Biology 1002B Lecture Notes - Lecture 19: Semiconservative Replication, Base Pair

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17 Apr 2012
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Gene Mutation
Telomerase will do the trick, it has its own template built in, a length of RNA that provides the needed template in order to
make DNA to elongate the three prime end.
o Telomerase can only add DNA complimentary to the what it had (the RNA template)
o Once it finishes one sequence it just moves down and does it again
o That’s why completed DNA has repeats at the end of their sequences
Telomere is the end of the DNA strand that had multiple repeating bases that the Telomerase binds to.
Now that telomerase has made and has added complimentary base pairs to the end the strand the DNA can then begin to
replicate at the newly formed three prime end.
Telomerase adds DNA to the end of chromosomes controlling the shortening caused my DNA replication
Telomerase does not prevent the mechanism that causes your chromosomes to shorten; DNA will shorten no matter what.
What telomerase does is adds DNA to the end of chromosomes such that the shorten doesn’t matter, its controlled.
Your telomere is a way for your cell to tell how old it is, it isn’t a way to protect the more valuable DNA it just tells the cell
the age of the DNA. Every time your DNA replicates your telomere becomes shorter.
Connection between mutations and DNA replication is the primer on the leading strand. There is no 3’ OH for the
polymerase to use to replace the primer. As a result your chromosomes will shorten at every replication!
Ames Test
A lab interested in discovering the different kinds of mutagens in the environment, Haffie’s lab was mostly interested in discovering
the mutagens in wine. So they used the Ames test.
Bacterial Test (salmonella)
o The bacteria are mutants at the beginning of the test
o His (-) cannot make their own histodene
o When you plate them on minimal medium they will die
Except there are some that do survive
o Revertants - normal, wild type (no mutants)
o Reversion going form mutant back to normal, it is still a mutation
The Ames test helps realize that it is really easy to find the revertants (HIS (+)) they are the only ones that will grow.
The ones that live on the plates are spontaneous mutants but they are normal
They have arisen because of a mutant that converted them from HIS (-) to HIS (+).
If you put these cells in the environment with a mutagenic
component you will dramatically increase the frequency of
revertants
Classic positive Ames test, frequency increases in
bacteria
Does it mean it causes mutation sin you, not necessarily
Ames test is like the first line of screening for mutagenic
compounds
How is it that HIS (-) turns to (+)? How is it that we are changing the
DNA sequence of the genome in these salmonella?
This is the only thing that is called a mutation in this course; a double stranded change in the DNA, that is the only thing that is a
mutation. Just because something messes with your DNA doesn’t mean that it can cause a mutation; it may damage DNA but that is
not a mutation. There is only two ways that you can change your DNA…
Base Substitutions
Insertion/Deletion/Reorganization
Tautomeric Shifts
Change base pairing rules
Spontaneous
Structures ketogroups and aminogroups depend on electron distribution of the whole molecule
Distribution of electrons of the whole molecule not stable
Thymine usually presents 2 hydrogen bonds to pair with adenine
o electron distribution can shift can form 3 hydrogen bonds
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