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Lecture 23

Biology 1002B Lecture 23: Class 23 and Class 24

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Biology 1002B
Denis Maxwell

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Class 23 Molecular convergence Synonymous vs nonsynonymous mutations Synonymous mutations dont result in changes in amino acid sequences; neutral Occur at a relatively constant rate over evolutionary time Nonsynonymous mutations change amino acid sequence; advantageousdeleterious Characteristics of the neutral theory of molecular evolution Previously we thought that all mutations caused a change in protein sequence; this was the selection theory could be advantageous or deleterious (mostly deleterious) Now, comparing nucleotide to amino acid sequence has shown that most mutations are actually neutral (synonymous) This is gene evolution without natural selection Relationship between frequency of amino acid substitutions in given proteins vs. time since common ancestor Neutral mutations occur at a constant rate over time; number of neutral substitutions should be proportional to time Can find the number of neutral differences in cytochrome c gene; based on that we can tell how long ago species diverged Reflects a constant rate of mutation; more differences if two species diverged long ago Most substitutions are neutral; not advantageousdeleterious By counting the number of differences in a gene, you could use that as a molecular clock to find out how long ago two species shared a common ancestor To figure out when two species (distantly related) diverged, you look at the of amino acid substitutions and the time since their last common ancestor Relative rates of accumulation of synonymous vs. nonsynonymous mutations. Synonymous (silent) mutations occur at HIGHER rate than nonsynonymous (replacement) mutations synonymous mutations dont have an effect on the proteins; meanwhile nonysnonymous mutations are likely deleterious and will cause organisms to die before propagation Rate of nucleotide substitutions is also much higher in introns and pseudogenes because they dont have a protein product necessarily, but they may change phenotype in some untraditional way Variables that affect the rate of evolution of a particular protein Degree of constraint of a protein effects its rate of evolution If the protein is constrained in its shape (e.g. cytochrome c), it cant afford to change at all so it has low rates of mutations You cannot change the primary sequence of cytochrome c without having a negative effect on the protein (it is highly constrained and has evolved very little over evolutionary time) Hemoglobin isnt as constrained since you can change a couple of amino acids
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