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BGY Chapter_5.doc

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BIOL 1020

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Chapter 5: Mutation and Genetic Variation (pages 143 – 163) The genetic variation that natural selection and other evolutionary forces act on originates in mutation. Meiosis can be responsible for this because of the crossing over process can result in new grouping of alleles. However, meiosis reshuffles existing alleles into new combinations. Therefore, only mutation can create new alleles and new genes. Once this variation is produce, then selection, drift, and migration can act. 5.1. Where New Alleles Come From • The Nature of Mutation o Mutation is any change in DNA. Genes are made of DNA, so changes in DNA create changes in genes o Consider the mutation for the human gene for hemoglobin that results in sickle-cell  In 1958, Vernon Ingram showed that the difference between normal and sickle-cell hemoglobin was due to a single amino acid change at position number 6 in the protein chain.  Instead of having glutamic acid in this position, the sickle cell allele has valine; caused by a single base substitution in the hemoglobin gene.  A change like this is called a point mutation which is a change in a single base pair in DNA and occurs when errors in DNA replication or repair are not fixed properly. o Point mutations are caused by one of two processes:  Random errors in DNA synthesis  Random errors in the repair of sites damaged by chemical mutagens or high-energy radiation o If DNA polymerase mistakenly substitutes a purine for another purine or a pyrimidine for another, the resulting point mutation is known as transition. o If a purine is substituted for a pyrimidine or a pyrimidine for a purine, the mutation that results is called a transversion.  Transitions are most common o Changes in the first or second position of a codon almost always change the amino acid specified by the resulting mRNA; however, due to the redundancy of the genetic code, changes in the third position frequently produce no change at all.  Point mutations that results in an amino acid change are called replacement substitutions.  Those that result in no change in the amino acid coded are silent site substitutions  Both types result in new alleles • Mutation Rates o Read about this research done on page 148 o The research suggests that the mutation rate per cell divisions is approximately equal in most or all organisms o Suggested that natural selection had led to a single, common mutation rate o Read about Recent Work on pages 148 – 149 (insignificant material)  Early results suggest that mutation rates may exceed two new mutations per individual per generation and vary among population and species  Mutation rates vary due to variation in the structure of enzymes involved in DNA replication and repair. High mutation rates may be selectively advantageous in novel or rapidly changing environments • The Fitness Effects of Mutations o Read page 150 (no need to memorize the material; just read it)  The vast majority of mutations reduce fitness slightly or are neutral (e.g. silent mutation) with respect to fitness 5.2 Where New Genes Come From Gene mutations are probably the most important source of new genes. Most duplications result from one of two processes: • Retrotransposition – the process begins when a processed messenger RNA (lacks introns but has a poly [A] tail), is reverse-transcribed by the enzyme reverse transcriptase to form a double stranded segment of DNA. • Unequal Crosser – this is more common. It is a chance mistake caused by the proteins involved in managing the genetic recombination that occurs during meiosis. o See Fig. 5.6 on page 153 o Begins when homologous chromosomes do not synapse correctly during prophase of meiosis I.  Results in one chromosome that contains a deletion and one that contains a redundant stretch of DNA • Rates of Gene Duplication o Estimates suggest that the average rate of genes that duplicate and increase to high frequency in populations at 0.01 per gene per million years.  This means that in species with a genome containing 10 000 genes, a gene is duplicated and increases to high frequency every 10 000 years on average • The Fate of Duplicated Gene
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