Genetics BIOL 2120 - lecture notes - Mutation and DNA Repair

I. Mutation
     A. Change in DNA sequence
     B. Causes [two possibilities]
          1. Random - all capable of changing
          2. Physiologically induced - only some can change
     C. Fluctuation test (table 16.1)
          1. One large population, should produce same number of mutants
          2. Many small populations
               a. if mutations random, each culture will yield different numbers of mutants
          3. Results show mutations random; environmentally acquired traits are not inherited
II. Genetic fine structure (fig. 16.2, 16.3)
     A. Complementation: are two mutants in the same gene or different genes?
          1. Construct cis and trans heterozygotes
          2. If same gene, trans heterozygotes will still be mutant
          3. Mutants in same gene will not complement
          4. Cistron - unit of function; one-cistron-one-RNA
     B. If mutants at same site, no recombination; most mutants in one gene are at different places
     C. Fine-structure mapping - number of sites for mutation and recombination
          1. Benzer used rII mutants of T4 phage
          2. rII will not grow on lambda phage
          3. Locate within deletions- look for recombinants (fig. 16.7)
          4. All mutations arranged linearly
          5. Mutation and recombination can occur at nucleotide, not gene, level
          6. Some bases are more likely to mutate; these are hot spots
     D. Intra-allelic complementations - when functional protein is multimeric (fig. 16.9)
     E. Colinearity - sequence of bases arranged same way as amino acids (fig. 16.11)
          1. Yanofsky - trp operon - map mutants in one gene
          2. Correlate mutant position with altered amino acid in similar position in protein
III. Mutations
     A. X-ray induced similar in nature to spontaneous mutations
     B. Mutation rate
          1. Varies with gene and region of gene
          2. Mutations can revert
     C. Point mutation - single changes
     D. Frameshift mutations - add or subtract bases (figs. 16.12, 16.13)
          1. Change reading frame
          2. Change many amino acids
          3. May cause or alter stop signals
     E. Back mutation/suppression
          1. Correct original
          2. Second site restores function - separable by recombination
     F. Conditional-lethal
          1. Nutritional
          2. Temperature sensitive
     G. Spontaneous - some are result from tautomeric shift (figs. 16.14 - 16.18)
          1. Transition: purine to purine or pyrimidine to pyrimidine
          2. Transversion: purine to pyrimidine or pyrimidine to purine
     H. Chemical mutagens (fig. 16.19 - 16.23)
          1. 5-bromouracil: AT to GC shift
          2. 2-aminopurine: AT to GC shift
          3. Nitrous acid: AT to GC shift
          4. EMS (ethyl methane sulfonate), EES (ethyl ethane sulfonate): transversions by loss of base
          5. Acridine: small insertions, deletions
               a. Intragenic suppresion - second insertion or deletion that restores reading frame
               b. Back mutation - second mutation that restores identical gene sequence
     I. Misalignment (Fig. 16.24)
          1. Run of same base misalign - hot spots
          2. Forms small frameshifts
     J. Intergenic suppression
          1. Mutation in second gene
          2. Often tRNA mutants in anticodon
          3. Often inefficient and selected against in nature
     K. Mutators, antimutators - usually in enzymes for DNA synthesis, repair
IV. DNA repair
     A. Loss of ability to repair can lead to
          1. Mutations
          2. Cell death (apoptosis)
          3. Cancer
     B. Damage reversal: split dimers, methyl groups
     C. Excision repair: detect bulges and remove a few bases (Fig. 16.29)
          1. UV damage repair
               a. Cut out dimers; people with xeroderma pigmentosum lack excision repair enzymes
          2. AP repair: remove base from sugar
     D. Mismatch repair
          1. Recognize faulty pairing
          2. Recognize new strand; it is not yet methylated
     E. Some transcriptionally active genes are preferentially repaired
     F. Postreplicative repair: fill in gap caused by faulty duplication
          1. RecA (Fig. 16.32, 16.34)
               a. Binds SS DNA
               b. Invades DS DNA in search of complementary region
               c. Endonuclease nicks complementary DNA (so RecA steals the strand)
               d. Finally DNA polymerase and ligase fill in where DNA was removed
          2. SOS - many genes with consensus promoter (Fig. 16.35)
               a. Kicks in when high UV light or other mutagens
               b. LexA usually binds to SOS promoter (and 17 other genes) and prevents transcription
               c. RecA + LexA leads to inactive/degraded LexA,
               d. Leads to transcription of SOS repair enzyme (and other genes)
                    (1) inhibits DNA replication so more time for repair
                    (2) turns on other genes involved in repair
                    (3) repair is not always correct - panic situation in which many errors are made
V. Recombination (Fig. 16.36)
     A. Involves breakage - reunion of both strands
     B. Holliday model (fig. 16.36)
          1. Break both strands of each duplex
          2. broken strands pair with complement in other duplex

For questions, comments and additional information, contact mfhicks@pstcc.edu
Last Updated: June 24 2001
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