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Lecture 20

2013-03-21 Lecture 20 Molecular Convergence

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Western University
Biology 1002B
Tom Haffie

Sequence Conservation: - i.e. Chlamy and subject is Volvox: - highly conserved: same base is found in both - sometimes not very conserved - at level of amino acid: highly levels of conservation Lecture 20 - Molecular Convergence Chlamy Volvox Chlamy Volvox Chlamy Volvox Chlamy Volvox Synonymous mutations Change in nucleotide sequence, but DOES NOT CHANGE amino acid sequence = synonymous mutation (silent) - at red: that's non-synonymous - it changes the amino acid sequence, making it diff. b/t volvox and chlamy Neutral theory of molecular evolution Selection Theory: All + b/t A and S in red mutations would affect fitness above indicates that - most mutations will be they're different deleterious (harmful) and a smaller proportion will be - these mutations don't advantageous have any affect on the protein in this area - as if neutral Neutral Theory: Many mutations have no affect - lots of mutations seem to be neutral - can start using molecular sequence data to look at how sequences change from one species to another - the number of diff. b/t protein sequences of diff. species are roughly PROPORTIONAL to the time since those species diverged 1▯ - diverged a long time ago - can expect more differences than b/t species that diverged more recently - if you do a lot of sequence analysis, results in chart below 1.2 billion years ago: separation of plants and animals - there are more differences in cytochrome c of animals and plants than differences b/t organisms that diverged more recently - notice: change by function of time are all linear rates (straight lines, constant rate) Neutrality and the idea of a molecular clock - difficult to explain rates are constant - take constant rate with bunches of different selection theory proteins - can organize to - better supported display a molecular clock by neutral theory - most mutations - i.e. 2 unknown organisms, have no effect sequenced hemoglobin and there is a different rate found about 110 AA for each of the proteins, substitutions but there is a constant - we find that there are rate for each about 110 diff. b/t these 2 organisms - go on map at 110, find - sequence hemoglobin b/t a variety of animals - find a substitution at position so and so out of so and so amino acids that these 2 genes diverged a little more - look at chart for approx. when the 2 species diverged than 600 million years Synonymous vs nonsynonymous rate ago - synonymous mutation rate is higher - neutral mutations that DON'T affect amino acid sequence occurs faster Silent substitution > Replacement substitution 2▯ alpha globin: protein required for hemoglobin, heme binds to this and carries oxygen - histone H4: histone nucleosome complex, structure of chromosome rate of histone mutations is VERY SLOW, but rate of a-globin mutations is quite fast the difference in rates of mutations due to: - histones more sensitive to changes - constraint - some proteins can't be changed very much w/o changing the function and structure, and making it deleterious Degree of constraint dictates rate of evolution -i.e. Cytochrome C, histone - can't change the sequences much without affecting the function of the protein - some proteins are highly constrained (can NOT change them much), and some if the rate of mutation is the same, than we can use the weakly constrained (can change them a lot) number of diff. that's occurred b/t 2 organisms over evolutionary time to deduce when they diverged Molecular convergence - is it not possible •  Why is similarity between sequences considered to be evidence of that these homology? sequences can converge to be similar? - phenotype can be similar due to homology or convergence - this similarity can also be applied to proteins? 3▯ - i.e. look at glsA from volvox and chlamy - similar across entire sequence at nucleotide level - would not expect Functional convergence that if two organisms were •  There is no reason for two proteins to share high id- location of cysteins distantly related their entire sequences... is there? (disulfide (S-S) bonding) and then converged - amino acids necessary to the same for catalysis functional - DNA binding domains, phenotype receptor binding sites - WOULD expect similarities in
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