The Genetic Code - Online Tutor, Practice Problems & Exam Prep

The genetic code consists of 64 codons, each representing a specific amino acid or a start/stop signal. Codons are nucleotide triplets formed from four nitrogenous bases: uracil, cytosine, adenine, and guanine. Among these, 61 codons code for amino acids, while three serve as stop signals. The start codon, AUG, initiates protein synthesis and codes for methionine. Understanding the codon chart is essential for translating mRNA sequences into proteins, highlighting the relationship between nucleotides and amino acids in biological processes.

The Genetic Code Concept 1

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 Example 1

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

• A. 6
• B. 12
• C. 18
• D. 36

Here’s what students ask on this topic:

What is the genetic code and how is it structured?

The genetic code is a set of rules by which information encoded in genetic material (DNA or RNA sequences) is translated into proteins by living cells. It consists of 64 codons, which are nucleotide triplets formed from four nitrogenous bases: uracil (U), cytosine (C), adenine (A), and guanine (G). Each codon specifies a particular amino acid or a start/stop signal. Among these, 61 codons code for amino acids, while three serve as stop signals. The start codon, AUG, initiates protein synthesis and codes for methionine. Understanding the codon chart is essential for translating mRNA sequences into proteins, highlighting the relationship between nucleotides and amino acids in biological processes.

How many codons are there and what do they represent?

There are 64 codons in the genetic code. Each codon is a sequence of three nucleotides, and they represent either a specific amino acid or a start/stop signal. Out of the 64 codons, 61 code for the 20 amino acids used in protein synthesis, while the remaining three codons (UAA, UGA, and UAG) serve as stop signals, indicating the end of protein synthesis. The start codon, AUG, not only codes for the amino acid methionine but also signals the beginning of translation in mRNA.

What is the significance of the start codon AUG in the genetic code?

The start codon AUG is crucial in the genetic code because it serves two primary functions. Firstly, it signals the initiation of protein synthesis, marking the point where the ribosome begins translating the mRNA into a protein. Secondly, AUG codes for the amino acid methionine. If AUG is found at the beginning of an mRNA strand, it acts as a start signal. However, if it appears within the interior of the mRNA strand, it simply codes for methionine. This dual role makes AUG essential for proper protein synthesis and function.

How do stop codons function in the genetic code?

Stop codons play a critical role in the genetic code by signaling the termination of protein synthesis. There are three stop codons: UAA, UGA, and UAG. These codons do not code for any amino acids. Instead, they instruct the ribosome to stop translating the mRNA sequence, effectively ending the elongation of the polypeptide chain. This ensures that proteins are synthesized to their correct lengths and prevents the addition of unnecessary amino acids, which could disrupt protein function.

How can a single amino acid be coded by multiple codons?

A single amino acid can be coded by multiple codons due to the redundancy of the genetic code, a feature known as degeneracy. For example, the amino acid valine is coded by the codons GUU, GUC, GUA, and GUG. This redundancy helps protect against mutations, as a change in one nucleotide may still result in the same amino acid being incorporated into the protein. This flexibility ensures that proteins can be synthesized accurately even if minor errors occur in the DNA or mRNA sequences.