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Lecture 20

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Western University
Biology 1001A
Tom Haffie

Synonymous vs nonsynonymous mutations - ● Synonymous mutation: Changes in nucleotide sequence that don’t change amino acid ○ Silent mutations; no effect in phenotype ● Nonsynonymous mutation: changes in nucleotide sequence that change amino acid ○ Replacement substitutions; phenotype affected Characteristics of the neutral theory of molecular evolution - ● Old theory was Selection theory which stated that all mutations would affect fitness, be it advantageous or deleterious ● Neutral theory of molecular evolution:A large portion of mutations that occur are neutral Relationship between frequency of amino acid substitutions in given proteins vs. time since common ancestor - ● Molecular sequence data can be used to look at how sequences change from one species to another ● Number of differences between protein sequences of different species are proportional to the time since those species diverged ● Fewer differences between frequency of amino acid substitutions if divergence of the two species was more recent Relative rates of accumulation of synonymous vs. nonsynonymous mutations - ● Mutations occur at a constant rate as they seem to be mostly deleterious or neutral ● Synonymous mutations (silent substitutions) occur at a much higher rate compared to nonsynonymous mutations (replacement substitution) ● Both the rates are relatively constant Variables that affect the rate of evolution of a particular protein - ● Some proteins, if changed, will mess up the function thus are constrained ● Not many changes can be made to certain proteins without affecting the structure and function deleteriously thus the rate of evolution of them are much lower ● Proteins which do not have very specific structure of function and can take changes without completely ruining have weaker constraints thus can evolve at higher rates Deduce time of divergence given number of amino acid changes in particular protein - ● By using the number of amino acid substitutions differences between species, since the rate of mutations seem to be generally constant, the rate can be used to determine the time Characteristics of the "molecular" clock - ● The molecular clock (based on the molecular clock hypothesis (MCH)) is a technique in molecular evolution that uses fossil constraints and rates of molecular change to deduce the time in geologic history when two species or other taxa diverged. ● Its graph has on the y-axis, the number of amino acid substitutions, and on the x axis, millions of years since divergence Regions of two unr
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