D’YOUVILLE COLLEGE
BIOLOGY 102 - INTRODUCTORY BIOLOGY II
LECTURE # 30
BACTERIAL GENES
1.
Sources of Genetic Variability in Prokaryotes:
• mutation: rare event (1 in 10 million) involving change in a particular gene
- intestinal populations of bacteria may grow 20 billion new cells each day; this
would result in 2000 mutations/day in a particular gene; # of accumulated
mutations/day in the entire bacterial genome would be a staggering 9 million
- deleterious (harmful) mutations will kill or severely debilitate their
owners; beneficial ones will tend to accumulate, producing a significant source of
genetic variability
• recombination - various strategies for acquisition of genes from different
strains
2.
Mechanisms of Bacterial Recombination:
• transformation: uptake of extracellular DNA followed by “synapsis” with
host chromosome and some crossing over (recall Griffith experiment (fig. 16 – 2 & ppt. 1)
• transduction: bacteriophages are ‘unwitting’ vehicles for transfer of
bacterial genes from one bacterium to another; two types:
- generalized (virulent ‘phages are vehicles): some bacterial genome packaged
in viral capsid during lytic cycle; subsequent infection of another bacterium transfers
bacterial genes; new genes incorporated by new host chromosome (fig. 27 – 11 & ppts. 2
& 3)
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Bio 102
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- specialized (temperate ‘phages are vehicles): some adjacent bacterial genes
accompany prophage excision as a temperate phage resumes lytic cycle; genes transfer
to new host upon infection; powerful vehicle for genetic recombination (ppts. 2 & 3)
• conjugation: transfer of genes between different mating types (discussed
with plasmids below)
3.
Plasmids: “accessory” chromosomes first found in bacteria
• sex factor (F-plasmid): facilitates conjugation (sex pilus formation (fig. 27 –
12 & ppt. 4) facilitates connection & gene transfer between donor & recipient)
- occurrence of conjugation was suspected from experiments showing
recombinant strain in cultures of mixture of two different nutrient-requiring strains
(ppt. 5)
- F+ mating type (donor) has sex factor; F- mating type (recipient) lacks sex
factor; F- converted to F+ following conjugation; transfer of sex factor (F plasmid) causes
conversion (fig. 27 – 13a & ppts. 6)
- Hfr strains are donors with integrated (episomic) sex factor; conjugate
more frequently, transfer chromosomal genes + incomplete sex factor); gene
recombination occurs in recipient, e.g. met-/bio- strain x thr-/leu- strain:
recombinant strain grows on minimal medium (fig. 27 – 13b & ppt. 7)
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Bio 102
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• R-plasmids carry genes for antibiotic resistance; some have genes for sex pili
and can transfer like F-plasmids
4.
Transposons: are genes or groups of genes that can move to a new location on a
plasmid or within the main genome (transposition)
• insertion sequence codes for transposase (enzyme that catalyzes
transposition)(ppts. 8 & 9)
• composite transposon: one or more chromosomal genes sandwiched between two
insertion sequences; transferred from chromosome to plasmid (or vice versa) or from
one plasmid to another; mechanism for concentration of antibiotic resistance genes
in R- plasmid (ppt. 10)