Microbial Genetics and Molecular Biology
Domain 4 covers information flow and bacterial gene transfer. The exam asks for accurate base pairing, enzyme roles and the distinction between mutation and recombination.
Information flow
DNA is an antiparallel double helix; A pairs with T and G with C. Replication is semiconservative: helicase unwinds, primase makes an RNA primer, DNA polymerase III extends 5′ to 3′, DNA polymerase I replaces the primer, and ligase seals nicks. Transcription uses RNA polymerase; RNA contains uracil instead of thymine. Translation occurs at 70S ribosomes: mRNA codons are read 5′ to 3′, tRNA carries anticodons, AUG is the start codon and UAA, UAG and UGA are stops.
Mutation and gene transfer
Point mutations are silent, missense or nonsense; insertions and deletions cause frameshifts. Mutagens include ultraviolet light (thymine dimers), alkylating agents and base analogues; the Ames test screens for mutagens. Transformation is uptake of naked DNA (Griffith; competent cells). Conjugation requires direct contact through an F pilus and transfers plasmids. Transduction is bacteriophage-mediated. Plasmids are extrachromosomal and often carry antibiotic resistance. The lac operon is inducible: the repressor binds the operator, lactose relieves repression, and glucose causes catabolite repression.
Worked example
A codon changes from UAC (tyrosine) to UAA. Reasoning: UAA is a stop codon, so translation terminates early; this is a nonsense mutation and usually gives a truncated, non-functional protein. If UAC changed to UAU, which also codes for tyrosine, the mutation would be silent.
Traps
Transformation needs no cell contact; conjugation does. Transduction requires a phage. Not every mutation changes phenotype. RNA contains uracil, not thymine. The lac operon is induced by lactose, not repressed by it. Do not confuse genotype with phenotype.