Genetic Code - Online Tutor, Practice Problems & Exam Prep

Genetic Code

In this video, we're going to begin our lesson on the genetic code. The genetic code is just a table that reveals how DNA and RNA encode the sequence of amino acids in a protein. The genetic code is the link between nucleic acids like DNA and RNA and the amino acids of a protein. The genetic code is relatively universal or relatively consistent across all organisms but can have some differences between different species occasionally. The genetic code analyzes one codon at a time. Recall that a codon is a three nucleotide sequence found in the messenger RNA, the mRNA. Each codon, each three nucleotide sequence, specifies or reveals one particular amino acid. We use the genetic code to analyze one codon at a time, revealing one amino acid at a time. We'll be able to talk about exactly how to use the genetic code in our next lesson video. So, I'll see you all there.

How to Use the Genetic Code

In this video, we're going to talk about how to use the genetic code. And so notice down below over here on the right-hand side of our image, what we have is the genetic code. And so how exactly do you use this genetic code? Well, it turns out that using the genetic code is really just a 3-step process. And notice that we have each of those 3 steps numbered down below here. And so the very first step of using the genetic code is actually to use the coding DNA sequence to reveal the mRNA sequence. And so really this is the process of transcription using DNA to make mRNA. And so, of course, we know from our previous lesson videos that the mRNA sequence is going to be exactly the same as the coding DNA sequence, except for the fact that it's going to be replacing all of the t's or thymines with Us or uracils in the mRNA. And so if we take a look at our image down below over here on the left-hand side, notice that we're showing you a DNA molecule, which we know has 2 strands. One of the strands is going to be the DNA coding strand, which is going to be this top strand here, and the other strand is going to be the DNA template strand, which will be the bottom strand here. And so, of course, we know from our previous lesson videos that the mRNA sequence is going to be exactly the same as the DNA coding strand, which is the top strand. So the top strand is the one that we want to focus predominantly on. And so again, it's going to be exactly the same as this top strand except it's going to be replacing all of the t's or thymines with u's. And so as we start to reveal the mRNA sequence, we can see here we start off with a t g. And so we will put a, and it's tempting to put a t here as the mRNA, but we need to remember that in mRNA there are no T's and that the T's will be replaced with U's or uracils. And then of course we have the g. So then we can continue to do that just u literally rewriting the coding sequence except replacing the t's with u's. And so when you do that, you get c a u, you get a c c here, you get u g u at this these positions, and then you get u a a here. And so now we've revealed the mRNA sequence, and step 1 is complete.

Now in step 2, what we need to do is identify the 3-nucleotide coding frames within the mRNA transcript. The mRNA molecule is sometimes referred to as the mRNA transcript. And so identifying the 3-nucleotide coding frames includes identifying the start codon, which is typically where the, the first codon of the, mRNA, and also identifying the stop codon as well, which is typically going to be the last codon. And so, if we take a look at our image down below, we can kind of see step 2 has already been done for us because these vertical dotted lines that you see at these positions are kind of separating our mRNA into these 3-nucleotide codons. And so you can see here that we have the first codon would be AUG, then we have the second codon would be CAU, and then we have ACC and UGU, and then last but not least, UAA. So those are the codons, and notice that they've already been identified because we have separated them with these dotted lines. So step 2 is complete. And so in step 3, all we need to do is identify the amino acid that corresponds with each of the codons until a stop codon is reached. And so the stop codon is going to stop the process because it does not actually code for an amino acid and that's why the process stops at these stop codons. And so, of course, in order to identify the amino acids that each codon corresponds to, we need to use the genetic code over here. And so the genetic code is going to show the very first letter of the codon on the left-hand side of the table. So you can see the first letter of the codon here is going to be here. And the first letter of the codon will limit us to a specific row. Then, of course, the second letter of the codon is going to be across the top. And so across the top here, we will find the second letter of the codon, and the second letter of the codon is going to limit us to a specific column. And so between the first and second letters, it will limit us to a specific box where they overlap. And so, at this point, we would be looking at just one specific box here. And then the third letter of the codon is going to be over on the right-hand side. So you can see the 3rd letter is over here, and that would limit us to a specific position within the box so that it would reveal a specific codon. And so the genetic code shows all of the possible codons and linking these codons to amino acids. And so here in our example, what we're going to do is determine the polypeptide sequence or the protein sequence down below here, from the following DNA sequence. And so we already have used the DNA to reveal the mRNA sequence, so now we need to use the mRNA codons to reveal the amino acids in the polypeptide. And so we'll do this one codon at a time starting with the very first codon here, AUG. And so the very first letter of this codon is A. So the first letter of the codon is over on the left-hand side, and A is going to limit us to a specific row. Then the second letter of this codon is U, and U is going to be, the second letter of the codon is across the top, and U is going to limit us to a specific column. And so, where these two overlap would be right here in the middle, and so where this yellow region is. So we can kind of get rid of this to see it easier. So now we're focused on this particular box that's highlighted in yellow. Then we look at the 3rd and final codon letter of the codon, which is G. And the third letter of the codon is going to limit us to a specific position within the box, and that takes us right here. Since we have the G, you can kinda trace it all the way over and see that it's right here, AUG. And so AUG is, the codon is found right here. And it turns out that AUG is typically the, the what they call the start codon. It's the very first codon of most proteins. And so, it codes for an amino acid called methionine, which is abbreviated with the letters MET. And so this first codon here, the start codon, is going to be MET. And notice that this polypeptide sequence, this amino acid sequence, is going to be revealed from the N-terminal of the protein to the C-terminal of the protein. And so methionine is the very first one. So now we just need to repeat this process for all of the other codons. So we'll do this again and, relatively slow here with CAU. The very first letter of the codon is C. So the first letter of the codon is going to limit us to a specific row. Then the second letter of the codon is A, and the second letter of the codon is across the top, and A is right here. It limits us to a specific and see this is the box that we're focusing on. And then the third letter of the codon is U here. And so U, the 3rd letter of the codon, is going to limit us to a specific position, CAU.

A particular triplet of bases in the template strand of DNA is 5′-AGT-3′. What would be the corresponding codon for the mRNA that is transcribed?

• A. 3′-UCA-5′.
• B. 3′-UGA-5′.
• C. 5′-TCA-3′.
• D. 3′-ACU-5′.

A particular triplet of bases in the coding sequence of DNA is AAA. The anticodon on the tRNA that binds the mRNA codon is ________.

• A. TTT.
• B. UUA.
• C. UUU.
• D. AAA.

Which of the following sequences of nucleotides are possible in the template strand of DNA that would code for the polypeptide sequence Phe–Leu–Ile–Val?

• A. 5′–TTG–CTA–CAG–TAG–3′.
• B. 5′–AUG–CTG–CAG–TAT–3′.
• C. 3′–AAA–AAT–ATA–ACA–5′.
• D. 3′–AAA–GAA–TAA–CAA–5′.

What amino acid sequence will be generated, based on the following mRNA codon sequence? 5′–AUG–UCU–UCG–UUA–UCC–UUG–3′

• A. Met-Arg-Glu-Arg-Glu-Arg.
• B. Met-Glu-Arg-Arg-Glu-Leu.
• C. Met-Ser-Leu-Ser-Leu-Ser.
• D. Met-Ser-Ser-Leu-Ser-Leu.

Here’s what students ask on this topic:

What is the genetic code and why is it important?

The genetic code is a universal table that links DNA and RNA sequences to specific amino acids in proteins. It is crucial because it provides the instructions for assembling proteins, which are essential for all cellular functions. The genetic code operates through a three-step process: transcription of DNA to mRNA, identifying codons in the mRNA, and translating these codons into amino acids. Each codon, a three-nucleotide sequence, corresponds to an amino acid, with start and stop codons marking the beginning and end of protein synthesis. Understanding this process is fundamental for grasping how genetic information is expressed and how proteins are synthesized.

How does the genetic code translate mRNA into amino acids?

The genetic code translates mRNA into amino acids through a three-step process. First, the DNA coding sequence is transcribed into mRNA, replacing thymine (T) with uracil (U). Second, the mRNA sequence is divided into codons, which are three-nucleotide sequences. Each codon corresponds to a specific amino acid. Third, the genetic code table is used to match each codon with its corresponding amino acid. For example, the codon AUG codes for methionine, which is often the start codon. This process continues until a stop codon is reached, signaling the end of protein synthesis.

What are start and stop codons in the genetic code?

Start and stop codons are specific sequences in the genetic code that signal the beginning and end of protein synthesis. The start codon is typically AUG, which codes for the amino acid methionine. This codon marks the initiation of translation. Stop codons, such as UAA, UAG, and UGA, do not code for any amino acid. Instead, they signal the termination of translation, indicating that the protein synthesis process should stop. These codons are essential for ensuring that proteins are synthesized correctly and efficiently.

How universal is the genetic code across different organisms?

The genetic code is relatively universal across all organisms, meaning that the same codons generally code for the same amino acids in different species. However, there are some exceptions and variations. For example, certain organisms, such as mitochondria and some protozoa, have slight differences in their genetic codes. Despite these variations, the overall structure and function of the genetic code are remarkably consistent, allowing for a common framework for understanding genetic expression and protein synthesis across diverse life forms.

What is the role of mRNA in the genetic code?

mRNA, or messenger RNA, plays a crucial role in the genetic code by serving as the intermediary between DNA and protein synthesis. During transcription, the DNA coding sequence is transcribed into mRNA, which carries the genetic information from the nucleus to the ribosome. The mRNA sequence is then divided into codons, each consisting of three nucleotides. These codons are read by the ribosome, which uses the genetic code to translate them into specific amino acids, ultimately forming a protein. mRNA ensures that the genetic instructions encoded in DNA are accurately conveyed to the protein synthesis machinery.