Here we're going to start talking about the translation process and kind of give you a sense of how a gene is translated into a protein. We're not going to get into the machinery of it yet, that'll be in the fourth recording. Here we're just talking about like a kind of big picture how it works and how different mutations can lead to changes mutations in DNA can lead to changes in a person's phenotype what you observe for example how does a mutated gene result in something like cancer I mentioned that the words of the RNA language were codons these are three base pair sequences in each codon can be translated into an amino acid And this code, this translation pattern, is nearly universally conserved, which means it is true for almost all living things that a UUU is translated into a phenylalanine. And so let's talk about how to use this table because it's important that you understand how. This is going to be a huge part of your recitation on transcription and translation.
and then it's definitely going to come up on the exam as well. So for any codon, you take the first letter right here, and that tells you which row you're looking at. So for example, if we have the first letter C, then we're going to look in this row.
And then for the second letter, that's going to give us our column. If the second letter is A, then we look in this column. This row, this column, we're in this box.
And then we go to the third letter. Let's say it is C. CAC codes for the amino acid histidine. And you can do that for any three-letter pair. There are a couple of special codons that do not code for an amino acid.
Those are our stop codons UGA and UAA and UAG. These instead of telling the cell to add a new amino acid will tell the cell to stop translation. Another special codon is AUG. This codes for methionine and methionine is always the first amino acid in a protein.
So AUG is always going to be the starting point for translation. and it's going to be at the very beginning of the gene typically. So each three-letter sequence codes for an amino acid with the exception of our stop codons and then the AUG is always our start codon it's where we start translation. Something you may have noticed on the last slide looking at the table is that while every codon is translated into an amino acid in some cases there are multiple codons that are translated into the same amino acid. For example, CCC and CCU are both translated into proline.
And it's often the case that this happens with the last base pair. So the first two letters in the codon are sometimes enough to tell you which amino acid we're going to produce. This idea that the last codon is not essential or is flexible.
called the wobble effect and so when it comes to different types of mutations sometimes those mutations won't have any real effect on the protein that gets made this is called a silent mutation so let's walk through this example here with proline imagine in our DNA sequence we have GGG well the complementary mRNA to GGG is CCC. So this is our codon that gets produced for our mRNA. And then normally when this mRNA goes and gets translated, we make a proline.
If instead there is a mutation in our DNA, so that instead of GGG, we have GGA, then when that codon is transcribed into RNA, instead of CCC, we'll get CCU. However, CCU also is translated into a proline. So this is a mutation that has no impact on the protein that gets made.
Down here is another example, or it's spelling out kind of the same one, right? So we have our DNA where there is GGG, and then after transcription, we get a codon that is CCC. That is translated into a proline to make our protein.
If instead that Last G was mutated into an A or an adenine. Then after transcription, we have CCU, but that CCU is also translated into a proline. So no effect on the protein in the case of a silent mutation because of this wobble effect. Another type of mutation is a missense mutation.
And in this scenario, the alteration in DNA is going to cause a change in the protein. um here we have our dna and we are going to focus on uh g a u down here so we have our dna let me get a pen out so we can really focus in um let's go with red all right so we have our dna and normally in the wild type we have c t and that gets transcribed into GAU. The GAU is translated into aspartic acid. If we mutate our DNA so that instead of CTA, we have CAA, then the codon that is produced after transcription is no longer GAU, but GAU. UU and GU you is not translated as aspartic acid but instead is translated as valine so in this case the mutation changes the amino acid that gets produced and as a result changes the protein that gets produced now a lot of times these kind of mutations will affect a protein's function, but the protein will still get made.
One caveat to that is if there is a missense mutation in the start codon, in that case, the mRNA won't be translated because you need that AUG start codon intact in order for translation to happen. But most of the time, missense mutations won't completely destroy a protein. You could also get some, you know, pretty wild results if there's a missense mutation in a stop codon but either way missense mutations typically will lead to a functional protein but still a mutated one that may not function properly and still cause problems in the cell the next type of mutation is a nonsense mutation in this case the amino acid or the dna is mutated in a way that the codon is changed and now instead of coding for an amino acid is going to code for a stop codon.
So here we're focused on the second amino acid, sorry the second codon ACC and ACC is transcribed into UGG which is translated into tryptophan. If we mutate that second cytosine into a thymine then it becomes ATC. our transcript becomes UAG and that is a stop codon.
So here instead of making you know this whole protein we are just making methionine and that's it. So a nonsense mutation is typically going to be truncated or shortened and often nonsense mutations unless it happens very late in the gene sequence. Often nonsense mutations will produce a protein that is not functional.
Similarly, you can have a loss of stop mutation. And so here, if we focus on the last codon, ATT, normally gets transcribed into UAA, which is a stop codon. If instead we mutate that last adenine into a guanine, then... We're going to produce a UAC. Sorry, my video is blocking that so I can't accurately draw it, but you'll be able to see it on the recording.
So you have a UAC, and instead that is translated into an amino acid. And so now, where the protein should have been stopped, we keep producing amino acids until, by chance, we find another stop codon. Again, often this is going to cause such a dramatic change in the shape of the protein that it won't be able to function.
Or... it may create a protein with an entirely new function because it is so different from our original polypeptide that was produced. And finally we have frame shift mutations. And frame shift mutations are not usually exclusive with nonsense, missense, and silent mutations. What I mean by that is you can have a frame shift mutation that causes a nonsense mutation or a frame shift mutation that causes a missense mutation.
A frame shift mutation is instead of changing a base pair, we are inserting or deleting one or more bases. As long as it's not a multiple of three, this will result in a frame shift mutation. So here, looking at the third codon, Yeah, we have GAG and that is transcribed into CUC.
CUC is translated into leucine. However, if instead we insert a T between, oops, I made a mistake. If instead we insert a T between that C and G, now the reading frame shifts so that instead of reading a RGAG, we are now reading a TGA, and that TGA becomes, it's transcribed into ACU, which instead of leucine is threonine.
And now because of that insertion, we've bumped, right, before let me do a different color, before this G, A, G was our codon. But now this G that used to be the third codon, sorry, it used to be the third base pair in this codon, now that G is instead going to be the first base pair in the next. codon so when we read g g c whereas before that was a g g g and so after this frame shift everything else is affected we we shift what is called the reading frame normally we're you know we're reading three bases at a time but if you insert one well now that's included in this three so we bump down each subsequent codon is changed. So again, often these frame mutations will cause a loss of stop mutation and will have a dramatic effect on the protein's shape and therefore its function. However, a kind of like quirky bit of frame shift mutations is if you were to insert three bases or delete three bases your impact on the protein that gets made will be significantly less than if you just insert one or two or something that is not a multiple of three because there the the frame shift doesn't happen you're just inserting one new amino acid or deleting one as a whole so frame shift mutations uh very interesting and again not mutually exclusive with the other mutations we talked about the others loss of stop you stop, missense, nonsense, those are mutually exclusive, right?
You can't have a mutation in one amino acid that is both a silent and nonsense, right? It has to be one or the other. But you can have frameshift mutations that are also nonsense or also missense.
I'm going to close out this recording by talking about our translator in this. process of translation, and that is a molecule called tRNA or transfer RNA. um transfer rna is a few things you should be familiar with the bottom so first their shape they take this like um kind of like clover or shamrock shake shape uh where you have three bulbs uh and then like a stem coming off of it at the tip of this stem there is an amino acid binding site so an amino acid will bind a tRNA at this site all the time and when it is bound we say that that amino acid is charged.
The amino acid that binds this tRNA will be dependent on a region of the tRNA that is called the anticodon. That's highlighted here in yellow. This anticodon will be complementary to a codon. So the CCG here would be complementary to uh let's match it up because this is five prime to three prime so the three prime would be g g c so if you look at that table the table reads five prime to three prime i know it's a little confusing um so if you look at the table for a c g g that would be the i don't know off the top of my head but that would be the codon um that is bound by this t rna and therefore it'll tell you you which amino acid binds this specific tRNA.
These tRNAs are going to have interactions with the ribosome and they form non-covalent, temporary non-covalent bonds with part of the ribosome in order to add their amino acid to the growing polypeptide chain. But we'll get into that in the next. recording where we talk about the uh like moving parts of translation