00:15:52,390 --> 00:15:54,670 this in this configuration, actually 294 00:15:54,670 --> 00:15:57,880 shows you that the anticodon loop 295 00:15:57,880 --> 00:16:00,730 is antiparallel to the codon loop 296 00:16:00,730 --> 00:16:03,650 to make that good hydrogen-bonding network. 297 00:16:03,650 --> 00:16:06,790 So that's why I like to be consistent in the way I 298 00:16:06,790 --> 00:16:08,230 render this structure. 299 00:16:08,230 --> 00:16:13,195 So the anticodon loop of the RNA complements that triplet codon 300 00:16:13,195 --> 00:16:15,550 in the messenger RNA, all right? 301 00:16:15,550 --> 00:16:16,406 Yes? 302 00:16:16,406 --> 00:16:17,894 AUDIENCE: [INAUDIBLE]. 303 00:16:17,894 --> 00:16:19,393 What's between the G and T? 304 00:16:19,393 --> 00:16:21,310 BARBARA IMPERIALI: What's between the G and T? 305 00:16:21,310 --> 00:16:21,810 Hold on. 306 00:16:21,810 --> 00:16:25,282 I'm going to-- what's between the G and the-- 307 00:16:25,282 --> 00:16:26,529 AUDIENCE: On the loop there. 308 00:16:26,529 --> 00:16:27,737 BARBARA IMPERIALI: Over here? 309 00:16:27,737 --> 00:16:28,930 AUDIENCE: No, above that on the left. 310 00:16:28,930 --> 00:16:30,222 BARBARA IMPERIALI: On the left. 311 00:16:30,222 --> 00:16:32,740 Oh, this guy? 312 00:16:32,740 --> 00:16:34,180 So cool you ask that. 313 00:16:34,180 --> 00:16:38,620 So this guy is what's known as a funny base. 314 00:16:38,620 --> 00:16:42,882 It's actually-- that's the symbol for it 315 00:16:42,882 --> 00:16:43,840 that you've picked out. 316 00:16:43,840 --> 00:16:46,210 And it's a base known as pseudouridine. 317 00:16:50,920 --> 00:16:56,020 And it turns out that these unusual bases show up 318 00:16:56,020 --> 00:16:58,210 in RNA sequences. 319 00:16:58,210 --> 00:17:01,030 Pseudouridine has an interesting structure 320 00:17:01,030 --> 00:17:04,480 where the bond to the ribose ring 321 00:17:04,480 --> 00:17:06,940 is not a carbon-nitrogen bond, but, rather, 322 00:17:06,940 --> 00:17:07,960 a carbon-carbon bond. 323 00:17:07,960 --> 00:17:09,460 And it's a bit more stable. 324 00:17:09,460 --> 00:17:13,690 So, in RNA, there's some of these other unusual bases. 325 00:17:13,690 --> 00:17:17,500 And pseudouridine is the most common of the unusual ones. 326 00:17:17,500 --> 00:17:19,630 It can still hydrogen bond, but it 327 00:17:19,630 --> 00:17:22,869 tends to show up in these sort of different loops 328 00:17:22,869 --> 00:17:24,527 and turn-type places, OK? 329 00:17:24,527 --> 00:17:25,569 Thanks for noticing that. 330 00:17:25,569 --> 00:17:26,227 Yeah? 331 00:17:26,227 --> 00:17:28,060 AUDIENCE: So do you get the G in parentheses 332 00:17:28,060 --> 00:17:29,640 on the yellow part [INAUDIBLE]? 333 00:17:29,640 --> 00:17:31,960 BARBARA IMPERIALI: The G, which is in parentheses, 334 00:17:31,960 --> 00:17:34,960 designates that even those sort of bulges 335 00:17:34,960 --> 00:17:37,810 between the real loops can vary in length. 336 00:17:37,810 --> 00:17:39,490 So it could be more than one. 337 00:17:39,490 --> 00:17:42,120 So that addresses-- comes back to that other question. 338 00:17:42,120 --> 00:17:44,920 But that stuff in between has variables. 339 00:17:44,920 --> 00:17:50,920 It has variable bulges and variable shapes associated 340 00:17:50,920 --> 00:17:54,010 with the synthetase enzymes that I'll introduce in a minute 341 00:17:54,010 --> 00:17:56,680 that it recognizes, all great questions. 342 00:17:56,680 --> 00:18:01,160 OK, all right, cool. 343 00:18:01,160 --> 00:18:06,290 OK, so [HUMMING] we've got all of that. 344 00:18:06,290 --> 00:18:10,300 So now let's move ourselves so we've looked at-- 345 00:18:10,300 --> 00:18:12,250 we know the messenger well. 346 00:18:12,250 --> 00:18:14,740 We're starting to understand the tRNAs. 347 00:18:14,740 --> 00:18:16,960 What we need to move on to now is 348 00:18:16,960 --> 00:18:19,600 sort of the most important part of the game, which 349 00:18:19,600 --> 00:18:23,360 is really taking a look at the genetic code. 350 00:18:23,360 --> 00:18:30,478 So this table is but one rendition of the genetic code. 351 00:18:30,478 --> 00:18:32,770 Sometimes, you'll see it in different shapes and sizes. 352 00:18:32,770 --> 00:18:35,560 In a second, I'll show you one of the other renditions. 353 00:18:35,560 --> 00:18:37,930 But it is the absolute-- 354 00:18:37,930 --> 00:18:50,940 the sort of Rosetta Stone for translating 355 00:18:50,940 --> 00:18:59,460 messenger RNA to amino acid sequence using codons. 356 00:18:59,460 --> 00:19:03,120 So this sort of-- whoa. 357 00:19:03,120 --> 00:19:04,890 Getting a little-- I love translation. 358 00:19:04,890 --> 00:19:05,130 I'm sorry. 359 00:19:05,130 --> 00:19:07,255 I'm getting a little bit excited about translation. 360 00:19:07,255 --> 00:19:11,700 OK, so there are a few features of this genetic code. 361 00:19:11,700 --> 00:19:13,770 Number one, you won't have to remember it. 362 00:19:13,770 --> 00:19:15,960 It would be foolish for us to think you could, 363 00:19:15,960 --> 00:19:18,960 but there are characteristics about the genetic code that 364 00:19:18,960 --> 00:19:19,840 are very important. 365 00:19:19,840 --> 00:19:22,110 But, first of all, let me sort of calibrate you. 366 00:19:22,110 --> 00:19:26,310 This would be the first of the three letters in the codon. 367 00:19:26,310 --> 00:19:28,530 And, by the way, the genetic code 368 00:19:28,530 --> 00:19:31,440 gives you the identities of what are 369 00:19:31,440 --> 00:19:37,950 known as the codons, which is how we designate 370 00:19:37,950 --> 00:19:40,290 the triplet of nucleotides. 371 00:19:44,720 --> 00:19:47,680 So you'll hear a lot about codons and anticodons 372 00:19:47,680 --> 00:19:48,730 of course. 373 00:19:48,730 --> 00:19:53,530 So the way you read this table is you read the first letter. 374 00:19:53,530 --> 00:19:57,160 So all of these begin with U. All of these begin with C 375 00:19:57,160 --> 00:19:58,240 and so on. 376 00:19:58,240 --> 00:20:02,710 Then you read the second letter, and there's those designations. 377 00:20:02,710 --> 00:20:07,420 So, if I'm going here, here, it's going to be starting UU. 378 00:20:07,420 --> 00:20:09,310 And then, within each block, there 379 00:20:09,310 --> 00:20:13,690 are the four alternatives for the other four bases. 380 00:20:13,690 --> 00:20:16,120 And then the third one, basically, 381 00:20:16,120 --> 00:20:17,860 just designates those. 382 00:20:17,860 --> 00:20:22,450 So you can read, for each amino acid, what three-letter codon 383 00:20:22,450 --> 00:20:24,170 would correspond to it. 384 00:20:24,170 --> 00:20:25,850 Some people quite like-- 385 00:20:25,850 --> 00:20:27,110 oops. 386 00:20:27,110 --> 00:20:28,600 I got rid of it too fast. 387 00:20:28,600 --> 00:20:31,420 Some people quite like this other rendition 388 00:20:31,420 --> 00:20:35,160 where the first of the three letters is in the center. 389 00:20:35,160 --> 00:20:40,120 So it's either G, U, A, or C. Then the second one comes out. 390 00:20:40,120 --> 00:20:42,460 It's in the brown circle. 391 00:20:42,460 --> 00:20:47,650 And then the third one is the third letter in the codon. 392 00:20:47,650 --> 00:20:51,640 And then that tells you what amino acid that corresponds to. 393 00:20:51,640 --> 00:20:56,390 And we'll generally tend to just stick with this one table 394 00:20:56,390 --> 00:20:57,590 and stick with it. 395 00:20:57,590 --> 00:21:01,120 So you might as well get used to that particular table. 396 00:21:01,120 --> 00:21:04,750 Now there's a couple of characteristics in the table. 397 00:21:04,750 --> 00:21:08,530 The first thing to notice is that, within that table, 398 00:21:08,530 --> 00:21:12,130 there is a codon to start. 399 00:21:12,130 --> 00:21:13,510 And it's AUG. 400 00:21:13,510 --> 00:21:16,720 And the one thing that's sort of sent to drive you crazy 401 00:21:16,720 --> 00:21:28,960 is that AUG codon equals start, but it also equals methionine. 402 00:21:28,960 --> 00:21:33,640 And, in bacteria, it equals a modified version of methionine. 403 00:21:33,640 --> 00:21:37,690 So, if the ribosome is reading the messenger RNA, 404 00:21:37,690 --> 00:21:42,100 it will look for the first one of these and start reading. 405 00:21:42,100 --> 00:21:44,470 But, once you find another one of these further 406 00:21:44,470 --> 00:21:47,260 into the sequence, it will put in methionine. 407 00:21:47,260 --> 00:21:48,640 Methionine is fairly rare. 408 00:21:48,640 --> 00:21:51,240 There may only be one or two more in the protein. 409 00:21:51,240 --> 00:21:54,790 And I hate the illogic of that, but, nevertheless, it 410 00:21:54,790 --> 00:21:56,020 is the case. 411 00:21:56,020 --> 00:21:59,120 In some organisms, you start with different amino acids, 412 00:21:59,120 --> 00:22:02,800 but the most common start is the codon for methionine 413 00:22:02,800 --> 00:22:03,770 is the start. 414 00:22:03,770 --> 00:22:07,270 So what that means is that every protein you translate 415 00:22:07,270 --> 00:22:09,370 has a methionine at one of the termini. 416 00:22:09,370 --> 00:22:11,180 Which terminus would it be at? 417 00:22:11,180 --> 00:22:12,160 AUDIENCE: [INAUDIBLE] 418 00:22:12,160 --> 00:22:20,050 BARBARA IMPERIALI: Yeah, OK, so that's 419 00:22:20,050 --> 00:22:23,650 the details with methionine and the start codon. 420 00:22:23,650 --> 00:22:26,500 Then there are several stop codons. 421 00:22:26,500 --> 00:22:29,080 And I've shown you the three of them there. 422 00:22:29,080 --> 00:22:32,050 The stop codons tend to be used variously. 423 00:22:32,050 --> 00:22:35,620 Some are more predominant in some organisms than other. 424 00:22:35,620 --> 00:22:37,960 And some of them respond differently 425 00:22:37,960 --> 00:22:40,090 when the process of stopping occurs, which 426 00:22:40,090 --> 00:22:41,980 we'll talk about in a second. 427 00:22:41,980 --> 00:22:47,930 But the next important thing to notice about the genetic code 428 00:22:47,930 --> 00:22:49,960 is that it's what's called degenerate. 429 00:22:56,323 --> 00:22:57,990 Now, when you call something degenerate, 430 00:22:57,990 --> 00:23:00,323 it seems like sort of a really nasty thing to call them. 431 00:23:00,323 --> 00:23:02,460 And it doesn't mean, oh, it's so bad. 432 00:23:02,460 --> 00:23:03,510 It's just a degenerate. 433 00:23:03,510 --> 00:23:08,627 It just means it is specific, but it's not-- 434 00:23:08,627 --> 00:23:09,210 wait a minute. 435 00:23:09,210 --> 00:23:11,210 I've got the right wording here because I always 436 00:23:11,210 --> 00:23:12,390 get this sort of-- 437 00:23:12,390 --> 00:23:18,390 degenerate, ah, it's not ambiguous. 438 00:23:18,390 --> 00:23:19,813 It's degenerate. 439 00:23:19,813 --> 00:23:20,730 That's the best thing. 440 00:23:20,730 --> 00:23:23,680 But it's not ambiguous. 441 00:23:26,570 --> 00:23:28,760 What this means is there are all, 442 00:23:28,760 --> 00:23:33,110 for several amino acids, more than one codon that specify it. 443 00:23:38,900 --> 00:23:41,700 And let's take a look at some of the examples 444 00:23:41,700 --> 00:23:43,360 of that degeneracy. 445 00:23:43,360 --> 00:23:46,530 It's at its most extreme with residues 446 00:23:46,530 --> 00:23:50,490 such as leucine where there are six codons that specify 447 00:23:50,490 --> 00:23:52,200 leucine. 448 00:23:52,200 --> 00:23:55,080 Alanine has four different codons. 449 00:23:55,080 --> 00:23:57,630 They all specify alanine. 450 00:23:57,630 --> 00:23:59,760 Lysine has two codons. 451 00:23:59,760 --> 00:24:02,850 And it's generally appreciated that the residues that 452 00:24:02,850 --> 00:24:07,350 have more codons tend to be the more common in your messenger 453 00:24:07,350 --> 00:24:11,310 RNAs, in your final protein sequence, because you'll find, 454 00:24:11,310 --> 00:24:13,930 in your protein, a lot of leucines. 455 00:24:13,930 --> 00:24:17,550 So we need a few more codons to stack the deck in favor 456 00:24:17,550 --> 00:24:19,720 of putting in more leucines. 457 00:24:19,720 --> 00:24:21,610 But so it's ambiguous-- 458 00:24:21,610 --> 00:24:23,610 it's degenerate. 459 00:24:23,610 --> 00:24:26,010 But, what I mean by it's not ambiguous, 460 00:24:26,010 --> 00:24:28,680 you know what it's going to code for. 461 00:24:28,680 --> 00:24:33,690 The other place where degenerate codons can become important 462 00:24:33,690 --> 00:24:35,910 is that there is species specificity. 463 00:24:39,582 --> 00:24:42,040 So I'm going to write that in here and explain what I mean. 464 00:24:47,510 --> 00:24:51,680 And what that means is that some organisms might prefer 465 00:24:51,680 --> 00:24:54,320 two or three of the degenerate codons, 466 00:24:54,320 --> 00:24:57,110 and others may prefer a couple of the others. 467 00:24:57,110 --> 00:24:59,480 And what that means, once in the laboratory, 468 00:24:59,480 --> 00:25:01,490 is it's really annoying if we want 469 00:25:01,490 --> 00:25:05,650 to express a protein in a really convenient bacterial system 470 00:25:05,650 --> 00:25:08,030 that we've taken from a mammalian system. 471 00:25:08,030 --> 00:25:11,210 Our codon mix may not be the same. 472 00:25:11,210 --> 00:25:13,490 And so there are companies now that 473 00:25:13,490 --> 00:25:16,670 actually will fix the codons in a gene for you 474 00:25:16,670 --> 00:25:19,550 to make them compatible with a different organism 475 00:25:19,550 --> 00:25:20,450 for expression. 476 00:25:20,450 --> 00:25:22,580 So it has huge practical implications 477 00:25:22,580 --> 00:25:23,990 to be quite honest. 478 00:25:23,990 --> 00:25:27,930 And it can be very annoying in the laboratory. 479 00:25:27,930 --> 00:25:34,700 OK, and then so codon usage varies amongst organisms. 480 00:25:34,700 --> 00:25:37,850 All right, so now, one last thing, so we've 481 00:25:37,850 --> 00:25:41,530 talked about the genetic code. 482 00:25:41,530 --> 00:25:45,220 It's the code that's going to be embedded within the messenger 483 00:25:45,220 --> 00:25:47,440 RNA. 484 00:25:47,440 --> 00:25:50,470 The last thing I want to do is, basically, explain to you 485 00:25:50,470 --> 00:25:52,330 one more time, when you're looking 486 00:25:52,330 --> 00:25:54,940 to read what your amino acids that get put in 487 00:25:54,940 --> 00:25:57,520 may be, you're going to look at the codon. 488 00:25:57,520 --> 00:26:01,050 And it will tell you exactly the amino acid. 489 00:26:01,050 --> 00:26:03,340 A long time ago, I used to be confused 490 00:26:03,340 --> 00:26:05,650 because I thought I should be looking at the anticodon. 491 00:26:05,650 --> 00:26:07,422 And I was trying to translate everything, 492 00:26:07,422 --> 00:26:08,380 and it was a real mess. 493 00:26:08,380 --> 00:26:10,690 But it's the genetic code in that box 494 00:26:10,690 --> 00:26:12,760 that I just showed you is written down 495 00:26:12,760 --> 00:26:16,480 for maximum clarity and ease of use. 496 00:26:16,480 --> 00:26:20,320 So, whenever you see a particular three-letter code 497 00:26:20,320 --> 00:26:22,690 on the messenger, you will then be 498 00:26:22,690 --> 00:26:25,750 able to know what amino acid it would code. 499 00:26:25,750 --> 00:26:27,320 So this is kind of interesting. 500 00:26:27,320 --> 00:26:31,800 It just reinforces to you that the codon and the anticodon 501 00:26:31,800 --> 00:26:33,920 are antiparallel. 502 00:26:33,920 --> 00:26:37,490 And so what I want to do is, basically, in this diagram, 503 00:26:37,490 --> 00:26:41,620 you would be reading from the 5 prime to the 3 prime end, 504 00:26:41,620 --> 00:26:45,820 as the transfer RNAs attach to the messenger RNA. 505 00:26:45,820 --> 00:26:51,310 And that would give you a codon here that would be AUC. 506 00:26:51,310 --> 00:26:53,260 If you'd written this the wrong way round, 507 00:26:53,260 --> 00:26:54,670 it would look like CUA. 508 00:26:54,670 --> 00:26:57,580 So you really want to be reading 5 prime to 3 prime 509 00:26:57,580 --> 00:26:58,700 in the codon. 510 00:26:58,700 --> 00:27:01,990 And then you can go to your favorite genetic code map 511 00:27:01,990 --> 00:27:05,080 and say this thing is read 5 prime to 3 prime. 512 00:27:05,080 --> 00:27:10,690 And the amino acid that gets put in is isoleucine, OK? 513 00:27:10,690 --> 00:27:15,250 So you'll need to be able to do that quite readily. 514 00:27:15,250 --> 00:27:19,780 All right, next portion, those monster-- 515 00:27:19,780 --> 00:27:21,460 loading the amino acid. 516 00:27:21,460 --> 00:27:24,920 OK, so, as I said, this is a many building block, many parts 517 00:27:24,920 --> 00:27:25,420 thing. 518 00:27:25,420 --> 00:27:26,980 So we've got the transfer RNAs. 519 00:27:26,980 --> 00:27:31,540 We know where we load them onto the amino acids. 520 00:27:31,540 --> 00:27:35,620 We know where we load the amino acids onto the transfer RNA. 521 00:27:35,620 --> 00:27:38,020 But we do not know how that is done. 522 00:27:38,020 --> 00:27:40,840 So I want to show you briefly how that occurs. 523 00:27:40,840 --> 00:27:45,730 So let me just start by drawing transfer RNAs the way 524 00:27:45,730 --> 00:27:54,210 I usually draw them in a very sort of cartoon form. 525 00:27:54,210 --> 00:28:00,450 And, to attach an amino acid to the 3 prime end of the transfer 526 00:28:00,450 --> 00:28:04,380 RNA, you have an amino acid residue-- 527 00:28:04,380 --> 00:28:07,355 we're just going to go R here-- 528 00:28:07,355 --> 00:28:10,830 carboxylic acid, amine. 529 00:28:10,830 --> 00:28:14,130 Actually, I'm going to draw them in their appropriate charged 530 00:28:14,130 --> 00:28:15,230 states. 531 00:28:15,230 --> 00:28:17,970 And what I need to be able to do is faithfully 532 00:28:17,970 --> 00:28:25,640 fix this amino acid to the 3 prime OH of the transfer RNA. 533 00:28:25,640 --> 00:28:28,250 And what we do is we need adenosine 534 00:28:28,250 --> 00:28:34,070 triphosphate to activate this chemistry. 535 00:28:34,070 --> 00:28:39,768 And then the OH at the 3 prime end reacts with the amino acid. 536 00:28:39,768 --> 00:28:41,060 So I'm just going to draw that. 537 00:28:41,060 --> 00:28:44,990 I'm leaving out steps because, otherwise, it's too many, 538 00:28:44,990 --> 00:28:46,400 and you'll be cross with me. 539 00:28:59,080 --> 00:29:02,920 So you attach through an ester to the amino acid 540 00:29:02,920 --> 00:29:06,610 from the 3 prime end of these transfer RNA. 541 00:29:06,610 --> 00:29:07,690 That's what's done. 542 00:29:07,690 --> 00:29:10,510 The ATP makes this chemistry feasible, 543 00:29:10,510 --> 00:29:12,570 but there's one more player here. 544 00:29:12,570 --> 00:29:15,560 And that's the enzyme that brings them all together, 545 00:29:15,560 --> 00:29:31,600 which is known as an aminoacyl-tRNA synthetase, 546 00:29:31,600 --> 00:29:36,880 meaning it's an enzyme that makes an aminoacyl-tRNA. 547 00:29:36,880 --> 00:29:38,270 And it synthetases it. 548 00:29:38,270 --> 00:29:41,690 So that's how its name gets subtracted. 549 00:29:41,690 --> 00:29:45,370 So, with reference to an earlier question, what I'm showing 550 00:29:45,370 --> 00:29:48,010 you here are different synthetases 551 00:29:48,010 --> 00:29:50,680 for different amino acids that show you 552 00:29:50,680 --> 00:29:54,130 that there's a recognition not just for the amino acid 553 00:29:54,130 --> 00:29:55,600 that's being loaded, but, rather, 554 00:29:55,600 --> 00:29:58,100 for the entire transfer RNA. 555 00:29:58,100 --> 00:30:00,280 So some of these look quite different. 556 00:30:00,280 --> 00:30:04,450 The isoleucine one interacts in one way. 557 00:30:04,450 --> 00:30:06,580 The valine one is a little different. 558 00:30:06,580 --> 00:30:08,860 And the glutamine one is different again. 559 00:30:08,860 --> 00:30:13,630 So they vary in the way they interact with the transfer 560 00:30:13,630 --> 00:30:14,180 RNAs. 561 00:30:14,180 --> 00:30:16,750 So the transfer RNAs are specific 562 00:30:16,750 --> 00:30:19,660 for the amino acid that is loaded onto them, 563 00:30:19,660 --> 00:30:22,840 but also for the synthetase that does the loading. 564 00:30:22,840 --> 00:30:24,940 That's how you get the specificity. 565 00:30:24,940 --> 00:30:27,940 Does that address your question from earlier? 566 00:30:27,940 --> 00:30:28,480 Hello? 567 00:30:28,480 --> 00:30:29,840 OK. 568 00:30:29,840 --> 00:30:30,840 That makes sense, right? 569 00:30:30,840 --> 00:30:33,760 So they fall into a lot of different families, 570 00:30:33,760 --> 00:30:35,470 but they're quite varied when you 571 00:30:35,470 --> 00:30:38,770 look at specific interactions between the synthetase 572 00:30:38,770 --> 00:30:40,110 and the amino acid. 573 00:30:40,110 --> 00:30:42,820 And, on those synthetases, there will also 574 00:30:42,820 --> 00:30:46,420 be specificity for the amino acid side 575 00:30:46,420 --> 00:30:50,380 chain of the amino acid that gets loaded. 576 00:30:50,380 --> 00:30:52,660 Now let's look at the ribosome components 577 00:30:52,660 --> 00:30:55,390 because they're the last big monsters. 578 00:30:55,390 --> 00:31:00,580 Ribosomes have a small and a large-- 579 00:31:00,580 --> 00:31:03,340 that's the large and a small subunit. 580 00:31:03,340 --> 00:31:06,880 They are made up, as I mentioned over here, 581 00:31:06,880 --> 00:31:17,700 of RNA and protein, so small and large subunits. 582 00:31:17,700 --> 00:31:21,220 And I've shown them there in two different colors. 583 00:31:21,220 --> 00:31:24,010 The prokaryotic ribosomes are pretty 584 00:31:24,010 --> 00:31:26,170 different from the eukaryotic ones. 585 00:31:26,170 --> 00:31:29,170 There's a higher proportion of RNA 586 00:31:29,170 --> 00:31:32,710 in the prokaryotic ones than the eukaryotic ones, which 587 00:31:32,710 --> 00:31:34,730 is kind of interesting. 588 00:31:34,730 --> 00:31:36,970 And these complexes are so big, and they're 589 00:31:36,970 --> 00:31:39,970 made up of so many protein and RNA 590 00:31:39,970 --> 00:31:42,550 strands, that we don't so much measure them 591 00:31:42,550 --> 00:31:44,860 by the number of those components, 592 00:31:44,860 --> 00:31:47,470 but, rather, by what's known as their sedimentation 593 00:31:47,470 --> 00:31:50,590 coefficient, which gives us a sense of the weight 594 00:31:50,590 --> 00:31:53,120 of the module or the complex. 595 00:31:53,120 --> 00:31:54,455 So the small subunit-- 596 00:31:58,260 --> 00:32:01,850 oh, he's back [INAUDIBLE]. 597 00:32:01,850 --> 00:32:07,380 The small subunit would have a 30S sedimentation coefficient. 598 00:32:07,380 --> 00:32:10,010 The S stands for Svedberg units. 599 00:32:10,010 --> 00:32:13,490 It's how fast its sediments in an ultra centrifuge. 600 00:32:13,490 --> 00:32:17,840 And the large subunit would have a 50S sedimentation 601 00:32:17,840 --> 00:32:18,920 coefficient. 602 00:32:18,920 --> 00:32:21,800 And those correspond to a certain number of daltons. 603 00:32:21,800 --> 00:32:25,668 If you see that S term after a number, 604 00:32:25,668 --> 00:32:26,960 that's what it's talking about. 605 00:32:26,960 --> 00:32:29,002 It's talking about the sedimentation coefficient, 606 00:32:29,002 --> 00:32:31,775 which gives us a sense of the size of the complex. 607 00:32:35,140 --> 00:32:38,950 And, when you start to bring all the pieces together now, 608 00:32:38,950 --> 00:32:42,820 what we can see on this slide is the messenger, the transfer 609 00:32:42,820 --> 00:32:48,280 RNAs, and the ribosomal all to scale in such a way 610 00:32:48,280 --> 00:32:50,110 that it can really explain it. 611 00:32:50,110 --> 00:32:54,580 So what I show you here is the small and large subunit. 612 00:32:54,580 --> 00:32:58,270 In orange-- well, that's kind of a burnt orange-- 613 00:32:58,270 --> 00:33:01,030 is a sneaky little bit of the messenger. 614 00:33:01,030 --> 00:33:03,640 In yellow are the transfer RNAs. 615 00:33:03,640 --> 00:33:05,410 And there's one more unit on here 616 00:33:05,410 --> 00:33:07,330 that I won't describe too much. 617 00:33:07,330 --> 00:33:11,230 It's a protein factor that helps all the processes occur. 618 00:33:11,230 --> 00:33:15,610 Generally, it's thought to help the loaded tRNA come 619 00:33:15,610 --> 00:33:18,790 to the ribosome, get it in place, and then go away. 620 00:33:18,790 --> 00:33:20,380 So it's some of these extra helper 621 00:33:20,380 --> 00:33:22,330 proteins that are involved. 622 00:33:22,330 --> 00:33:24,070 OK, so let's build a protein because I 623 00:33:24,070 --> 00:33:26,200 know it's the moment we've all been waiting for, 624 00:33:26,200 --> 00:33:29,770 and we're going to walk through how those pieces come together. 625 00:33:29,770 --> 00:33:32,860 It's a very clunky animation, but I'm very proud of myself 626 00:33:32,860 --> 00:33:34,340 because I did it myself. 627 00:33:34,340 --> 00:33:36,700 So I'm just going to show you how these things happen, 628 00:33:36,700 --> 00:33:40,120 as you assemble a chunk of polypeptide chain 629 00:33:40,120 --> 00:33:41,310 from a messenger. 630 00:33:41,310 --> 00:33:43,450 And so here's the messenger. 631 00:33:43,450 --> 00:33:45,970 It's being read 5 prime to 3 prime. 632 00:33:45,970 --> 00:33:49,720 What happens, first of all, is that the small subunit 633 00:33:49,720 --> 00:33:52,480 kind of floats along, looking for the place that 634 00:33:52,480 --> 00:33:56,350 would be the ribosome binding site that I mentioned here, 635 00:33:56,350 --> 00:33:59,920 and then sliding its way to position the start 636 00:33:59,920 --> 00:34:03,520 codon in the right place to start the synthesis. 637 00:34:03,520 --> 00:34:06,820 Once that happens, the methionine 638 00:34:06,820 --> 00:34:11,710 that's on its tRNA, the start one, gets into place. 639 00:34:11,710 --> 00:34:14,370 And, at the same time, the large subunit, 640 00:34:14,370 --> 00:34:17,469 completing the ribosome complex, comes together. 641 00:34:17,469 --> 00:34:22,120 So you now have large and small ribosomal complexes 642 00:34:22,120 --> 00:34:25,600 stuck onto that messenger, ready to carry on. 643 00:34:25,600 --> 00:34:27,850 And, in each case, you're translocating 644 00:34:27,850 --> 00:34:30,469 through the messenger RNA. 645 00:34:30,469 --> 00:34:34,000 And, in each step, you're bringing in a tRNA 646 00:34:34,000 --> 00:34:38,840 that's loaded with an amino acid where the anticodon 647 00:34:38,840 --> 00:34:41,350 of the amino acid-- and I've got those little letters shown 648 00:34:41,350 --> 00:34:42,610 on the bottom here-- 649 00:34:42,610 --> 00:34:45,460 is complementary to the codon that's 650 00:34:45,460 --> 00:34:47,420 within the messenger RNA. 651 00:34:47,420 --> 00:34:50,530 So we can start building this protein. 652 00:34:50,530 --> 00:34:52,150 AUG is that start codon. 653 00:34:52,150 --> 00:34:54,190 Methionine is the first amino acid. 654 00:34:54,190 --> 00:34:56,409 And it's always at the N-terminus. 655 00:34:56,409 --> 00:34:59,080 Then there's another amino acid comes in. 656 00:34:59,080 --> 00:35:03,550 The codon was UUU, and that corresponds to phenylalanine. 657 00:35:03,550 --> 00:35:05,410 And then the next thing that happens 658 00:35:05,410 --> 00:35:09,040 is there's a movement such that a new bond gets-- 659 00:35:09,040 --> 00:35:12,880 a new amide bond is formed between the methionine 660 00:35:12,880 --> 00:35:14,020 and the phenylalanine. 661 00:35:16,960 --> 00:35:18,920 And that new bond is an amide bond. 662 00:35:18,920 --> 00:35:20,240 It's not any of the others. 663 00:35:20,240 --> 00:35:25,360 So you're literally intercepting this complex on the transfer 664 00:35:25,360 --> 00:35:28,488 RNA with the amine of a new amino acid. 665 00:35:28,488 --> 00:35:29,780 That's how that comes together. 666 00:35:29,780 --> 00:35:33,160 I'm not going to worry you too much with the chemical details, 667 00:35:33,160 --> 00:35:36,310 but a lot of them have been illuminated by having 668 00:35:36,310 --> 00:35:38,270 the structure of the ribosome. 669 00:35:38,270 --> 00:35:41,980 And it shows you, in fact, that it's not proteins that 670 00:35:41,980 --> 00:35:43,570 are catalyzing that reaction. 671 00:35:43,570 --> 00:35:45,170 It's nucleic acids. 672 00:35:45,170 --> 00:35:49,150 So now I'm just going to move you on through the synthesis, 673 00:35:49,150 --> 00:35:53,560 in each case, building that polypeptide chain 674 00:35:53,560 --> 00:35:56,770 and then moving, translocating. 675 00:35:56,770 --> 00:35:57,640 This guy leaves. 676 00:35:57,640 --> 00:35:59,080 The ribosome moves along. 677 00:36:02,770 --> 00:36:05,980 This was a good Saturday afternoon's work. 678 00:36:05,980 --> 00:36:07,923 We've got to use up all those tRNAs. 679 00:36:07,923 --> 00:36:10,090 It doesn't get interesting until you get to the end. 680 00:36:10,090 --> 00:36:14,440 So we're using them, but now we come and hit a stop codon. 681 00:36:14,440 --> 00:36:15,910 We have UAG. 682 00:36:15,910 --> 00:36:17,410 So what happens here? 683 00:36:17,410 --> 00:36:21,460 The whole process slows down because you can either 684 00:36:21,460 --> 00:36:25,630 load what's known as a suppressor tRNA-- 685 00:36:25,630 --> 00:36:28,300 that's the one, the magenta one, that went up there-- 686 00:36:28,300 --> 00:36:32,710 or a protein release factor can come in and bind. 687 00:36:32,710 --> 00:36:36,250 But, in either case, there's no new amino acid to come in, 688 00:36:36,250 --> 00:36:39,850 and translation finishes and releases 689 00:36:39,850 --> 00:36:44,170 the protein in its complete form. 690 00:36:44,170 --> 00:36:46,780 Now the reason I want to differentiate 691 00:36:46,780 --> 00:36:51,160 between the release factor and an RNA that 692 00:36:51,160 --> 00:36:54,070 has the complementary stop codon is 693 00:36:54,070 --> 00:36:58,240 because the RNAs that are known as the suppressor RNAs 694 00:36:58,240 --> 00:37:00,670 have now been completely hijacked 695 00:37:00,670 --> 00:37:03,550 to make an enhanced genetic code where 696 00:37:03,550 --> 00:37:06,970 we can load lots of different amino acids using 697 00:37:06,970 --> 00:37:08,650 the suppressor RNAs. 698 00:37:08,650 --> 00:37:11,050 And, if anyone would like to chat to me about this, 699 00:37:11,050 --> 00:37:13,990 I'd love to because I think it's a fascinating field, 700 00:37:13,990 --> 00:37:16,110 but it's a bit beyond the scope of. 701 00:37:16,110 --> 00:37:20,290 OK, so this entire process took the following 702 00:37:20,290 --> 00:37:24,010 where you've got small and large subunits, all the elongation 703 00:37:24,010 --> 00:37:26,950 factors and initiation and release. 704 00:37:26,950 --> 00:37:28,630 And then, in each case, the energy 705 00:37:28,630 --> 00:37:30,850 is actually not provided by ATP. 706 00:37:30,850 --> 00:37:32,890 It's provided by GTP. 707 00:37:32,890 --> 00:37:37,120 But where ATP is important is in loading the amino acids 708 00:37:37,120 --> 00:37:39,290 onto the transfer RNAs. 709 00:37:39,290 --> 00:37:43,780 This occurs at about a rate of 20 amino acids per second, 710 00:37:43,780 --> 00:37:49,210 meaning you're reading about 60 bases per second, which 711 00:37:49,210 --> 00:37:52,750 is pretty consistent with the rate of transcription, 712 00:37:52,750 --> 00:37:54,250 not the rate of replication. 713 00:37:54,250 --> 00:37:56,380 That's far faster. 714 00:37:56,380 --> 00:38:00,400 OK, sometimes, when you're making a lot of a protein, 715 00:38:00,400 --> 00:38:05,920 you will see that ribosomes line up on the messenger RNA. 716 00:38:05,920 --> 00:38:07,600 And you'll have many proteins being 717 00:38:07,600 --> 00:38:10,600 made at once and at different stages in the game. 718 00:38:10,600 --> 00:38:13,810 And this is a lovely electron micrograph that actually 719 00:38:13,810 --> 00:38:15,970 shows this process in action. 720 00:38:15,970 --> 00:38:19,150 And, when you have a lot of ribosomes on one messenger, 721 00:38:19,150 --> 00:38:20,560 we call them polysomes. 722 00:38:20,560 --> 00:38:21,760 They have that name. 723 00:38:21,760 --> 00:38:26,980 OK, good, all right, so what I want to do now-- so 724 00:38:26,980 --> 00:38:28,390 does everyone feel good about how 725 00:38:28,390 --> 00:38:32,530 you translate a messenger into a protein and the various moving 726 00:38:32,530 --> 00:38:33,490 parts? 727 00:38:33,490 --> 00:38:36,070 You would always know the amino acid structures, 728 00:38:36,070 --> 00:38:37,630 have the genetic code. 729 00:38:37,630 --> 00:38:39,850 Just be familiar with reading it and making 730 00:38:39,850 --> 00:38:43,030 sure you could pick out which amino acid might 731 00:38:43,030 --> 00:38:47,590 be incorporated in response to which particular codon. 732 00:38:47,590 --> 00:38:50,020 So, when proteins are made on the ribosome, 733 00:38:50,020 --> 00:38:51,500 they have a bit of a choice. 734 00:38:51,500 --> 00:38:53,920 They can get made and fold beautifully 735 00:38:53,920 --> 00:38:55,930 into active proteins. 736 00:38:55,930 --> 00:38:57,570 Those proteins could be modified. 737 00:38:57,570 --> 00:39:00,520 They could go to different places in the cell. 738 00:39:00,520 --> 00:39:03,010 Occasionally, proteins misfold. 739 00:39:03,010 --> 00:39:05,380 Maybe the rate of synthesis is too fast, 740 00:39:05,380 --> 00:39:07,240 or the environment isn't right. 741 00:39:07,240 --> 00:39:10,750 So there will need to be mechanisms whereby proteins 742 00:39:10,750 --> 00:39:13,660 get degraded if they're not folded properly, 743 00:39:13,660 --> 00:39:15,610 but that's the story for another day. 744 00:39:15,610 --> 00:39:18,880 It's not always perfect, but what is known now 745 00:39:18,880 --> 00:39:21,940 is that, as proteins are emerging from the ribosome, 746 00:39:21,940 --> 00:39:25,000 they're starting to fold almost immediately 747 00:39:25,000 --> 00:39:27,790 from that N-terminus to, ultimately, 748 00:39:27,790 --> 00:39:29,800 attain their compact shape. 749 00:39:29,800 --> 00:39:31,300 The last thing I want to talk to you 750 00:39:31,300 --> 00:39:34,090 about today is what happens-- what 751 00:39:34,090 --> 00:39:37,640 are the types of errors we get in translation. 752 00:39:37,640 --> 00:39:41,170 Now translation, like transcription and regulation, 753 00:39:41,170 --> 00:39:44,470 has some editing mechanisms to fix errors, 754 00:39:44,470 --> 00:39:47,590 but, occasionally, there are errors in the DNA that 755 00:39:47,590 --> 00:39:49,750 put different amino acids. 756 00:39:49,750 --> 00:39:54,190 And the editing at that stage, by the way, to fix errors 757 00:39:54,190 --> 00:39:57,010 is editing when you've loaded the wrong amino acid 758 00:39:57,010 --> 00:39:58,690 onto a tRNA. 759 00:39:58,690 --> 00:40:03,190 But what happens when the DNA message, the DNA starting 760 00:40:03,190 --> 00:40:06,550 point, is wrong, which means the messenger is wrong, 761 00:40:06,550 --> 00:40:11,320 which then means we get a different translated proteins? 762 00:40:11,320 --> 00:40:14,800 So I'm just going to give you a couple of terms here. 763 00:40:14,800 --> 00:40:17,740 When we have here-- and, on all these slides, 764 00:40:17,740 --> 00:40:20,320 I'm going to show you the double-stranded DNA. 765 00:40:20,320 --> 00:40:22,900 I'm going to show you the messenger that gets made 766 00:40:22,900 --> 00:40:24,460 and the protein that comes forward. 767 00:40:24,460 --> 00:40:28,090 And they're all lined up so you can follow them really nicely, 768 00:40:28,090 --> 00:40:31,270 always writing 5 prime to 3 prime, except when 769 00:40:31,270 --> 00:40:32,950 we have double-stranded because we 770 00:40:32,950 --> 00:40:36,830 have to put the bottom strand in a different order. 771 00:40:36,830 --> 00:40:39,220 First of all, this entire chunk would 772 00:40:39,220 --> 00:40:41,840 be called the reading frame. 773 00:40:41,840 --> 00:40:44,350 It's the portion of DNA that's going 774 00:40:44,350 --> 00:40:48,115 to be read and transcribed into the messenger RNA. 775 00:40:50,930 --> 00:40:52,733 And, when you look at the two strands, 776 00:40:52,733 --> 00:40:54,400 you're going to have to figure out which 777 00:40:54,400 --> 00:40:56,560 one is the template strand. 778 00:40:56,560 --> 00:41:00,580 And you would figure that out by knowing that you read 3 prime 779 00:41:00,580 --> 00:41:05,720 to 5 prime, but that you transcribe 5 prime to 3 prime, 780 00:41:05,720 --> 00:41:06,220 right? 781 00:41:06,220 --> 00:41:09,150 So you could recognize which of the two strands 782 00:41:09,150 --> 00:41:11,470 you're going to make the messenger out of. 783 00:41:11,470 --> 00:41:13,780 Once you get the messenger, you come straight 784 00:41:13,780 --> 00:41:16,750 to the genetic code because you can use the genetic code 785 00:41:16,750 --> 00:41:18,340 to translate each of these. 786 00:41:18,340 --> 00:41:19,810 Does that make sense? 787 00:41:19,810 --> 00:41:21,250 So this would be a situation where 788 00:41:21,250 --> 00:41:22,990 you have a wild-type enzyme. 789 00:41:22,990 --> 00:41:27,520 Everything is transcribed and translated properly. 790 00:41:27,520 --> 00:41:29,620 Occasionally, though, there are errors 791 00:41:29,620 --> 00:41:33,530 that will introduce defects into the ultimate protein. 792 00:41:33,530 --> 00:41:37,480 The first type of error is a nonsense mutation, 793 00:41:37,480 --> 00:41:41,530 which might be leaving out a base pair, 794 00:41:41,530 --> 00:41:43,870 inserting one, substituting one. 795 00:41:43,870 --> 00:41:47,650 And this would, ultimately, cause an error 796 00:41:47,650 --> 00:41:52,760 in the DNA that then causes an error in the messenger RNA. 797 00:41:52,760 --> 00:41:56,620 So let's say we delete by mistake, 798 00:41:56,620 --> 00:41:59,720 or we're missing the C-G base pair. 799 00:41:59,720 --> 00:42:02,520 Then the messenger RNA is now different. 800 00:42:02,520 --> 00:42:06,430 And a lot of the base pairs-- the bases slide up 801 00:42:06,430 --> 00:42:07,700 to fill the gap. 802 00:42:07,700 --> 00:42:11,560 So we have what's known as a frame-shift mutation. 803 00:42:11,560 --> 00:42:13,260 We've shifted things. 804 00:42:13,260 --> 00:42:17,890 And what we've done by doing that is introduce wrong codons. 805 00:42:17,890 --> 00:42:21,670 So the first codon was read properly. 806 00:42:21,670 --> 00:42:25,210 The next one was read properly, even though there 807 00:42:25,210 --> 00:42:29,050 was a mistake, but then, all of a sudden, we get in this mess 808 00:42:29,050 --> 00:42:32,410 where one of the later codons carries a mistake 809 00:42:32,410 --> 00:42:34,580 and becomes a stop codon. 810 00:42:34,580 --> 00:42:36,220 OK, does everybody see that? 811 00:42:36,220 --> 00:42:39,730 So that would be called a frame-shift mutation that 812 00:42:39,730 --> 00:42:44,260 introduces a nonsense mutation and puts a stop codon 813 00:42:44,260 --> 00:42:46,510 into your messenger RNA, OK? 814 00:42:49,690 --> 00:42:52,270 The next types of mistakes are ones 815 00:42:52,270 --> 00:42:54,880 that are what are called silent mutations. 816 00:42:54,880 --> 00:42:58,900 It doesn't matter, for example, if you made an error 817 00:42:58,900 --> 00:43:03,790 in the DNA, which ended up with an error in the messenger RNA, 818 00:43:03,790 --> 00:43:07,420 if you still code the same amino acid, right? 819 00:43:07,420 --> 00:43:08,620 I go to the genetic code. 820 00:43:08,620 --> 00:43:10,690 I'm like, oh my goodness, I've got a mutation. 821 00:43:10,690 --> 00:43:11,800 Oh, it's fine. 822 00:43:11,800 --> 00:43:16,430 CCC codes for proline, but CCA also codes for proline. 823 00:43:16,430 --> 00:43:19,970 So that's called a silent mutation, OK? 824 00:43:19,970 --> 00:43:24,080 Then the last ones are the ones where 825 00:43:24,080 --> 00:43:27,350 we start to encounter errors in DNA that 826 00:43:27,350 --> 00:43:30,530 result in errors in proteins that may cause 827 00:43:30,530 --> 00:43:32,580 genetically inherited diseases. 828 00:43:32,580 --> 00:43:34,040 So let's take a look at that. 829 00:43:34,040 --> 00:43:38,090 Here it's a missense mutation where 830 00:43:38,090 --> 00:43:40,220 we've got an error in the DNA, which 831 00:43:40,220 --> 00:43:44,130 has resulted in an error in the messenger RNA. 832 00:43:44,130 --> 00:43:45,800 So, instead of valine-- 833 00:43:45,800 --> 00:43:48,410 instead of leucine, we've put in valine. 834 00:43:48,410 --> 00:43:51,380 That's not so bad because it's quite what 835 00:43:51,380 --> 00:43:53,420 we would call conservative. 836 00:43:53,420 --> 00:43:55,650 They're sort of similar amino acids. 837 00:43:55,650 --> 00:43:57,620 They have similar personalities. 838 00:43:57,620 --> 00:44:01,340 So this is a missense mutation, but it 839 00:44:01,340 --> 00:44:05,810 doesn't cause any dramatic changes, probably, in the DNA. 840 00:44:05,810 --> 00:44:07,370 And then the last one I want to show 841 00:44:07,370 --> 00:44:11,840 you is a missense mutation that is non-conservative 842 00:44:11,840 --> 00:44:14,070 and causes a serious defect. 843 00:44:14,070 --> 00:44:17,150 And this takes you back to the beginning of the class 844 00:44:17,150 --> 00:44:19,940 where we've put a mistake in the sequence 845 00:44:19,940 --> 00:44:23,045 where we've changed a glycine to an arginine. 846 00:44:23,045 --> 00:44:24,620 And that's a big change. 847 00:44:24,620 --> 00:44:28,210 And I just want to remind you of the situation in hemoglobin 848 00:44:28,210 --> 00:44:33,140 when we had a missense mutation, and we incorporated a valine 849 00:44:33,140 --> 00:44:36,560 instead of a glutamic acid, just through one change 850 00:44:36,560 --> 00:44:39,920 in the DNA, which made one change in the messenger, which 851 00:44:39,920 --> 00:44:43,340 put a drastic change in the protein that caused sickle cell 852 00:44:43,340 --> 00:44:44,570 anemia. 853 00:44:44,570 --> 00:44:46,070 So missense mutations are where you 854 00:44:46,070 --> 00:44:48,050 put in the wrong amino acids. 855 00:44:48,050 --> 00:44:51,410 And those are the ones where you end up, in a lot of cases, 856 00:44:51,410 --> 00:44:53,520 with inherited diseases. 857 00:44:53,520 --> 00:44:55,550 Nonsense mutations are not so bad 858 00:44:55,550 --> 00:44:58,040 because you probably just truncate the protein. 859 00:44:58,040 --> 00:45:02,640 You don't-- nonsense mutations aren't so bad because you end 860 00:45:02,640 --> 00:45:06,150 up with a truncated protein, which would be degraded. 861 00:45:06,150 --> 00:45:09,317 The missense mutations are the more serious ones 862 00:45:09,317 --> 00:45:11,400 because you end up with a full length protein that 863 00:45:11,400 --> 00:45:13,110 might have a mistake in it. 864 00:45:13,110 --> 00:45:15,570 And then that would affect the function. 865 00:45:15,570 --> 00:45:17,620 Am I being clear enough to everyone? 866 00:45:17,620 --> 00:45:18,120 Yeah? 867 00:45:18,120 --> 00:45:18,620 Good. 868 00:45:18,620 --> 00:45:22,440 OK, I am going to tell you that I'm handing over the baton 869 00:45:22,440 --> 00:45:25,350 to my colleague, Professor Martin. 870 00:45:25,350 --> 00:45:27,270 He'll take over on Monday. 871 00:45:27,270 --> 00:45:28,680 Mouse is pretty happy. 872 00:45:28,680 --> 00:45:30,630 He's pretty excited about genetics. 873 00:45:30,630 --> 00:45:32,770 And these will be the lectures that will occur. 874 00:45:32,770 --> 00:45:33,270 What? 875 00:45:33,270 --> 00:45:34,830 You haven't seen him, have you? 876 00:45:34,830 --> 00:45:36,962 He's keen on genetics, yeah. 877 00:45:36,962 --> 00:45:37,848 AUDIENCE: Great. 878 00:45:37,848 --> 00:45:39,390 BARBARA IMPERIALI: OK, and that's it. 879 00:45:39,390 --> 00:45:43,350 Don't forget my office hours on Monday if you need them. 880 00:45:43,350 --> 00:45:44,850 I think this field is fascinating. 881 00:45:44,850 --> 00:45:47,160 Once you get used to the mechanics of it, 882 00:45:47,160 --> 00:45:50,160 it's really cool to think of how you go from DNA 883 00:45:50,160 --> 00:45:53,390 to RNA to folded proteins.