Microbial genetics _Ch

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Microbial genetics _Ch Powered By Docstoc
					Microbial genetics (Ch. 7) Part 3 -- Creating genetic diversity • • • • • • Types of mutations Mutagens Transduction Conjugation Transformation Transposons

Types of mutations (Fig. 7.22)
• Mutations are changes in the sequence of the genome • Point mutations are changes in only one basepair (bp) • AT-> GC, transition • AT-> CG, TA, transversion

Mutagens (Fig. 7.22, 7.23)
• UV light causes pyrimidine dimers • Chemical mutagens are analogs of normal bases or cause a chemical change that causes misreplication of DNA


Frameshift mutagens (Fig. 7.24)
• Frequently mutagens like ethidium bromide intercalate into DNA and cause DNAP to insert or delete bases


Horizontal gene transfer among prokaryotes via transformation
• Horizontal genetic transfers occurs within a population of the same generation (vs. vertical) • Typically occurs with only 1% of a population • Occurs cross-species and cross-genera, i.e., can pass genes to unrelated organisms • Transformation, transduction and conjugation all cause horizontal gene transfer • Many factors that contribute to pathogenesis may be transferred via these processes

Transformation (Fig. 7.30)

• Recipient cells (competent) take up DNA from environment and incorporate pieces into the chromosome • Some pathogens (Haemophilus, Neisseria, Streptococcus, Bacillus, Staphylococcus) are naturally competent • Competent cells can be artificially produced by injuring 6 cell membranes (Biotechnology)

Transduction (Fig. 7.31)
• Carried out by bacteriophage that sometimes package the host’s DNA by mistake • Phage also can transduce genes when they insert themselves into the chromosome and later pop out with attached host DNA

Bacterial conjugation (Fig. 7.32)
• Depends on specialized plasmids (F) that can transfer themselves from one cell into another (often unrelated genetically) via F pilus • Some transfer themselves to eukaryotic cells (Ti plasmid, R751) • Some R plasmids also transfer antibiotic resistance genes


Transposons (Fig. 7.35)

• Jumping genes first discovered by Barbara McClintock in corn • Transposons are mobile genetic elements that carry a gene that carries out the transposition and may carry other jeans as well • Transposons can be carried by plasmids or phage 10

Transposon structure (Brock, Fig. 10.29)


Mechanism of transposition (Fig. 7.36)
• Many of these sequences carry drug resistance genes and many drug resistant plasmids carry several transposons