Genetics 3301

Chapter 14: Mutation, Repair & Recombination

Point mutations:

Ä Spontaneous versus induced mutations; Mutagens and the frequency with which they generate mutations (T-14-1); Mutagen dosage versus mutations induced (14-2).

Ä Base substitutions mutations ≠ transitions and transversions (T14-2); Base additions and deletions (T14-2); Molecular consequences of point mutations (T14-2, 14-3, 14-4).

Ä Tautomeric isoforms of bases and base analogs cause mutations due to altered pairing properties; Mechanism of transition base replacement via tautomeric shifts in the isomeric forms of natural bases from keto to enol or imino (14-5, 14-6); Inducing transition base replacements with modified bases such as 5-bromouracil and 2-amino purine (14-7, 14-8).

Ä Base alteration mutants modify a base causing specific mispairing of bases; Mechanism of base alteration using alkylating agents such as EMS and NG to generate transition mutations (14-9); Intercalating agents such as proflavin, acridine orange and ICR compounds mimic bases and cause single nucleotide insertion and deletion mutations (14-10).

Ä Base damage mutations result from damage to bases that eliminate base pairing; Lack of base pairing stalls polymerase and mutations result from the SOS repair mechanism; Base damage mutations arise due to the action of error prone polymerases (14-11); Error prone polymerases are induced by pyrimidine dimers generated by UV light and apurinic sites generated by aflatoxin (14-12, 14-13, 14-14); SOS dependent mutagens include most carcinogens, and lead to transitions and transversions.

Spontaneous mutations:

Ä Are mutants produced spontaneously and randomly or do physiological changes cause mutations to occur? - the Luria and Delbr¸ck fluctuation test (14-15); Replica plating from cells grown on nonselective media to selective media (14-16, 14-17).

Ä Spontaneous mutations caused by depurination (transitions or transversions), deamination and oxidized bases (transversions) (14-18, 14-19, 14-20); Errors in replication generate mutations via base substitutions (tautomeric shifts) or by base addition/deletion at repeated sequences (14-21); replication errors at repeated sequences lead to hot spots for mutation (14-22, 14-23); Fragile X syndrome and replication errors at trinucleotide repeats (14-24).

Repair Mechanisms:

Ä Direct reversal of pyrimidine dimers via photolyases (14-26) and alkyl groups added to guanine via alkyltransferases.

Ä Repairing altered bases via base excision repair involves base removal by DNA glycosylases, excising DNA around the removed base and repair via polymerase (14-27).

Ä Nucleotide excision repair can repair abnormal bases that distort the double helix such as pyrimidine dimers or aflatoxin bound guanine (14-28).

Ä Postreplication repair using the mismatch repair system (14-30).

Ä Repairing double strand beaks via nonhomologous end joining (14-32) and by homologous recombination (14-33).

Meiotic crossing-over:

Ä A double strand break is required to initiate homologous crossing-over; Heteroduplexes form during homologous recombination and can lead to altered ratios of alleles during meiosis by gene conversion; The double strand break model for homologous recombination (14-34).

Key terms: Know all of these.

Problems: 1 - 9, 11 -14, 17, 19, 22, 24, 25.