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.
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.
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.
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.