Microbiology, part 36: Genetics - Mutations

Hi, I'm Cathy with Level Up RN. In this video, we will be discussing mutations. And at the end of the video, I'm going to give you a 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 on mutations 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 are likely to get tested on in your micro class.

A mutation is a change in the DNA sequence of an organism. Mutations can occur spontaneously through errors that occur during DNA replication. We also have induced mutations which are caused by exposure to chemicals or radiation. Mutations are classified based on how they affect the DNA molecule. Let's go through the key types of mutations that you need to know. A point mutation is a mutation that affects a single nucleotide in a DNA strand, where one base is substituted with another. A point mutation can result in a silent mutation, a missense mutation, or a nonsense mutation. Let's take a look at an example of each of these types of mutations.

With a silent mutation, we have a base substitution that does not alter the amino acid sequence due to degeneracy of the genetic code. So up here, we have the template DNA without the mutation. It has the nucleotides CAT, GGG, and ATG. When this gets transcribed into mRNA, this results in the nucleotides GUA, CCC, and UAC. Using the genetic code chart, we see that GUA codes for the amino acid valine, CCC codes for the amino acid proline, and UAC codes for the amino acid tyrosine. So our amino acids are valine, proline, and tyrosine. Now, let's say that a mutation causes the third nucleotide, which was supposed to be thymine, to be replaced with adenine. With transcription, the third nucleotide in the mRNA strand then becomes uracil instead of adenine. So the first codon in the mRNA strand is now GUU as opposed to GUA. So what happens during translation? Well, if we look up GUU in the genetic code chart, we see that it still codes for the amino acid valine. So even though we had a mutation in the DNA, it did not alter the amino acid sequence because the codons GUA and GUU both code for valine. So our amino acid sequence of valine, proline, and tyrosine remains unchanged.

Next, let's take a look at a missense mutation, which does cause a different amino acid to be incorporated into the polypeptide. So we have the same template DNA strand that we saw in the last example with the nucleotides CAT, GGG, and ATG. This gets transcribed into mRNA, which results in the nucleotides GUA, CCC, and UAC. Like we did with the last example, if you look up these codons in the genetic code chart, we see that this sequence codes for the amino acids, valine, proline, and tyrosine. Now, let's say that a mutation causes the fourth nucleotide, guanine, to be replaced with thymine. With transcription, the fourth nucleotide in the mRNA strand now becomes adenine as opposed to cytosine. So what happens during translation? Well, the first and third codons in the mRNA strand did not change, so they still code for valine and tyrosine. However, the second codon in the mRNA strand did change, and it is now ACC as opposed to CCC. And if we look up ACC in the genetic code chart, we see that it codes for threonine instead of proline. So that is going to alter the polypeptide. Instead of valine, proline, and tyrosine, we now have valine, threonine, and tyrosine. The effect of a missense mutation can be significant or minimal depending on a variety of factors.

Okay. Let's now move on to a nonsense mutation, which is where a codon that codes for an amino acid gets replaced with a stop codon. So here we have the same template DNA that we saw with the last two examples with the nucleotides CAT, GGG, and ATG. This gets transcribed into mRNA, which results in the nucleotides GUA, CCC, and UAC. Like we did with the last two examples, if you look up these codons in the genetic code chart, we see that this sequence codes for the amino acids, valine, proline, and tyrosine. In this example, a mutation causes the last nucleotide to change from guanine to cytosine. With transcription, the last nucleotide in the mRNA strand becomes guanine instead of cytosine. So let's see what happens in translation. Well, the first two codons are unchanged, so they still code for valine and proline. However, the last codon is now UAG instead of UAC. And if we look up UAG in the genetic code, we see that it is a stop codon. So instead of valine, proline, and tyrosine, we now end up with a polypeptide with only two amino acids, valine and proline. In most cases, with this type of mutation, the resulting protein will be nonfunctional. So our cool chicken hint to help you remember this is nonsense mutations result in nonfunctional proteins.

In addition to mutations that cause one nucleotide to be replaced with another, we can also have a mutation that causes one or more nucleotides to be added or deleted from the DNA strand. When this happens, this results in what we call a frameshift mutation, which causes a change in the reading frame and alters the amino acid at the location of the mutation and every amino acid after the mutation. So let's take a look at an example of a frameshift mutation caused by the deletion of a nucleotide in the DNA strand. So we again have the same DNA template strand, which gets transcribed and then translated, which produces a polypeptide with the amino acids, valine, proline, and tyrosine. But in this example, the second nucleotide in the DNA strand is removed entirely, which shifts everything over to the left. So instead of the nucleotides being CAT, GGG, and ATG, we now have CTG, GGA, and TGT. And after transcription, we get GAC, CCU, and ACA, which is very different from the mRNA strand that was transcribed from the unmutated DNA. So let's see what happens during translation. If we look up GAC, this codes for aspartate. CCU codes for proline, and ACA codes for threonine. So you can see that two of the three amino acids have changed. Instead of valine, proline, and tyrosine, we now have aspartate, proline, and threonine. Like nonsense mutations, proteins made from a DNA with a frameshift mutation are also typically nonfunctional.

All right. It's quiz time. And in this particular quiz, I want you to name that mutation. You guys ready? Number one, this mutation causes one nucleotide to be replaced with another but does not alter the amino acid sequence. The answer is, a silent mutation. Number two, this type of mutation is caused by the insertion or deletion of a nucleotide, which alters the reading frame and causes the wrong amino acids to be added to the polypeptide. The answer is, a frameshift mutation. Number three, this mutation causes a codon that codes for an amino acid to be replaced with a stop codon. The answer is, a nonsense mutation. All right. That's it for this video. Hope you did great with that quiz, and I hope you found this video to be helpful. Take care, and good luck with studying.