Review: Point Mutations, Triplet Repeats, Chromosomal Mutations

by Georgina Cornwall, PhD

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    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.

    About the Lecture

    The lecture Review: Point Mutations, Triplet Repeats, Chromosomal Mutations by Georgina Cornwall, PhD is from the course Introduction to Medical Genetics.

    Included Quiz Questions

    1. Abnormal hemoglobin molecules stack up and cause the cell to sickle.
    2. Glutamine is present in place of Valine in the alpha chain of hemoglobin.
    3. Valine is present in place of Glutamine in the alpha chain of hemoglobin.
    4. It is a type of silent mutation.
    5. Both alpha and beta chains are abnormal.
    1. Silent mutation
    2. Missense mutation
    3. Nonsense mutation
    4. Frameshift mutation
    5. Insertion mutation
    1. Translocation
    2. Deletion
    3. Duplication
    4. Inversion
    5. Insertion

    Author of lecture Review: Point Mutations, Triplet Repeats, Chromosomal Mutations

     Georgina Cornwall, PhD

    Georgina Cornwall, PhD

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