02-06_gene_dup - BIOLOGY 325H Molecular Evolution and Gene...

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BIOLOGY 325H Molecular Evolution and Gene Duplication 2/6/08 A central dogma of modern biology is that breeding populations acquite genetic diversity through mutations, and that natural selection and genetic drift act on the frequency of those mutant alleles over succeeding generations. Some newly arisen alleles are eventually lost; others spread through the population until they reach fixation, i.e. become the 'wild-type' allele for that population. In today's lecture, we will review the molecular basis of gene divergence, both in the context of speciation and following gene duplications. We will also look at gene trees, as a prelude to thinking about phylogenetic trees in Friday's lecture. Learning goals 1. In the coding region of a gene, why do only some nucleotide substitutions (= point mutations) alter the amino acid sequence of the protein? What names are used to distinguish substitutions that do alter protein sequence from those that do not? On average, which of these two types of mutation is more likely to become fixed over the course of future evolution? Why? 2. What does it mean when two genes are described as 'homologous'? You should appreciate that 'orthology' and 'paralogy' are two distinct types of homology, and be able to explain the difference. 3. What does it mean to say that a pair of orthologous genes diverge in two sibling species that arisen from a single speciation event? How could you quantitate the degree of divergence between these two genes? How could you quantitate the divergence of their products? In general, what is the relationship between the degree of divergence and the time that has elapsed since the speciation event? 4. What is a gene family, and how does it relate to the phenomenon of gene duplication? You should appreciate that a single gene duplication often occurs in a single species, but that a gene family can extend across many species. What chemical events must happen to the genomic DNA in order for gene duplication to occur? 5. Assume that a duplication event produces two identical copies of a gene that are initially identical in function. How do you expect their functions to change during subsequent evolution? You should be able to explain three different, stable outcomes that can arise from this situation. 6. In what sort of animal species did the primitive globin gene duplicate to produce
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02-06_gene_dup - BIOLOGY 325H Molecular Evolution and Gene...

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