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The causes of genetic mutations

Video transcript

so today we're going to talk about the causes of genetic mutations but first let's just do a quick review of the idea that mutations are mistakes in a cell's DNA and there are two main types of mutations that we see when we look at a cell's DNA and the first is called point mutations and that's when one DNA base is switched out for another which usually results in a change to one codon in the RNA sequence frameshift mutations are when the reading frame of the RNA is altered and while the actual nucleotides in the RNA sequence haven't changed that much the reading frame of the RNA strand has shifted meaning that many different RNA codons will change as a result and we're going to take a look at into what causes these point in frameshift mutations so point mutations are caused by base substitution which is when one DNA base is substituted for another and there are a couple of different types of base substitution a transition is when you have a substitution of adenine for guanine or vice-versa which is a swap between two purines or a substitution of cytosine for thymine or also vice-versa which is a swap between two parameters a trans version is when either adenine or guanine is swapped for either cytosine or thymine and in this type of base substitution you have either a purine being replaced with a pyrimidines a pyrimidine being replaced with a purine now the last kind of mutation that can lead to a point mutation is a miss pairing which some people call miss matching and that's when a DNA strand has a non Watson Crick base pairing normally a pairs with T in G pairs with C but when you have a miss pairing that's when a and C pair up or when G and T pair up and it's much more common for miss pairings to occur between a purine and pyrimidine as opposed to between two purines like a and G pairing up or to pyrimidines like C and T pairing up next we're going to talk about frameshift mutations so let's say that we have this DNA strand here with three repeating CDC units and an extra C on the end this would then be transcribed into an RNA strand with repeating g AG units and an extra g on the end and our three codons would be the 3 g AG units which were then each translate to a glutamate amino acid now one way you can as a frameshift mutation is through an insertion and that's when an extra DNA base finds its way into our sequence so here we have this extra cytosine base that I've underlined falling into our sequence and in this additional C base would lead to an extra G being thrown into our RNA sequence which we then shift the codon reading frame of our RNA strands during translation so now instead of three G AG codons we have just one G a G codon and 2 g GA codons with two extra bases on the end this would then code for one glutamate residue and two glycine residues instead of three glutamates the other way that you can cause a frameshift mutation is through a base deletion so in a deletion we drop off one of our bases from our original sequence so here I've dropped that first thiamine base and this would also result in a shift of the RNA reading frame now instead of having 3 g AG codons we have a GG G codon and 2 AGG codons which would lead to a protein with a glycine and two arginine amino acids so overall insertions and deletions can both lead to frameshift mutation now we can also talk about large-scale mutations which instead of being at the level of individual nucleotides are usually seen at the chromosomal level and can affect many genes instead of just a few base pairs so first we'll talk about translocation which is when a gene from one chromosome is swapped for another gene on a different chromosome now it's important to see that translocation refers to gene swapping between non homologous chromosomes which means that if this blue chromosome were chromosome 10 then the green one could be any chromosome aside from chromosome 10 and this is what sets translocation apart from the process of crossing over that occurs during meiosis between homologous chromosomes the next large-scale mutation we'll talk about is chromosomal inversion and that's when two genes on the same chromosome switch places so here are green and blue genes are being swapped and end up on different parts of the chromosome after the mutation now since both of these mutations don't always affect the individual nucleotides coding for a gene it's important to see that many of these types of mutations affect how it genes expression is regulated in addition to changing what the genes actually code for remember that the position of a gene on a chromosome partly determines how it's regulated and this could be due to histone configuration promoter regions or any other regulatory process so what did we learn well first we learned that small-scale mutations affect DNA at the nucleotide level and of these small-scale mutations we have point mutations which can be caused by transitions trends versions and miss pairings and we also have frameshift mutations which can be caused by insertions or deletions next we talked about large-scale mutations which affect the DNA at the chromosomal level and the two large-scale mutations we talked about where translocation and inversion