Frameshift Mutations

by Georgina Cornwall, PhD

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    00:00 a whole bunch of nonsense and it is not a functional protein. That is just looking at a single base change. What about if we have an insertion of a base or a deletion of a base? These types of mutations end up with a reading frame chain. If we look at our normal strand, we have our nucleotides in a row when they code for this specific sequence of amino acids. Everything is good and then one base gets inserted. We throw off the whole reading frame so that everything downstream of that insertion is changed most likely and so we have a whole bunch of missense amino acids and that protein will no longer fold in the way that it should have folded and so it will probably have a great effect.

    00:48 Of course it depends on what protein that this mutation occurred in and again if it occurred even in a coding region on the DNA. Now these sorts of mutations are the things that natural selection will work upon perhaps this was a good thing. Maybe it ended up creating an enzyme that worked more at warmer temperatures or something and that temperature is rising, so that it is a good thing. This is where new material arrives from upon which natural selection works. Let us say though in this case instead of just one nucleotide insertion, we had a triplet insertion. A triplet insertion would only change one amino acid and probably would not be that big of a deal unless we really changed a polar for a nonpolar or something like that and impacted the folding of the protein. Lots of different things could happen there. Sometimes we will see that there are triplet repeats. The most common triplet repeat is the CAG triplet repeat and we are not sure why but it seems like often this repeats will expand. So we call them expanding triplet repeats and they can be found in coding or noncoding regions of the genome. Fascinating topic because these tandem repeats seem to repeat themselves ending up with lots more glutamine, glutamine, glutamine in a row. Then we have them expanding over the lifetime of an individual and so the triplet expanding repeat grows and grows.

    02:26 We see often that this is associated with Huntington's disease. There are many other examples where triplet repeats come into play, but someone with Huntington's disease is gaining more and more triplet repeats and somehow that is manifesting itself we think in the progression of Huntington's disease. So, good topics are there.

    02:51 Now here is a quick summary of what we have looked at. We have seen we could have point mutations. We could have insertion mutations. We have covered these expansion repeats. Now

    About the Lecture

    The lecture Frameshift Mutations by Georgina Cornwall, PhD is from the course DNA Repair & Genetic Mutations.

    Included Quiz Questions

    1. A small deletion has removed the nucleotides that code for the signal sequence at the amino terminus of the protein.
    2. A missense mutation has caused premature termination during translation of this protein.
    3. A chromosomal segment that includes the gene for insulin has been inverted.
    4. A two-base deletion near the middle of the gene has altered the reading frame during translation of the protein.
    5. A missense mutation has altered the ribosome-binding sequence at the 5' end of the mRNA.
    1. Huntington’s disease leads to the degeneration of excretory and digestive systems due to heavy metal toxicity.
    2. The trinucleotide repeat expansions in genes or introns lead to triplet repeat expansion disorders.
    3. The expanded CAG repeats in the coding regions get translated into a polyglutamine tract ("polyQ") which leads to an increased aggregation.
    4. The expanded CAG repeats in the coding regions of few proteins lead to few neurologic disorders in humans.
    5. Few of the codon reiteration disorders fall under the category of insertion mutations.

    Author of lecture Frameshift Mutations

     Georgina Cornwall, PhD

    Georgina Cornwall, PhD

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