Explore the genetic code and how it is translated into a polypeptide. We’ll practice using the RNA codon chart and learn the basics of codon recognition.
Making Sense of Codons
The genetic code found in mRNA gives instructions for how to make our proteins. mRNA is a coded sequence of nucleotide bases that we call by the four letters, A, G, C, and U. Earlier, we took a look at an RNA codon chart, a guide for understanding how the letters code for the amino acid chain. mRNA is read by groups of three nucleotide bases called codons. There are 64 different codons, and each codes for a different amino acid or a stop signal. The start codon AUG codes for methionine and signals translation to begin. So, we know what the codon chart looks like. But, how can we actually use it to make sense of the genetic code?
Practice with Codons and Amino Acids
Let’s try some practice with the RNA codon chart. It’ll help us get an idea of how the genetic code is used to make a chain of amino acids. We’ll start with a sample code in a strand of mRNA, match the codons to the correct amino acids, and then build a polypeptide from the amino acid chain. Don’t worry, it won’t be hard. It’s just like deciphering a secret message, like you may have done when you were a kid.
Here’s our first gene on the mRNA strand. It reads, A U G A A G U G G U A G. But, I didn’t read it the way I should. Just like tRNA, we need to read it in the form of codons, three letters at a time. So, it really reads, AUG, AAG, UGG, and UAG. Let’s begin with AUG and find it on our codon chart. Which amino acid does AUG code for? Oh, it codes for methionine, and AUG is also the start codon. Well, that’s fitting, since this is the beginning of our gene. So, we’ll put down a methionine amino acid and call it the official start of our polypeptide.
Now let’s look at the next codon, AAG. Which amino acid does that go with? Here, it goes with lysine. So, we’ll put down a lysine right next to methionine. Look, we already have a polypeptide! We have two amino acids made into a chain.
Let’s take the next codon, UGG. Where is it on the codon chart? You know, it’s kind of a pain to go searching through all 64 codons every single time. I wonder if there’s a quicker way to find them? Oh, look! You can use the side bars on the codon chart to help you find the codons. On the left, you find the row that matches the first base in your codon. So for us, that would be U. Now we’re restricted to that row, and we look up top to find the column that matches our second base, a G. So, there’s the row that goes with all the codons that have the second letter G, and in that cell, we have all four codons that begin with U and G. We have UGU, UGC, UGA, and UGG. We want the codon UGG, which goes with the amino acid tryptophan. So, let’s lay down our third amino acid, tryptophan, right next to methionine and lysine.
The last codon in our mRNA strand is UAG. If we look at our chart, we see that UAG is one of the three stop codons. So, instead of adding another amino acid, we’re just going to stop. We’re done! We made a polypeptide consisting of three amino acids arranged in a specific order: methionine, lysine, and tryptophan. This exact arrangement was dictated by the code in the mRNA, which in turn came from the original code found in the DNA.
Introduction to Codon Recognition
This amazing process of matching codons to the correct amino acids is called codon recognition. The molecules that are in charge of building your polypeptides are designed to read the codons down the length of the mRNA strand. It’s just like how we read the code in groups of three and matched the codons in the chart. Of course, our molecules don’t have an actual chart that they use inside our cells. Instead, they have special parts that match up so that our polypeptides are built through molecular mechanics. It works similarly to a lock and key. We’ll save the more complicated stuff for another time. Just keep in mind that codon recognition is the ability of mRNA codons to be matched with the correct amino acids.
The Genetic Code is Degenerate and Universal
The RNA codon chart is a tool that we invented to help us understand codon recognition. By looking at the chart, we can see that each codon specifies one particular amino acid. But remember, each amino acid may have more than one codon that it is specified by. Because there are more codons than amino acids, we say that the genetic code is redundant. For example, both codons UUU and UUC specify the amino acid phenylalanine. Scientists have a special term for describing this idea. They say that codons are degenerate. Now, that doesn’t mean that codons are inferior or corrupted. It just means that elements with slightly different structures can perform the same exact function. So, codons are degenerate because different codons can give the same instructions about which amino acid is specified.
So, amino acids can be indicated by more than one codon. But, each codon only specifies one amino acid. For this reason, scientists say that the genetic code has no ambiguity. In other words, we can be sure that the codon UUC will always specify the amino acid phenylalanine. We never have to wonder whether UUC might indicate some other amino acid. In fact, this is true for almost every living thing. If you look at the genetic code for an animal, whether it’s a mammal, a bird, a reptile, or a fish, the codon UUC will always code for phenylalanine. This even holds true for plants, fungi, bacteria, and viruses. Scientists say that the genetic code is universal, which means it is used the same way for all the organisms on Earth.
Actually, there are some microbes and other living structures that tweak the code a little. But for the most part, we can assume that all living things use the same codons to specify the same amino acids. In fact, that’s why scientists have been able to produce some amazing organisms by inserting genes from one creature into another. We now have bacteria that manufacture human insulin, crops that are resistant to pests and disease, and even fluorescent glowing fish! All because our genetic code is universal. Without the universality of the genetic code, none of these achievements would be possible.
The genetic code is used in the process of translation to convert the mRNA codons into a chain of amino acids. The RNA codon chart helps us to identify which codons specify which amino acids. Codon recognition describes the ability of codons to match with the appropriate amino acids. Good codon recognition is essential to the correct assembly of polypeptides, which in turn leads to the production of the right proteins. The genetic code is degenerate, meaning that different codons can specify the same amino acid. Also, the genetic code is nearly universal, which means that across all organisms, the codon specifications for amino acids remain fairly consistent.
At the conclusion of this lesson, you’ll be able to:
- Explain how a polypeptide is translated from the genetic code
- Read a codon chart
- Define the terms ‘degenerate’ and ‘universal’ as they relate to the genetic code