Transcript Microbial genetics (Ch. 7) Part 3

Microbial genetics (Ch. 7)
Part 3 -- Creating genetic diversity
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Types of mutations
Mutagens
Transduction
Conjugation
Transformation
Transposons
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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
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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
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Frameshift mutagens (Fig. 7.24)
• Frequently mutagens
like ethidium bromide
intercalate into DNA
and cause DNAP to
insert or delete bases
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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
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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
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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
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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
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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)
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Mechanism of transposition (Fig. 7.36)
• Many of these
sequences carry drug
resistance genes and
many drug resistant
plasmids carry several
transposons
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