BIOL 2040 Lecture Notes - Lecture 3: Gene Duplication, Polyploid, Point Mutation

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13 Feb 2016
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Lecture 3: Mutation and selection Part 1
Mutation: a heritable change in genetic material that is not due to genetic recombination
- Introduces variation before any other evolutionary force can act
oNatural selection unable to operate without a mutation/variability
- Point mutation
oMutations that create a change in a single base pair
Does not effect previous or subsequent base pairs
oCauses:
Random copying errors in DNA synthesis
Ex. A ends up paired with G, rather than T
Random errors in repair following DNA damage
oTransition
DNA polymerase mistakenly substitutes a purine (A and G) for
another purine or a pyrimidine (T and C) for another pyrimidine
oTransversion
Purine is substituted for a pyrimidine, or vice versa
Transitions outnumber transversions
Transitions cause less disruption in DNA helix – less likely
to be recognized as an error = less likely to be corrected
oReplacement substitutions: point mutations that result in an amino acid
change
oSilent site substitutions: point mutations that result in NO amino acid
change
Possible due to redundancy of genetic code
- Serious point mutations
o
oBase insertion and deletion cause frame shifts
Every amino acid after insertion/deletion is changed
- Point mutation rates
oPer base pair, per generation = 10^-9
About 1 in 1 000 000 000
There are ~1000 – 10 000 bases in a gene
Per gene, per generation = 10^-5 – 10^-6
oAbout 1 in 1 000 000
With 10-100 000 genes in a genome…
Per genome, per generation = 0.1 – 1
mutations
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oEvery person, statistically
speaking, should have one
base pair that is not present in
either parent
- Gene duplication and deletion
oRetrotransposition: a processed mRNA molecule is reverse transcribed to
from a double-stranded segment of DNA
New DNA can become integrated into the chromosome –
chromosome now has a duplicated section of DNA
oUnequal cross-over: when homologous chromosomes do not synapse
correctly and during meiosis I prophase
Results in one chromosome containing a deletion of a full section
of DNA and the other having a redundant stretch of DNA
- Chromosome rearrangements: inversion
oChromosome breaks
Chromosome segment detaches, flips, then is reattached
Gene order becomes inverted
oAffect genetic linkage
Linkage: tendency for alleles of different genes to assort together
at meiosis – genes on the same chromosome that are close together
are more closely linked
oHeterozygous inversions: one chromosome contains an inversion, the
other does not
Genes along inversion cannot align properly with homologous
genes
Crossing over will be unsuccessful – result in missing
genes from each chromosome, and redundant genes on
each
- Chromosomal rearrangements: polyploidy and aneuploidy
oChanges in chromosome number
Polyploidy: complete doubling of all chromosomes
Aneuploidy: fission, fusion, or doubling of only some
chromosomes
Mutations produce new variation in:
- The DNA sequence of genes
oPoint mutation
- The number of genes
oDuplication, polyploidy
oNew genes
- The arrangement of the chromosomes
oPolyploidy, aneuploidy, inversion
Mutation rate and allele frequencies
- Mouse example:
oInitial gene pool allele frequency; A=0.9, a=0.1
Expected zygote genotype frequency; AA=0.81, Aa=0.18, aa=0.01
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