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Chapter 13 Textbook Notes

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BIOL 102
Daniel Lefebvre

Mutation – heritable change in the genetic material (order of DNA bases is changed permanently) Gene mutation – relatively small changes in the DNA sequence in a particular gene • Point mutation – affects single base pair (substitution, deletion, addition, missense) • Silent mutation/neutral mutation – does not affect function of protein • Nonesense mutation – changes normal codon to stop codon = truncated polypeptide • Frameshift mutation – addition or deletion of nucleotides that are not in multiples of 3 Sickle cell anemia – mutation in beta-globin gene which encodes polypeptide subunit that make up hemoglobin (common case: missense mutation alters the polypeptide sequence so 6 amino th acid is changed from hydrophilic glutamic acid to hydrophobic valine Except for silent mutations, new mutations are more likely to produce polypeptides that have reduced rather than enhanced function • Lederbergs – used replica plating to demonstrate that mutations are random events • Germ line – cells that give rise to gametes (eggs/sperm cells) • Somatic cells – constitute all cells of the body excluding germ-line cells • Genetic mosaic – and individual with somatic regions that are genetically different from each other (generally the earlier a mutation occurs, the larger the patch of mutated tissue) • Spontaneous mutation - result from abnormalities in biological processes (larger genes are more likely to incur a mutation than smaller genes) • Induced mutations – caused by environmental agents that enter cell, then alter DNA structure (increase mutation rate in comparison to spontaneous mutations) • Mutagens – chemical and physical substances/agents known to cause mutation and lead to change in DNA structure o Covalently modifying nucleotide structure (bases to not appropriately pair up) o An Interfere with DNA replication (addition or deletion) o Physical agents (i.e radiation) can create free radicals, cause DNA breakage or deletion Thymine dimer – a type of pyrimidine dimer which is a site at which two adjacent thymine bases become covalently cross-linked (proper base pairing doesn’t occur between template strand and incoming nucleotides when DNA polymerase attempts to replicate the dimer). This mispairing can cause gaps or incorporate incorrect bases. Ames test – testing method (developed by Bruce Ames in 1970’s) that can identify whether or not an agent is a mutagen DNA Repair: • Direct repair – repair enzyme recognizes an incorrect structure in the DNA and converts/corrects • Nucleotide excision repair (NER) – when a region encompassing several nucleotides in the damaged strand is removed from the DNA and the intact strand is used as a template for the resynthesis of a normal complementary strand; can fix many type of DNA damage (i.e. UV induced, chemically modified bases, missing bases, cross-linkage etc.) • In E. coli, the NER system is composed of 4 key proteins: UvrA, UvrB, UvrC and UvrD (Uvr comes from the fact that they are involved in ultraviolet light repair) Cell Cycle: 1. G (1irst gap) 2. S (synthesis of DNA, the genetic material) interphase 3. G (2econd phase) 4. M phase (mitosis and cytokinesis) • G 0hase – cells may exit the cell cycle and remain for long periods of time; substitutes for G1phase. Possibility to never divide again. • Length of cell cycle varies considerable among cell types (several mins  several months) • G1 is often longest and most vari
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