- Here is a catalogue of the kinds of mutations that can arise in DNA & there are
certain names for these & you have to understand what these names are.
- One idea would be that you have a substitution in the nucleotide & that is called
the base substitution & there are special names depending on what base gets
changed. When you talk about a transition, what happens in a transition is that one
type of purine is converted to another type of purine, in other words where an A
turns into a G or a G turns in to an A. Or in another type of transition, a pyrimidine
converted so one type of pyrimidine is converted to another type of pyrimidine so
C & T can be converted. Transitions are sticking with that group, purine to purine
or pyrimidine to pyrimidine. Behind shows us the base pairing, the A & T on the
two strands & the A & T would be converted to a base pairing of G & C because if
A is converted to G, then the G will pair with C that will happen.
- That is called a transition, another type of substitution is called a transversion &
in that case, a purine becomes a pyrimidine or a pyrimidine becomes a purine. Here
the idea can be an A to a C, or T to G. You have to know the difference between a
transition & a transversion & what the consequences are.
- As well there can be insertions or deletions, these can be really small or these can
be really large, these can be deleting one nucleotide base pair or they could be
inserting one pair. So these are one or more nucleotide pairs that are either added
or deleted from the sequence. When you’re talking about these mutations as a
group, they are called indel mutations, which is indel kind of contraction of
insertion or deletion.
- As well there is something called an inversion, in that case imagine you have a
segment of DNA that gets chopped out in two places, & it gets flips 180 degrees &
joins up, that will change the sequence & that is called an inversion.
- Another type is called a reciprocal translocation where two parts of the
chromosome will switch places, so a chunk of chromosome 1 goes into
chromosome 8 & a part of chromosome 8 goes into chromosome 1. That is usually
a much larger change.
- For today’s lecture, we will focus more on small insertion or deletion & base
- Here we have made a list of quite a few of the different types of mutations, so
how do they arise? How do we get those?
- In some cases these happen totally spontaneously, in the total absence of a
mutagen, a mutagen is something that causes your DNA to be mutated, we
probably know about them, there are different substances that do that. For example,
chemicals in cigarette smoke are known mutagens or other types of chemicals are
known as mutagens. Even UV light is a mutagen but some mutations just arise
spontaneously. They provide a kind of background rate of the mutations that we
have in our DNA, the background rate is around two to twelve *10-6 mutations per
gene per gamete.