Lecture21S11 - BIS101/Engebrecht Lecture21 5/19/11 Website...

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BIS101/Engebrecht Lecture21 5/19/11 Website for mutations: http://www.justinbadal.com/mutations/index.html DNA damage, repair and recombination In this class we have examined three processes in eukaryotes that provide genetic change: 1) Meiotic chromosome segregation (Mendel’s law of independent assortment) 2) Genetic recombination 3) Mutation!!!!! We discussed the relationship between DNA damage and mutation. DNA damage is any modification of the DNA that alters the chemical structure of the DNA double helix. This excludes modifications like methylation of bases where specific cellular enzymes are utilized (e.g. CpG methylation). DNA damage can occur at the bases, the sugars and the sugar-phosphate backbone. DNA damage can be cytotoxic, meaning it interferes with essential cellular processes like replication and transcription and therefore leads to cell death or it can be mutagenic, meaning is alters base pairing potential of the damaged base resulting in a change of nucleotide at a given position. If high level of DNA damage occurs, the cell could purposefully induce cell death to prevent abnormal cells from multiplying and resulting in cancer. A mutation is the heritable change in DNA sequence that results in a change in the sequence of the normal bases. Remember, a mutation may or may not result in a phenotype. The relationship between DNA damage and mutation is complicated and depends on the ability of the cell to either tolerate (meaning not fix the damage) or fix the damage either back to the normal base or to a change in the base (mutation). The ability to fix damage is dependent on a large number of genes that are used to sense and respond to DNA damage, we will discuss a number of these proteins. Are mutations adaptive (ala Lamark) or do they preexist and are selected (ala Darwin)? Luria and Delbruck did an experiment with E. coli and the lytic phage T1 to try to address this issue. They grew E. coli with T1 and looked at the number of resistant colonies that arose (i.e., E. coli mutants that were resistant to infection by T1). They reasoned that if mutations were adaptive there would not be a large variation in mutation frequency when a selective pressure is applied but if the mutations were preexisting then one would see a large variation (or fluctuation) in mutations frequency between different populations. They in fact did see a large fluctuation among the different populations and therefore concluded that mutations were preexisting and random. Figure 14-15 in 8 th ; Figure 15-5 in 9 th ; Figure 16-5 in10th show a cartoon of this idea. This remains a controversial issue – Dr. Roth on campus is performing research on this topic. This has important implication for evolution. This type of experiment can also tell you the mutation rate as described in your book; however, you do not need to know how to calculate this. Mutations
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This note was uploaded on 06/05/2011 for the course BIS 101 taught by Professor Simonchan during the Spring '08 term at UC Davis.

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Lecture21S11 - BIS101/Engebrecht Lecture21 5/19/11 Website...

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