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The different types of mutations

Video transcript

so today we're going to talk about the different types of genetic mutations that you would find in a cell but first I want to review the central dogma of molecular biology and how the genetic information of a cell is stored in the form of DNA which is then transcribed to form RNA and then translated to generate protein nucleotides from the DNA are transcribed they're complementary forms on RNA which are then read as codons or groups of 3 to code for specific amino acids in a larger protein now if you mutate one of the nucleotides on DNA like let's say turning this thymine base into an adenine base then that will affect the RNA sequence and ultimately the protein that follows so we say that mutations are mistakes in a cell's DNA that ultimately lead to abnormal protein production so what are the different types of mutations well the first type of mutations we're going to talk about are called point mutations now here I've just written at a random sequence of DNA which is just a repeating pattern of CTC which would code for a repeating sequence of G AG in the RNA strand and finally a protein sequence of 3 glutamate amino acids so a point mutation is when one of our DNA bases is replaced with another so in this example a thymine base is being replaced with an adenine base which leads to a change in one RNA nucleotide and ultimately a change in one amino acid another type of mutation is called frame shift which works a little differently so first I'll write out the same DNA RNA and protein sequences from before but now instead of changing one base to another I'm going to add one to the sequence and here I've thrown in this extra cytosine base that I've written in blue now naturally this change would lead to an additional guanine base being in the resulting messenger RNA sequence what's interesting is that this mutation will change the reading frame of the RNA remember that RNA is read in groups of three or codons when being translated to form protein but now since we've added an extra G here all of the codons coming after that extra G will look a little differently now instead of having 3 g AG codons we've swapped out 2 for GGA codons this means that two of our amino acids in the final protein will be changed and in this example they'll be changed from glutamate to glycine so you can see that frameshift mutations usually have more significant effects on the final protein than point mutations do now it's important to recognize that both of these mutations are classified and named for how they affect the cells DNA structure and aren't really named for how they affect the resulting protein now our next type of mutations are nonsense mutations and missense mutations let's say we have a DNA sequence that normally generates RNA and codes for a cysteine amino acid a nonsense mutation is any genetic mutation that leads to the RNA sequence becoming a stop codon instead now missense mutations are a little different and they're any genetic mutation that changes an amino acid from one to another so in this example our mutation is changing the resulting amino acid from a cysteine to a tryptophan now you can see that nonsense mutations probably affect the resulting protein a lot more than missense mutations do since that new stop codon that we're creating could chop off a huge section of the protein instead of just changing one amino acid to another so now we can divide the missense mutations even further into a bunch of smaller categories silent mutations are when the mutation doesn't actually affect the protein at all since many different RNA codons can code the same amino acid it's possible that the mutation might not affect the protein at all so in this example CCA CCG CCT and CCC in a section of DNA will all end up coding for glycine so if you change the third base it wouldn't affect the final protein conservative mutations are where the new amino acid is that the same type as the original so here I have a glutamate and an aspartate which are both acidic amino acids so a mutation that swapped out an aspartate for a glutamate would be a conservative mutation finally a non-conservative mutation is one with a new amino acid is of a different type from the original so here we have a serine amino acid which is a small polar amino acid it being replaced with phenylalanine which is a large nonpolar aromatic amino acid and this would be an example of a non-conservative mutation since serine and phenylalanine are different types of amino acids now I'll point out again that all of these mutations are classified and named for how they affect the resulting proteins and aren't really named for how they affect the cells DNA so let's look at a quick example sickle-cell disease is a disorder where hemoglobin or HB which is a protein found in human blood is mutated into a less active form which we're going to call HBS and it results from a single glutamate residue being converted into a valine residue now we can classify this mutation as a point mutation since only one DNA base is affected but we can also say that it's a non-conservative missense mutation since glutamate is being swapped out for valine and the two are different types of amino acids since glutamate is an acidic amino acid and valine is a non-polar one so what should we learn well first we learn that mutations originate at the DNA level but show their effects on the protein level and second we learn that we can classify different types of mutations by either their effects on DNA or their effects on protein in reference to DNA we have point and frameshift mutations and in reference to protein we have miss sense and nonsense mutations