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Lecture 20

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Biology 1002B
Tom Haffie

Lecture 20: Molecular convergence  Mutation resulting in not changing amino acids are called synonymous mutations o Mutation seems neutral  Neutral theory of molecular evolution o Selection theory  All mutations would affect fitness  Most mutations are deleterious and only a few are advantageous o Neutral theory  Half are deleterious and half is neutral and only a small portion is advantageous o The number of difference between protein sequences of different species are proportional the time since those species diverge  Neutrality and the molecular clock (important, check out slide) o The more time the two species diverge, the more differences between amino acids for a particular protein o It is a straight line, the change of a function of time is a straight line, rate o Seems to be a constant rate, because they are pretty neutral, a nice straight rate o There’s a protein of 100 amino acids, and there’s a substitution of amino acid, then we can look at the graph and we can find out how long ago did they diverge o Humans and chimpanzee, finding from the amino acid differences, we found that humans and chimpanzees diverged 500million years ago  Sequence conservation o The mutations that have no effect on the amino acid happens more faster than nonsynonymous mutations  Synonymous mutations are silence mutations  X  Degree of constraint dictates rate of evolution o Histone H4 is very slow o Alpha globin is very high o Why?  One might be more important (weak constraint)  Histone might be more sensitive (strong constraint)  Constraint, one change might be affect the function so can’t change the sequence very much  Molecular convergence o Why is similarity between sequences considered to be evidence of homology?  X  Functional convergence o There is no reason for two protein to share high identity across their entire sequences… is there? o Location of cysteins (disulfide (S-S) bonding)  Localized convergence , localized high similarity o Proteins are different my localized amino acids because they have active sites, that’s what defines one from another o Amino acids necessary for catalysis o DNA binding domains, receptor binding sites o Proteins converge, mutations cause nucleotides to change, but it doesn’t occur for the whole entire sequence  If you see sequences are similar across DNA you c
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