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Chapter 5

Chapter 5- Mutation and Genetic Variation.docx

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University of Waterloo
BIOL 359
Jonathan Witt

BIOL 359 Lecture 5 Janice Wong Evolution Mutation Chapter 5: Mutation and Genetic Variation • Meiosis reshuffles existing alleles into new combinations • Mutation is the only process that creates completely new alleles and new genes • Mutation is the ultimate source of genetic variation 5.1| Where New Alleles Come From The Nature of Mutation • A mutation is any change in the base sequence of DNA. • Genes are made of DNA, so changes in DNA create changes in genes • Complementary base pairing provided a mechanism for copying the hereditary material • Genes are stretches of DNA that code for a distinctive type of RNA or protein product • Alleles are versions of the same gene that differ in their base sequence Point Mutaitons • Point Mutation: create a change in a single base pair in DNA. They occur when errors in DNA replication or repair are not fixed properly o 1. Random errors in DNA synthesis o 2. Random errors in the repair of sites damaged by chemical mutagens or high- energy radiation  Both types result from reactions catalyzed by DNAP • Transition: a type of point mutation. Occurs when DNAP mistakenly substitutes a purine (A or G) for another purine, or pyrimidine (T or C) for another pyrimidine • Transversion: a type of point mutation if a purine is substituted for a pyrimidine or a pyrimidine for a purine. Ex. Substituting an A for a T • Transition is much more common • Hypothesis: transitions cause much less disruption in the DNA helix during synthesis, so are less likely to be recognized as an error and therefore less likely to be immediately corrected by DNAP or later by mismatch repair enzymes • If either type of base substitution occurs in the coding region of a gene, the mutation changes the codon read by the protein, called RNAP, that synthesizes RNA from the DNA template • Changes in the first or second position of a codon almost always change the genetic code • Changes in the third position frequently produce NO change at all • Replacement (nonsynonymous) substitutions: Point mutations that result in an AA change • Silent site (synonymous) substitutions: Point mutations that result in no change in the AA change Mutation Rates • Loss-of-function mutations: also known as knock out mutations. Changes in DNA that inactivate a gene, leading to a complete lack of gene product. Results in observable mutant phenotypes BIOL 359 Lecture 5 Janice Wong Evolution Mutation o Ex. Human dwarfism and haemophilia A (impaired blood clotting) • Natural selection had led to a single, common mutation rate Recent work: direct estimates of Mutation Rates • Researchers have begun to estimate mutation rates directly, by sequencing DNA from the same study population over time • Indels: small number of bases that had been inserted into or deleted from the genome • Mutation introduces a great deal of genetic variation into populations in every generation • Each population and each species has different mutation rate • Early results suggest that mutation rates 1) may exceed two new mutations per individual per generation, and 2) vary among populations and species Investigating Natural Selection on Mutation Rate • Enzymes responsible for copying and repairing DNA have been under intense selection • DNAP vary in their accuracy o There is a fundamental trade off between accuracy and speed: mutants that increased accuracy were slower than more error-prone mutants • Mutation rates vary due to variation in the structure of enzymes involved in DNA replication and repair • Higher mutation rates may be adaptive when organisms colonize new environments to which they are poorly adapted in • Individuals who are well adapted to their environment, then most mutations are likely to be deleterious-meaning they will lower fitness • Individuals with high mutations rates appear to be favoured when a population is in a novel or rapidly changing environment, where mutations are more likely to be beneficial The Fitness Effects of Mutations • Refer to Fig 5.5 • A) o Control populations were maintained in normal conditions of laboratory conditions, where large number of individuals competed for resources and a large sample of individuals were randomly chosen as the progenitors of the next generation o In the control populations, individuals with deleterious mutations would produce relatively few offspring and be less likely to be represented in the next generation o Hypothesis: vast majority of mutations are deleterious. If they are allowed to accumulate, fitness declines • B) o DNA sequences were inserted into random locations in the genomes of E.coli o Selection coefficient: the different between each experimental population and the control o The upwards of 70% of mutations have selection coefficients less than 2% o Because these mutations consisted of large insertions that were likely to knock out genes o Their average effect on fitness should be much greater than that of an average base substitution BIOL 359 Lecture 5 Janice Wong Evolution Mutation • The vast majority of mutations reduce fitness slightly or are neutral with respect to fitness • Few mutations are beneficial and that many mutations have little to no detectable effect on fitness • Neutral: alleles that have no effect on fitness • Silent site substitutions in DNA do not alter gene products and thus are not subject to natural selection • Summary: o Mutations are surprisingly frequent when considered on a per genome per generation basis o Mutation rate may vary considerably among populations and species o The vast majority of mutations are either neutral or deleterious 5.2| Where New Genes Come From Gene Duplication: can create new genes because it creates new DNA. Sources of Gene Duplication 1. Retrotransposition • (backward) (change in position) 2. Unequal Cross-Over • A change mistake caused by the proteins involved in managing the genetic recombination that occurs during meiosis • Fig. 5.6 • This happens when homologous chromosomes do not synapse correctly during prophase of meiosis I • Results in one chromosome that contains a deletion and one chromosome that contains redundant stretch of DNA • Retrotransposition and unequal crossover leave distinctive “footprints” in the genome • Retrotransposed genes lack introns and lack the nearby regulatory sequences found in the original gene • Unequal crossover result in genes that are duplicated and are foundback-to-back, or in tandem Rate of Gene Duplication • Gene duplications occur often enough to be an important source of genetic variation in populations over time The Fate of Duplicated Genes • When a DNA sequence is duplicated via retrotransposition or unequal crossover, the original gene should continue to produce a normal product • The duplicate
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