Microbial Genetics, Mutation and Genetic Engineering
Domain 6 tests the central dogma in microbes, mechanisms of genetic exchange, mutation types and the tools of recombinant DNA technology. Expect questions on conjugation, transformation and transduction, and on how restriction enzymes and vectors are used.
Core rules and concepts
- DNA replication is semiconservative; transcription makes mRNA; translation at 70S ribosomes uses codons. Plasmids are extrachromosomal circular DNA that can carry antibiotic-resistance genes.
- Horizontal gene transfer: transformation (uptake of naked DNA), conjugation (direct contact, often plasmid transfer through a pilus), transduction (bacteriophage-mediated).
- Mutations include silent (no amino acid change), missense (different amino acid), nonsense (premature stop) and frameshift (insertion/deletion not in multiples of three). Mutagens include UV, alkylating agents and base analogues.
- Recombinant DNA: restriction endonucleases cut specific sequences; DNA ligase joins fragments; vectors (plasmids, phages) carry inserts into host cells; selection uses antibiotic markers, and screening identifies the desired clone. PCR amplifies DNA in vitro.
Worked example
A point mutation changes a codon from UAC (tyrosine) to UAA (stop). Reasoning: UAA is a stop codon, so translation ends early and a truncated protein is produced. This is a nonsense mutation. If UAC changed to UAU, tyrosine would still be encoded, making it a silent mutation.
Mark-losing traps
- Transformation is not phage-mediated; transduction is.
- Restriction enzymes cut DNA; DNA ligase joins it. Do not swap their roles.
- A frameshift changes every downstream codon, not just one amino acid.
- Selection (survival on antibiotic) is not the same as screening (detecting the insert, e.g., by blue-white colony colour).