1 Independent Study Guide - Bacterial Genetics (Chapter 8) I. Genetic change in bacteria (p. 186-187) a. Two general mechanisms of genetic change in bacteria (figure 8.1): i. Mutation – alteration in existing DNA sequence ii. DNA transfer (horizontal gene transfer) – acquisition of DNA from another source b. Why study bacterial genetics? i. Model system c. Spontaneous mutations occur in all cells at a very low frequency (≈one per billion nucleotides) i. Bacteria quickly grow to high concentrations (10 9 /ml) in culture, making it possible to study rare occurrences d. Terms i. Phenotype – the observable characteristics of an organism ii. Genotype – the sequence of nucleotides in the DNA of an organism iii. Wild type – characteristics similar to the organism as it occurs in nature iv. Prototroph – a strain that grows on minimal medium (glucose-salts) v. Auxotroph – a strain that lacks the ability to synthesize a nutrient; as a consequence, that compound must be supplied in the growth medium II. Spontaneous Mutation (section 8.2) a. Mistakes made during replication i. Base substitution (figure 8.3) 1. Silent 2. Missense 3. Nonsense ii. Removal or addition of nucleotides 1. Frameshift mutation (figure 8.4) a. Generates an entirely different set of triplets b. Often, a stop codon is generated III. Transposable elements (transposons) (p. 189; p. 207) a. Segments of DNA that can "jump or hop" from one site to another i. Insertional inactivation of the gene in which the transposon lands ii. A transposon can insert elsewhere in the same DNA molecule, or into an entirely different DNA molecule iii. Some transposons simply “jump or hop”; others replicate then jump IV. Induced Mutation (section 8.3) a. Chemical mutagens i. Chemically modify purines and pyrimidines 1. Alter the base-pairing properties 2. Example: nitrous acid strips the amino groups from nucleotides ii. Base analogs 1. Resemble nucleotide bases; erroneously incorporated into DNA 2. Consequence of incorporation: analog base-pairs with a different nucleotide iii. Intercalating agents 1. Insert between base-pairs, pushing nucleotides apart; extra nucleotide may then be erroneously added during replication b. Biological mutagens i. Transposons c. Physical mutagens i. Ultraviolet irradiation- causes formation of covalent bonds (thymine dimers) between adjacent thymine bases ii. X rays -causes double- and single-stranded breaks in DNA