Genome Evolution

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  • Created on: 21-05-16 11:20
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Genome Evolution
Why is this relevant to the study of evolutionary biology of animals?
1. It tells us what we might expect evolution to do:
The limits of how a genome can change ultimately constrain what evolutionary changes
can take place and how fast it can occur.
2. It helps us interpret evolutionary patterns that we see today:
Evolution does not have the benefit of hindsight; past history determines the starting pint
for the evolution of any particular trait ­ leads to the question what is more important,
past or current events?
The same body shape is found in Isoxya and Gasteracantha spiders even though they are unrelated.
This is convergence.
Often evolution causes the same patterns to emerge over and over again.
Large changes can occur due to one small change in the genome.
Example: chirality in land snails is due to one base pair change.
DNA is highly coiled into chromosomes.
Access to the DNA requires uncoiling.
The amount of energy needed to uncoil DNA depends on the position on the chromosome.
The location of DNA on a chromosome will also effect how much recombination occurs.
Nuclear chromosomes are linear; cytoplasmic DNA such as that in mitochondria and chloroplasts is
What Kind of Changes Occur?
Single base pair changes
Chromosome breakages and fusions
Duplication of genes and entire genomes
Pseudogene formation
Horizontally transferred DNA
Single base pair changes are known commonly as Single Nucleotide Polymorphisms (SNPs).
May be synonymous, meaning they still encode the same amino acid, but this does not mean they are
Different codons for the same amino acid are not equally good because some tRNAs are rarer than
others, which will have an effect on how quickly protein synthesis can occur.
This leads to codon bias ­ selection for codons with more common tRNAs.
Example: in microorganisms such as yeast ( Saccharomyces cerevisiae ) glutamic acid uses CTT rather
than CTC.

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Selection against the rare codon is greatest in highly expressed genes, but weak or nonexistent in
rarely expressed genes (because there is less opportunity for natural selection to act on rarely
expressed genes).
A single gene leads yeast cells to cooperate against threats.
A social behaviour that mobilises yeast cells to cooperate in protecting each other from stress,
antibiotics and other dangers is driven by a single gene.…read more

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Evolution has constrained the amino acids used for 360 million years.
Duplication of entire genomes is common in plants but rare in animals.
One case where it has occurred is X. laevis (African toad) which is tetraploid.
This is thought to have arisen after duplication of the entire genome of the ancestor of X. tropicalis.
You don't need supplicates of every gene, so generally over time most of the duplicate DNA will gain
mutations and no longer work.…read more

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An example of gene conversion: Saccharomyces cerevisiae has two mating type alleles.
Each individual has a `master copy' of each allele, plus a working copy.
The working copy can be removed and replaced with the other type by using one of the master copies
as a template = mating type switching.
N.B. in this case both alleles happen to be on the same strand.
Repair (gene conversion) is not always random.…read more

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Transposable elements provide the necessary matches
Virogene ­ a strain found in baboons is very similar to that in cats.
Cats and baboons are not thought to be closely related, so this result suggests that there has been
horizontal gene transfer between the two.
Horizontal gene transfer occurred from mitochondria and chloroplasts to the nucleus.…read more

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E.g. a paper shows that Buchnera endosymbiotic bacteria of aphids have stopped their rapid
However we don't know the genome has stayed still, we only know no changes have persisted.
Pleiotropy (when one gene influences many traits) limits the types of genetic changes that can occur.
E.g. the melanocortin system (series of genes) determines colour patterns as well as other traits such
as immunity.
Darker wild vertebrates are more aggressive, sexually active and resistant to stress than lighter
individuals.…read more


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