Ch8 microbial genetics

Copyright © 2010 Pearson Education, Inc.
CH8: Microbial Genetics
 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?

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
Structure and Function of Genetic Material

Terminology
 Genomics: The molecular study of genomes
(sequencing and molecular characterization of
genomes).
 Genotype: The genes of an organism
 Phenotype: Expression of the genes

Figure 8.2
The Flow of Genetic Information

Figure 8.3b
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

Figure 8.3a
Semiconservative Replication

Figure 8.4
DNA Synthesis

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

Figure 8.5
DNA Synthesis

Figure 8.6
Replication of Bacterial DNA

Table 8.1
Important Enzymes
DNA Gyrase
DNA Helicase
DNA polymerase

Table 8.1

RNA and Protein Synthesis
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

Figure 8.7
Transcription

Figure 8.7
The Process of Transcription

Figure 8.7
ANIMATION Transcription: Overview
ANIMATION Transcription: Process
The Process of Transcription

Figure 8.11
RNA Processing in Eukaryotes

Figure 8.2
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

Figure 8.2
The Genetic Code
 64 total codons of
which 61 sense codons
encode the 20 amino
acids
 3 are non-sense codons
or stop codons.
 The genetic code is
degenerate
 tRNA carries the
complementary
anticodon

Figure 8.8
The Genetic Code

Figure 8.10
Simultaneous Transcription &
Translation

Figure 8.9-1

Figure 8.9-2

Regulation
 Constitutive genes are expressed at a fixed rate
(60-80%genes)
 Other genes are expressed only as needed
− Repressible genes: Always active , can be turned off
(repressed).
− Inducible genes: Inactive , needs to be turned on or
induced.
− Catabolite repression: Inhibition of the metabolism of
alternative carbon sources by Glucose.
 Regulation of protein synthesis saves (conserve) energy
for the organism.
The Regulation of Gene Expression

Figure 8.12
Operon
Control of Transcription
Consists of a promotor, an operator and series of
genes

• Inducible operons – must be activated by inducers
• Regulate catabolic pathway
• Lactose Operon
• Repressible operons – transcribed continually until
deactivated by repressors
• Regulate anabolic pathway.
• Tryptophan Operon
Operons

Figure 8.12
Induction

Figure 8.13
Repression

 Lactose present, no
Glucose
 Lactose + glucose
present
Figure 8.15
Catabolite Repression

Figure 8.14b

Mutation
 A change in the genetic material
 Mutation is a change in the base sequence of DNA.
 Mutations may be neutral, beneficial, or harmful
 Mutagen: Agent that causes mutations
 Spontaneous mutations: Occur in the absence of a
mutagen

 Base substitution
(point mutation)
 Missense mutation
Mutation
 Change in one base
 Result in change in
amino acid
Figure 8.17a, b

 Nonsense mutation
Mutation
 Results in a nonsense
codon
Figure 8.17a, c

Mutation
 Frameshift mutation  Insertion or deletion of
one or more nucleotide
pairs
Figure 8.17a, d

 Spontaneous mutation rate = 1 in 109
replicated
base pairs or 1 in 106
replicated genes
 Mutagens increase to one in 105
(10-5
) or one in
103
(10–3)
per replicated gene
 Point mutations concept map
The Frequency of Mutation

Figure 8.19a
Chemical Mutagens
Nucleoside analogs

Chemical mutagens
 Frameshift mutagens
 Cause small deletions or insertions
 Ex:Benzopyrene in smoke
 Aflatoxins
 Acridine orange
 Frameshift mutagens are often carcinogens.

Radiation
 Ionizing radiation (X rays and gamma rays)
causes the formation of ions that can react with
nucleotides and the deoxyribose-phosphate
backbone
 Causes physical breakdown of DNA

Figure 8.20
Radiation
 UV radiation
causes thymine
dimers

Figure 8.20
Repair
 Photolyases separate thymine dimers
 Nucleotide excision repair (can repair other
mutations also)

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

Figure 8.21
Replica Plating

Figure 8.22
Ames Test for Chemical Carcinogens
90% of the chemicals found to be mutagenic also have been
shown to be carcinogenic.

• Exchange of DNA segments composed of homologous
sequences
• Recombinants – cells with DNA molecules that
contain new nucleotide sequences
• Vertical gene transfer – organisms replicate their
genomes and provide copies to descendants
• Horizontal gene transfer – donor contributes part of
genome to recipient; three types
• Transformation
• Transduction
• Bacterial Conjugation
Genetic Recombination and Transfer

 Vertical gene transfer:
Occurs during
reproduction between
generations of cells.
 Horizontal gene
transfer: The transfer of
genes between cells of
the same generation.
Genetic Recombination

Figure 8.23
Genetic Recombination
 Exchange of
genes between
two DNA
molecules
 Crossing over
occurs when two
chromosomes
break and rejoin

Figure 8.24
Genetic Transformation
Griffith’s Experiment

• Griffiths experiment concluded transforming agent was
DNA; one of conclusive pieces of proof that DNA is genetic
material
• Cells that take up DNA are competent; results from
alterations in cell wall and cytoplasmic membrane that allow
DNA to enter cell
Transformation

Figure 8.26
Bacterial Conjugation
Transfer of DNA by cell to cell contact

Figure 8.28
Transduction by a Bacteriophage
• Generalized
transduction –
transducing phage
carries random DNA
segment from donor to
recipient
• Specialized
transduction – only
certain donor DNA
sequences are
transferred

Plasmids
 Conjugative plasmid: Carries genes for sex pili and
transfer of the plasmid
 Dissimilation plasmids: Encode enzymes for
catabolism of unusual compounds
 Bacteriocins plasmids
 R factors: Encode antibiotic resistance

Figure 8.29
R Factor, a Type of Plasmid
Carry genes that confer resistance to
antibiotics.

Figure 8.30a, b
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

Genes and Evolution
 Mutations and recombination provide diversity
 Fittest organisms for an environment are selected by
natural selection

Ch8 microbial genetics

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