Making Sense of the Genetic Code: Codon Recognition

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• published: 24 Mar 2016
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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. A gene code is read by dividing it into groups of three letters Reading Gene Codes 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 this 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. Here 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 this row, and we look up top to find the column that matches our second base, a G. So here's the row that goes with all the codons that have the second letter G, and here in this 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 mRNA, which in turn came from the original code found in DNA. Introduction to Codon Recognition Codon recognition is the process of matching codons to the corresponding amino acids 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.