Microbiology, part 35: Genetics - Translation

Hi, I'm Cathy with Level Up RN. In this video, we are going to talk about translation of a messenger RNA into a protein. And at the end of the video, I'm going to give you guys a little quiz to test your understanding of some of the key facts I'll be covering, so definitely stay tuned for that. And if you have our Level Up RN microbiology flashcards, go ahead and pull out your flashcards so you can follow along with me. And pay close attention to the bold red text on the back of the cards because those are the things that you definitely have to know for your micro class.

Let's first talk about the genetic code, which is a set of instructions that directs translation of an mRNA into amino acids. As part of the genetic code, we have codons, which are groups of three nucleotides that correspond to an amino acid. For example, UGG is a codon that corresponds to the amino acid tryptophan, and UUA is a codon that corresponds to the amino acid leucine. Now, there are a handful of codons in this genetic code that you definitely need to be familiar with. The first is AUG. AUG codes for the amino acid methionine, but is also the start codon that initiates the translation process. I would also be familiar with UAA, UAG, and UGA, which are nonsense or stop codons. So these codons do not code for any amino acids. Their role is to stop the translation process.

There are 64 different combinations of nucleotides in the genetic code, but there are only 20 amino acids. Therefore, there are multiple codons that code for the same amino acid. So for example, the codons CCU, CCC, CCA, and CCG all code for the amino acid proline. So this redundancy in the genetic code is referred to as degeneracy. You'll notice that for a lot of these codons, if we change the third position in the codon, it still produces the same amino acid. So for example, with the codon GGU, if we change the third position to a C, A, or G, it still produces the amino acid glycine. Because this is the case with many of the codons in the genetic code, we refer to that third position in the codon as the wobble position, meaning it can wobble or change, but chances are it's going to produce the same amino acid.

Now that you understand the genetic code, let's talk about the steps involved in translation, which is where a ribosome decodes an mRNA strand to produce a polypeptide, which is a chain of amino acids. Translation is initiated when the large ribosomal subunit, small ribosomal subunit, mRNA, and initiator tRNA come together to form the initiation complex at the start codon on the mRNA strand, which is AUG. The initiator tRNA, carrying the amino acid methionine, binds to the P site on the ribosome. Next, a tRNA carrying the second amino acid pairs with the next codon on the mRNA strand at the A site. The first and second amino acids are joined via a peptide bond. Methionine, which is the first amino acid, is released by the initiator tRNA, but it remains bound to the amino acid of the second tRNA, which is at the A site. The ribosome then moves forward by one codon, which is referred to as translocation. The initiator tRNA shifts to the E site and then detaches. So E for exit. The second tRNA moves to the P site, which opens up the A site for the next tRNA carrying another amino acid.

The ribosome continues to move forward in this fashion, adding new amino acids onto the growing polypeptide. Once the ribosome reaches a stop codon at the A site, such as UAA, seen in this image, then the polypeptide is released, and the large and small ribosomal subunits separate. Of note, after translation, the polypeptide does have to go through several processing steps before it becomes activated. This includes folding of the polypeptide as well as various chemical modifications.

All right. It's quiz time, and I've got five questions for you. You guys ready? Question number one, a blank is a group of three nucleotides that corresponds to an amino acid. The answer is, codon. Number two, what is the start codon found on the mRNA strand that initiates translation? The answer is, AUG. Number three, what is meant by degeneracy of the genetic code? This means that there is redundancy in the genetic code, such that multiple codons code for the same amino acid. Number four, blank is the process in which the ribosome moves forward by one codon during translation. The answer is, translocation. And number five, when is the polypeptide released during translation? The answer is, when the ribosome reaches a stop codon. All right. That's it for this video. I hope you did great with that quiz. Take care, and good luck with studying.

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