Transcript Microbial Genetics

MICROBIOLOGY
WITH DISEASES BY TAXONOMY, THIRD EDITION
Chapter 7
Microbial Genetics
Lecture prepared by Mindy Miller-Kittrell, University of Tennessee, Knoxville
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Gene Mutation
Gene Mutation – a change in the sequence of bases
within a gene
 May have neutral, beneficial, or harmful effects (Almost
always harmful)
 May result in a change in amino acid or a nonfunctional
protein
 Some mutations are repaired by excising the mutant
nucleotides
 Rarely leads to a protein having a novel property that improves
ability of organism and its descendents to survive and
reproduce
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Types of Gene Mutations
Point mutations (most common) –
–A single base at one point in the DNA sequence is
inserted, deleted, or substituted by another
base
Frameshift mutations –
(a type of point mutation)
–One or several base pairs are deleted or
inserted into the DNA sequence
–Shifts the “translational reading frame”
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Types of Gene Mutations
Point mutations – 3 types
1. Silent Mutation - the change in the codon but no change in
amino acid; due to code redundancy
2.Missense Mutation - the change in the codon changes the
amino acid thus the protein
3.Nonsense Mutation - the change in the codon change
amino acid to Stop codon
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The effects of the various types of point mutations
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Figure 7.23
The Frequency of Mutation
• Spontaneous mutation rate
1 in 109 (10-9) replicated base pairs or
1 in 106 (10-6) replicated genes
• Mutagens (agents that cause a mutation) increase
mutation rate to 10–5 or 10–3 per replicated gene
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Mutations of Genes
Animation: Mutagens
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Mutagens
• Chemicals:
– Nitrous acid –
– mutates adenine to guanine
– Base analogs –
– mimic nitrogenous base; disrupt DNA and RNA replication and
cause point mutations
– ex. 5 bromo uracil
– Smoke/ethidium bromide
– causes insertion or deletion of base pairs
– Results in frameshift mutations
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Nitrogen Base Analog
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Mutagens
• Environment:
– Ionizing radiation – (X rays and gamma rays)
– Causes ion formations that react with bases and deoxyribosephosphate backbone
– induces breaks in chromosomes
– Nonionizing radiation –(UV rays)
– causes thymine to crosslink resulting in thymine dimers
– Cause errors in replication
and transcription
– The most common type
of mutation
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The action of a frameshift mutagen
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Figure 7.26
Transfer of Genetic Information
Horizontal gene transfer
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Gene transfer mechanisms
–Vertical Gene Transfer –
–Occurs in cell division; passing on gene’s to
offspring
–Horizontal Gene Transfer –
–Exchanging gene’s within the same
generation
–Results in a recombinant cell that has
received DNA from another cell
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Horizontal Gene Transfer
1.
2.
3.
4.
Transformation- transfer of naked DNA fragments
Transduction- mediated through a bacteriophage
Conjugation- transfer of DNA through a pilus
Transposition- migration of DNA “jumping
genes”
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Griffith’s Experiments of Transformation
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Figure 7.30
Transformation
•Transfer of naked
DNA fragments (from
environment) to cell
•Cells must be
competent
(able to take up the
DNA)
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Transduction
Mediated through a bacteriophage
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Figure 7.31
Bacterial Conjugation
Transfer of DNA through a pilus
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Figure 7.32a
Plasmids
Nucleoid region
Plasmid
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Figure 4.6a, b
Types of Plasmids
– F (fertility) factor
– Carries genes for sex pili and transfer of the plasmid
– R (resistance) factor
– Encodes antibiotic resistance
– Bacteriocin factor –
– Encodes for toxin that kills bacterial cell of the same or
similar species that lack that factor
– Virulence factor –
– Encode for enzymes, structures or toxins that make
bacteria pathogenic
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Conjugation
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Conjugation
Hfr plasmid Conjugation
Hfr plasmid
incorporates
directly into the
host genome so
increases
probability of
recombination
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Figure 7.33
Transposition
 Transposons are segments of DNA that can move from
one region of DNA to another
 They contain insertion sequences (palindromic) for
cutting and resealing DNA
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Figure 7.36
Transposons
a. Insertion sequence
b. Simple Transposon
c. Complex Transposon
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Figure 7.36a-c