Point Mutations

by Georgina Cornwall, PhD

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    00:00 Another thing we covered in quite a lot of detail are mutations, chromosome mutations and point mutations.

    00:07 So, I wanted to take a quick review here of how much of an impact a point mutation can have. This would be more specifically found in single-gene disorders. So point mutations will involve changing a particular amino acid.

    00:24 Here, you can see a string, part of the string of the amino acid chain and the nucleotides that code for it for hemoglobin B. You might recall that the hemoglobin molecule is made up of two alpha subunits and then two beta subunits. The problem can be in the mutation associated with the sickle cell just one nucleotide difference.

    00:52 We looked in this in a fair amount of detail before such that we see a substitution of valine for glutamine.

    01:01 When we do that, it changes the form of hemoglobin such that the beta subunits will sickle or they’ll fold in arm’s fold because they have a valine which has a different electronegativity than the glutamine.

    01:16 That causes the hemoglobin molecules to stack up. The hemoglobin molecules inside the red blood cells will cause them to sickle like this over here. So, that’s an example of a point mutation. Recall, there are a few different types of point mutations. We’re going to look at three of them here involving single nucleotides. You could have a silent mutation. A silent mutation is where we see that the amino acid isn’t changed at all even though one of the nucleotides is changed. So in this case, a U became a C and then we could have a nonsense mutation in which it codes for a stop codon. So instead of having a U in that spot, we have an A in that spot. It is nonsense because we get no more amino acids. Obviously, that’s going to have a more significant phenotypic effect or we could have a missense mutation which is like the hemoglobin B mutation that we just took a look at. That would result in having a different amino acid which may have a different polarity. I said electronegativity earlier. I didn’t mean that but a different polarity to the molecules, so it folds differently and has some of the phenotypic effects.

    02:44 So, point mutations can actually result in a frameshift mutation. 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. We will be looking at some situations where three nucleotides get inserted. That’s the case of the triplet repeat expansion. That’s significant to a couple of very important genetic disorders. So, if we insert this CAG repeat and it repeats and repeats and for some unknown reason, these repeats will expand from generation to generation in diseases like Huntington’s disease and Fragile X. So, the triplicate repeat will come up as a point mutation that will not quite fit the Mendelian inheritance patterns that we’ve been talking about.

    03:47 Then we look at chromosomal mutations. Again, we’ve seen this before. But they’re going to come into play fairly significantly during this course as we look at regions that are deleted. We end up with a chromosome that is different. We could have a section that is duplicated. Sometimes this happens from uneven crossing over during prophase I of meiosis. But it can happen in other means too with transposable elements, translocations of pieces of genome. So deletions and duplications and then we might even have inversions where one piece actually gets clipped out and moved and flipped back in upside down. So, you can see that the outcome is different chromosomes than we had initially. If these sorts of things happen within a gene, we may see point mutation but most generally, we’re going to see chromosomal mutations or duplications, deletions, and inversions on a much larger scale. So, we call them chromosomal mutations. The final chromosomal mutation that we’ll be considering are reciprocal translocations. That’s when we have two chromosomes that exchange reciprocally.

    05:07 However, they are not homologous chromosomes. So, we end up with completely different chromosomal products.

    05:13 You can see here, the top and the bottom chromosomes are different for some reason. Maybe it’s one chromosome's contacting another or maybe there’s a transposable element or some things moved and pieces are reciprocally exchanged between non-homologous chromosomes. We end up with different gene products. The effect that they have really depends on where this translocation happens. We’ll look at a particular example of Robertsonian translocation.

    05:45 We’ll look at in much more detail later on during the course. So, thank you so much for listening.

    05:51 I look forward to seeing you as we explore these types of mutations throughout the rest of the course.

    About the Lecture

    The lecture Point Mutations by Georgina Cornwall, PhD is from the course Introduction to Medical Genetics.

    Included Quiz Questions

    1. Abnormal hemoglobin molecules stack up over each other.
    2. Glutamine is present in place of Valine in the beta 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. Nonsense mutation
    2. Frameshift mutation
    3. Silent mutation
    4. Missense mutation
    5. Deletion mutation
    1. Translocation
    2. Deletion
    3. Duplication
    4. Inversion
    5. Insertion

    Author of lecture Point Mutations

     Georgina Cornwall, PhD

    Georgina Cornwall, PhD

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