Review: Point Mutations, Triplet Repeats, Chromosomal Mutations

00:01 Another thing we covered in quite a lot of detail are mutations.

00:04 Chromosome mutations and point mutations.

00:06 So I wanted to take a quick review here of how much of an impact a point mutation can have.

00:12 These would be more specifically found in single gene disorders.

00:19 So point mutations will involve changing a particular amino acid.

00:24 Here you can see a part of the string of the amino acid chain and the nucleotides that code for it for hemoglobin B.

00:34 You might recall that the hemoglobin molecule is made up of two alpha subunits, and then two beta subunits.

00:42 And the problem can be in the mutation associated with the sickle cell, just one nucleotide difference.

00:53 We looked at this at a fair amount of detail before and saw a substitution of valine for glutamate.

00:58 When that occurs, it changes the form of hemoglobin, such that the beta subunits will sickle or they will fold in ARMS fold, because they have a valine that has a different electronegativity than the glutamate.

01:09 And that causes the hemoglobin molecules to stack up and the hemoglobin molecules inside the red blood cells will cause them to sickle like this over here.

01:21 So that's an example of a point mutation recall.

01:25 There are a few different types of point mutations.

01:27 We're going to look at three of them here involving single nucleotides.

01:32 You can have a silent mutation.

01:34 Silent mutation is where we see that the amino acid isn't changed at all, even though one of the nucleotides is changed.

01:44 So in this case, a U became a C.

01:46 And then we could have a nonsense mutation, in which it codes for a stop code on.

01:53 So instead of having a U in that spot, we have an A in that spot.

01:59 And it is nonsense. because we get no more amino acids.

02:02 Obviously, that's going to have a more significant phenotypic effect.

02:07 Or we could have a missense mutation, which is like the hemoglobin B mutation that we just took a look at.

02:13 And that would result in having a different amino acid, which may have a different polarity.

02:20 I said electronegativity earlier I didn't mean that.

02:22 But a different polarity to the molecules so it folds differently, and has some of the phenotypic effects.

02:30 So point mutations can actually result in a frameshift mutation.

02:37 For instance, if we insert a particular base, or we delete a particular base, then you're shifting the reading frame by one nucleotide or two nucleotides, or three nucleotides.

02:51 And we will be looking at some situations where three nucleotides get inserted.

02:57 And that's the case of the triplet repeat expansion.

03:00 And that's significant to a couple of very important genetic disorders.

03:05 So, if we insert this C, A, G repeat and it repeats, and repeats.

03:10 And for some unknown reason, these repeats will expand from generation to generation in diseases like Huntington's disease and Fragile X.

03:21 So, the triplicate repeat will come up as a point mutation that will not quite fit them Mendelian inheritance patterns that we've been talking about.

03:34 Then, we look at chromosomal mutations.

03:37 Again, we've seen these before, but they're going to come into play fairly significantly.

03:42 during this course, as we look at regions that are deleted.

03:46 We end up with a chromosome that is different.

03:51 We could have a section that is duplicated.

03:54 And sometimes this happens from uneven crossing over during prophase, one of meiosis, but it can happen in other means too, with transposable elements, translocations of pieces of genome.

04:09 Deletions and duplications.

04:12 And then, we might even have inversions, where one piece actually gets clipped out and moved and flipped back in, upside down.

04:19 And so you can see that the outcome is different chromosomes than we had initially.

04:26 And if these sorts of things happen within a gene, we may see point mutation, but most generally, we're going to see chromosomal mutations, such as our duplications, deletions, and inversions on a much larger scale, so we call them chromosomal mutations.

04:43 The final chromosomal mutation that we'll be considering are reciprocal translocations.

04:48 And that's when we have two chromosomes that exchange reciprocally.

04:53 However, they are not homologous chromosomes, so we end up with completely different chromosomal products.

04:59 You can see here the top and the bottom chromosome are different.

05:02 For some reason, maybe it's one chromosomes contacting another or maybe there's a transposable element or something's moved and pieces are reciprocally exchanged between nonhomologous chromosomes, we end up with different gene products.

05:18 The effect that they have really depends on where this translocation happens.

05:25 And we'll look at a particular example of translocation.

05:30 We'll look at in much more detail later on during the course.

05:35 So thank you so much for listening.

05:36 I look forward to seeing you as we explore these types of mutations throughout the rest of the course.