Hey, everyone. So in this video, we're going to talk about the genetic code. Now the genetic code is the assignment of each codon to one of the 20 amino acids. And recall when we say codon, a codon is a nucleotide triplet, so three nucleotides that code for an amino acid or it acts as a start or stop signal. And here we're going to say that there are 64 total codons. Now, how do we come up with the 64? Well, here remember we have four of our nitrogenous bases: uracil, cytosine, adenine, and guanine. And for the triplet, that's three of these nitrogenous bases. So we have the first letter of the triplet, the second letter of the triplet, and the third letter of the triplet. So in each case, that's four. So \(4 \times 4 \times 4\) gives us 64 total codons. This is how we come up with this total number of codons. And we're also going to say here that a single amino acid can have multiple codons. If we take a look here, we have our chart which helps us connect these codons to different amino acids. How do we use this chart? Well, if we take a look, we have our first letter here on the left side u, and let's say we're looking for the second letter. So look up here. Let's say the second letter is a, so that'll put us here. All of these start with u a. And then the third letter is on this side here. Let's say the third letter was c. So we'd say it's u a c which gives us this. It would code for the amino acid Tyrosine. Let's say we wanted to do g u u. So g u u, the first letter is g, the second letter is u, the third letter is u. G U U codes for valine as our amino acid. Now, here, we're going to notice that some of these codons are in different colors that are not black. So here we have A U G in green and then these three red ones. UAA, UGA, and UAG. So here we're going to say that AUG at the start of mRNA acts as a start signal, a start codon. Now, it codes for the amino acid methionine. So here, if AUG is found within the interior of an mRNA strand, it codes for the amino acid methionine. So again, it has two functions. If AUG starts off the mRNA strand, it acts as a start signal. If it's found in the interior of the mRNA strand, then it codes for Methionine. Now, remember this number of 64 total codons. Of these 64 total codons, 61 of them are assigned to amino acids and three of them act as stop signals or stop codons. They don't code for any particular amino acid. As you can see here, it's blank here and it's blank here because those three in red are our stop codons or stop signals. Now, how do you remember them? All you have to remember is on the left side you have u, and then on the top and on the right, it's just g. So this represents our first letter of our codon, the second letters and the third letters. So we'd have UAA here. Actually, we'll do it in red to signify the stop signal. So, UAA, UGA, and UAG. Those are our three stop signals or stop codons. So remember, on the left side is u, and then it's just g on the top, g on the right, and then match up the first, second, and third letter of our codon to get the overall codon for that particular stop signal or stop codon. Alright. So just keep this in mind in terms of how we utilize this chart. Remember, AUG represents a start codon or start signal. If it's at the beginning of an mRNA strand, if it's found on the interior, then it codes for Methionine instead.
The Genetic Code - Online Tutor, Practice Problems & Exam Prep
The Genetic Code Concept 1
Video transcript
The Genetic Code Example 1
Video transcript
Here it says, determine the polypeptide sequence from the mRNA sequence given below. So step 1 is we're gonna break the given mRNA sequence into codons. Remember, those are nucleotide triplets. So here every 3, we're gonna split it. So that gives me GGCCAUGCC AUG UAU
. Step 2, we're gonna identify the amino acids for each codon and write the peptide sequence. Right. So, here we're gonna utilize our chart. So, we have GG, and then C, which gives us glycine. Then we have CAU, so CA, and then U gives us histidine. Next, we have ACC, and then C gives us threonine. AUG, well, that gives us methionine. Remember, if AUG is at the start of the mRNA sequence, it's a start codon, or start signal. But when it's within the mRNA chain itself, in the interior, it codes for the amino acid methionine. And then finally, we have UAU, so UA, and then U gives us tyrosine. So this would be our peptide sequence that we're able to determine from the codons that we were given initially. Alright. So this is how you'd utilize this chart in order to map out the peptide sequence.
Determine the number of bases in the information strand for the gene that codes for the peptide below:
Pro–His–Gly–Gly–Lys–Arg
6
12
18
36
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