Repair of Mutations Since mutations can be potentially fatal, the cell has several enzymatic repair mechanisms in place to find and repair damaged DNA • DNA polymerase – proofreads nucleotides during DNA replication • Mismatch repair – locates and repairs mismatched nitrogen bases that were not repaired by DNA polymerase • Light repair – for UV light damage • Excision repair – locates and repairs incorrect sequence by removing a segment of the DNA and then adding the correct nucleotides The Ames Test 1 Any chemical capable of mutating bacterial DNA can similarly mutate mammalian DNA Agricultural, industrial, and medicinal compounds are screened using the Ames test Indicator organism is a mutant strain of Salmonella enterica that has lost the ability to synthesize histidine This mutation is highly susceptible to back-mutation The Ames Test 2 (d): Courtesy of Xenometrix AG Positive and Negative Effects of Mutations Mutations leading to nonfunctional proteins are harmful, possibly fatal Organisms with mutations that are beneficial in their environment can readily adapt, survive, and reproduce – these mutations are the basis of change in populations Any change that confers an advantage during selection pressure will be retained by the population Concept Check: (9) Which of the following mutations would cause a frameshift mutation? A.Silent mutation B.Missense mutation C.Nonsense mutation D.Deletion mutation Concept Check: (10) Which of the following mutations would cause a frameshift mutation? A.Silent mutation B.Missense mutation C.Nonsense mutation D.Deletion mutation Answer: D DNA Recombination Events Genetic recombination – occurs when an organism acquires and expresses genes that originated in another organism 3 means for genetic recombination in bacteria: • Conjugation • Transformation • Transduction Conjugation 1 Transfer of a plasmid or chromosomal fragment from a donor cell to a recipient cell via direct contact Gram-negative cell donor has a fertility plasmid (F factor) that allows the synthesis of a conjugative pilus Donor (F+ cell) transfers fertility plasmid through pilus to recipient (F− cell), which becomes F+ cell Some F+ cells become Hfr cells (high frequency of recombination) Conjugation 2 Physical Conjugation (1) The pilus of donor cell (top) attaches to receptor on recipient cell and retracts to draw the two cells together. This is the mechanism for gram-negative bacteria. Conjugation 3 High-frequency recombination – donor’s fertility plasmid is integrated into the bacterial chromosome When conjugation occurs, a portion of the chromosome and a portion of the fertility plasmid are transferred to the recipient Conjugation 4 F Factor Transfer (2) Transfer of F factor, or conjugative plasmid Conjugation 5 Hfr Transfer (3) High frequency (Hfr) transfer involves transmission of chromosomal genes from a donor cell to a recipient cell. The donor chromosome is duplicated and transmitted in part to a recipient cell, where it is integrated into the chromosome.