Download Microbiology

Chapter 8
Microbial Genetics
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Lectures prepared by Christine L. Case
Q&A
 E. coli is found
naturally in the
human large
intestine, and there
it is beneficial.
However, the strain
designated E. coli
O157:H7 produces
Shiga toxin. How
did E. coli acquire
this gene from
Shigella?
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Structure and Function of Genetic
Material
Learning Objectives
8-1 Define genetics, genome, chromosome, gene,
genetic code, genotype, phenotype, and
genomics.
8-2 Describe how DNA serves as genetic information.
8-3 Describe the process of DNA replication.
8-4 Describe protein synthesis, including
transcription, RNA processing, and translation.
8-5 Compare protein synthesis in prokaryotes and
eukaryotes.
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Terminology
 Genetics: The study of what genes are, how they
carry information, how information is expressed, and
how genes are replicated
 Gene: A segment of DNA that encodes a functional
product, usually a protein
 Chromosome: Structure containing DNA that
physically carries hereditary information; the
chromosomes contain the genes
 Genome: All the genetic information in a cell
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Terminology
 Genomics: The molecular study of genomes
 Genotype: The genes of an organism
 Phenotype: Expression of the genes
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Determine Relatedness
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Clinical Focus, p. 223
Determine Relatedness
 Which strain is more
closely related to the
Uganda strain?
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Strain
% Similar to
Uganda
Kenya
71%
U.S.
51%
E. coli
Figure 8.1a
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Genetic Map of the Chromosome of E.
coli
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Figure 8.1b
The Flow of Genetic Information
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Figure 8.2
DNA
 Polymer of nucleotides:
Adenine, thymine,
cytosine, and guanine
 Double helix associated
with proteins
 "Backbone" is
deoxyribose-phosphate
 Strands are held together
by hydrogen bonds
between AT and CG
 Strands are antiparallel
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Figure 8.3b
Semiconservative Replication
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Figure 8.3a
DNA Synthesis
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Figure 8.4
DNA Synthesis
 DNA is copied by DNA polymerase






In the 5'  3' direction
Initiated by an RNA primer
Leading strand is synthesized continuously
Lagging strand is synthesized discontinuously
Okazaki fragments
RNA primers are removed and Okazaki fragments joined
by a DNA polymerase and DNA ligase
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Table 8.1
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Table 8.1
DNA Synthesis
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Figure 8.5
Replication of Bacterial DNA
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Figure 8.6
Replication of Bacterial DNA
ANIMATION DNA Replication: Overview
ANIMATION DNA Replication: Forming the Replication Fork
ANIMATION DNA Replication: Replication Proteins
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Check Your Understanding
 Give a clinical application of genomics. 8-1
 Why is the base pairing in DNA important? 8-2
 Describe DNA replication, including the functions of
DNA gyrase, DNA ligase, and DNA polymerase. 8-3
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Transcription
 DNA is transcribed to make RNA (mRNA, tRNA, and
rRNA)
 Transcription begins when RNA polymerase binds to
the promoter sequence
 Transcription proceeds in the 5'  3' direction
 Transcription stops when it reaches the
terminator sequence
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Transcription
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Figure 8.7
The Process of Transcription
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Figure 8.7
The Process of Transcription
ANIMATION Transcription: Overview
ANIMATION Transcription: Process
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Figure 8.7
RNA Processing in Eukaryotes
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Figure 8.11
Translation
 mRNA is translated in
codons (three
nucleotides)
 Translation of mRNA
begins at the start
codon: AUG
 Translation ends at
nonsense codons:
UAA, UAG, UGA
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Figure 8.2
The Genetic Code
 64 sense codons on
mRNA encode the 20
amino acids
 The genetic code is
degenerate
 tRNA carries the
complementary
anticodon
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Figure 8.2
The Genetic Code
ANIMATION Translation: Overview
ANIMATION Translation: Genetic Code
ANIMATION Translation: Process
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The Genetic Code
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Figure 8.8
Simultaneous Transcription &
Translation
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Figure 8.10
The Process of Translation
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Figure 8.9
The Process of Translation
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Figure 8.9
The Process of Translation
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Figure 8.9
The Process of Translation
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Figure 8.9
The Process of Translation
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Figure 8.9
The Process of Translation
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Figure 8.9
The Process of Translation
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Figure 8.9
The Process of Translation
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Figure 8.9
Check Your Understanding
 What is the role of the promoter, terminator, and
mRNA in transcription? 8-4
 How does mRNA production in eukaryotes differ
from the process in prokaryotes? 8-5
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The Regulation of Gene Expression
Learning Objectives
8-6 Define operon.
8-7 Explain the regulation of gene expression in
bacteria by induction, repression, and catabolite
repression.
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Regulation
 Constitutive genes are expressed at a fixed rate
 Other genes are expressed only as needed
 Repressible genes
 Inducible genes
 Catabolite repression
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Operon
ANIMATION Operons: Overview
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Figure 8.12
Induction
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Figure 8.12
Induction
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Figure 8.12
Repression
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Figure 8.13
Repression
ANIMATION Operons: Induction
ANIMATION Operons: Repression
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Figure 8.13
Catabolite Repression
(a) Growth on glucose or lactose alone
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(b) Growth on glucose and lactose
combined
Figure 8.14
 Lactose present, no
glucose
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 Lactose + glucose
present
Figure 8.15
Check Your Understanding
 What is an operon? 8-6
 What is the role of cAMP in catabolite repression?
8-7
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Mutation: Change in the Genetic
Material
Learning Objectives
8-8 Classify mutations by type.
8-9 Define mutagen.
8-10 Describe two ways mutations can be repaired.
8-11 Describe the effect of mutagens on the mutation
rate.
8-12 Outline the methods of direct and indirect
selection of mutants.
8-13 Identify the purpose of and outline the procedure
for the Ames test.
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Mutation




A change in the genetic material
Mutations may be neutral, beneficial, or harmful
Mutagen: Agent that causes mutations
Spontaneous mutations: Occur in the absence of a
mutagen
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Mutation
 Base substitution
(point mutation)
 Missense mutation
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 Change in one base
 Result in change in
amino acid
Figure 8.17a, b
Mutation
 Nonsense mutation
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 Results in a nonsense
codon
Figure 8.17a, c
Mutation
 Frameshift mutation
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 Insertion or deletion of
one or more nucleotide
pairs
Figure 8.17a, d
The Frequency of Mutation
 Spontaneous mutation rate = 1 in 109 replicated
base pairs or 1 in 106 replicated genes
 Mutagens increase to 10–5 or 10–3 per replicated
gene
ANIMATION Mutations: Types
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Chemical Mutagens
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Figure 8.19a
Chemical Mutagens
ANIMATION Mutagens
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Figure 8.19b
Radiation
 Ionizing radiation (X rays and gamma rays) causes
the formation of ions that can react with nucleotides
and the deoxyribose-phosphate backbone
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Radiation
 UV radiation
causes thymine
dimers
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Figure 8.20
Repair
 Photolyases separate thymine dimers
 Nucleotide excision repair
ANIMATION Mutations: Repair
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Figure 8.20
Selection
 Positive (direct) selection detects mutant cells
because they grow or appear different
 Negative (indirect) selection detects mutant cells
because they do not grow
 Replica plating
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Replica Plating
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Figure 8.21
Ames Test for Chemical Carcinogens
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Figure 8.22
Ames Test for Chemical Carcinogens
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Figure 8.22
Check Your Understanding
 How can a mutation be beneficial? 8-8
 How are mutations caused by chemicals? By
radiation? 8-9
 How can mutations be repaired? 8-10
 How do mutagens affect the mutation rate? 8-11
 How would you isolate an antibiotic-resistant
bacterium? An antibiotic-sensitive bacterium?
8-12
 What is the principle behind the Ames test? 8-13
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Genetic Transfer and Recombination
Learning Objectives
8-14 Differentiate horizontal and vertical gene
transfer.
8-15 Compare the mechanisms of genetic
recombination in bacteria.
8-16 Describe the functions of plasmids and
transposons.
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Genetic Recombination
 Vertical gene transfer:
Occurs during
reproduction between
generations of cells.
 Horizontal gene
transfer: The transfer of
genes between cells of
the same generation.
ANIMATION Horizontal Gene Transfer: Overview
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Genetic Recombination
 Exchange of
genes between
two DNA
molecules
 Crossing over
occurs when two
chromosomes
break and rejoin
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Figure 8.23
Genetic Recombination
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Figure 8.25
Genetic Transformation
ANIMATION Transformation
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Figure 8.24
Bacterial Conjugation
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Figure 8.26
Conjugation in E. coli
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Figure 8.27a
Conjugation in E. coli
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Figure 8.27b
Conjugation in E. coli
ANIMATION F Factor
ANIMATION Chromosome Mapping
ANIMATION Conjugation: Overview
ANIMATION Hfr Conjugation
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Figure 8.27c
Transduction by a Bacteriophage
ANIMATION Generalized
Transduction
ANIMATION Specialized
Transduction
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Figure 8.28
Q&A
 E. coli is found naturally
in the human large
intestine, and there it is
beneficial. However, the
strain designated E. coli
O157:H7 produces
Shiga toxin. How did
E. coli acquire this gene
from Shigella?
Copyright © 2010 Pearson Education, Inc.
Plasmids
 Conjugative plasmid: Carries genes for sex pili and
transfer of the plasmid
 Dissimilation plasmids: Encode enzymes for
catabolism of unusual compounds
 R factors: Encode antibiotic resistance
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R Factor, a Type of Plasmid
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Figure 8.29
Transposons
 Segments of DNA that can move from one region of
DNA to another
 Contain insertion sequences for cutting and resealing
DNA (transposase)
 Complex transposons carry other genes
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Figure 8.30a, b
Transposons
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Figure 8.30c
Transposons
ANIMATION Transposons: Overview
ANIMATION Transposons: Insertion Sequences
ANIMATION Transposons: Complex Transposons
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Check Your Understanding
 Differentiate horizontal and vertical gene transfer.
8-14
 Compare conjugation between the following pairs:
F+  F–, Hfr  F–. 8-15
 What types of genes do plasmids carry? 8-16
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Genes and Evolution
Learning Objective
8-17 Discuss how genetic mutation and
recombination provide material for natural
selection to act upon.
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Genes and Evolution
 Mutations and recombination provide diversity
 Fittest organisms for an environment are selected by
natural selection
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Evolution
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Clinical Focus, p. 223
Evolution
 Which strain is more
closely related to the
Uganda strain?
 How did the virus
change?
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Strain
% Similar to
Uganda
Kenya
71%
U.S.
51%
Check Your Understanding
 Natural selection means that the environment favors
survival of some genotypes. From where does
diversity in genotypes come? 8-17
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