Microbial Genetics - Portland State University：微生物遗传学-波特兰州立大学.pptVIP

Microbial Genetics WHY? terms Genotype refers to genetic makeup Phenotype refers to expression of that genetic makeup Heritable traits must be encoded in DNA Mutations occur randomly Mutants resistant to an antibiotic Kinds of mutants Auxotroph: loss of biosynthetic enzyme Cold- or temperature-sensitive: enzyme function is restricted to narrower temperature range Drug resistance or sensitivity: permeability, degradation, or site of action Morphology: change in capsule or other feature leads to change in colony Catabolic: loss of a degradative enzyme leads to, for example, inability to ferment a carbohydrate Virus-resistant: loss of surface feature that is a virus receptor Results of point mutations Frame-shift mutations Reversions Same-site reversion: may be true revertant (same sequence) or just restore activity Second-site reversion Frame-shift Production of another enzyme that fulfills the function Mutagens Analogs for bases 5-bromo-uracil for thymine (5BU can pair with G as well as with A) 2-aminopurine for adenine (2AP can pair with C as well as with T) Intercalating agents (ethidium bromide) UV (260 nm) causes dimerization of adjacent thymines (photoreactivation) Ionizing radiation causes chemical changes to the DNA Generation of free radicals that can react with DNA SS or DS breaks in the DNA molecule Transposable genetic elements Recombination Recombination Genetic exchange: transformation Genetic exchange: plasmid transfer Small, usually circular, independently replicating DNA molecules Generally, G- plasmids replicate as does chromosomal DNA, G+ plasmids by “rolling circle” replication Genes of replication control, timing initiation on plasmid (ori) Some plasmids integrate (F+, Hfr) Most are double-stranded About 1- 100kb Code for: R-factors (R-plasmids) : antibiotic resistance, heavy metal resistance Virulence plasmids : adhesins, hemolytic factors, toxin, Ti, bacteriocins Degradation, tol, nah, Plasmid copy # Compatibility (inc) Avery Experiment Tra