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Chapter 7 Microbial Genetics
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The Structure and Replication of Genomes
• Genetics
• Study of inheritance and inheritable traits as expressed
in an organism's genetic material
• Genome
• The entire genetic complement of an organism
• Includes its genes and nucleotide sequences
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• The Structure of Prokaryotic Genomes
• Prokaryotic Chromosomes
• Main portion of DNA, along with associated proteins and
RNA
• Prokaryotic cells are haploid (single chromosome copy)
• Typical chromosome is a circular molecule of DNA in the
nucleoid
The Structure and Replication of Genomes
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• The Structure of Prokaryotic Genomes
• Plasmids
• Small molecules of DNA that replicate independently
• Not essential for normal metabolism, growth, or
reproduction
• Can confer survival advantages
• Many types of plasmids:
• Fertility factors
• Resistance factors
• Bacteriocin factors
• Virulence plasmids
The Structure and Replication of Genomes
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The Structure and Replication of Genomes
• DNA Replication
• Other Characteristics of Bacterial DNA Replication
• Bidirectional
• Gyrases and topoisomerases remove supercoils in DNA
• DNA is methylated
• Control of genetic expression
• Initiation of DNA replication
• Protection against viral infection
• Repair of DNA
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Figure 7.7 The bidirectionality of DNA replication in prokaryotes.
Origin Parental strand
Daughter strand
Replication forks
Replication proceeds in both directions Termination
of replication
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• The Relationship Between Genotype and
Phenotype
• Genotype
• Set of genes in the genome
• Phenotype
• Physical features and functional traits of the organism
• Genotype determines phenotype
• Not all genes are active at all times
Gene Function
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• The Transfer of Genetic Information
• Transcription
• Information in DNA is copied as RNA
• Translation
• Polypeptides synthesized from RNA
• Central dogma of genetics
• DNA transcribed to RNA
• RNA translated to form polypeptides
Gene Function
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Figure 7.8 The central dogma of genetics.
3′
5′
3′
3′
5′
5′
Transcription
Translation
by ribosomes
NH2 Methionine Arginine Tyrosine Leucine Polypeptide
mRNA
DNA
(genotype)
Phenotype
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Figure 7.9 The events in the transcription of RNA in prokaryotes.
RNA polymerase attaches
nonspecifically to DNA and
travels down its length until
it recognizes a promoter
sequence. Sigma factor
enhances promoter
recognition in bacteria.
Upon recognition of the
promoter, RNA polymerase
unzips the DNA molecule
beginning at the promoter.
Unzipping of DNA, movement of RNA polymerase
"Bubble"
Attachment of RNA polymerase
Sigma factor
RNA polymerase
Promoter Terminator
DNA 3′
5′
3′
5′
Template
DNA strand
5′
3′
5′
3′
Initiation of transcription
"Bubble"
5′
3′
Triphosphate ribonucleotides
align with their DNA
complements, and RNA
polymerase links them
together, synthesizing RNA.
No primer is needed. The
triphosphate ribonucleotides
also provide the energy
required for RNA synthesis.
Growing RNA molecule
(transcript) 5′
3′
5′
3′
3′
5′
Template
DNA
strand
Elongation of the RNA transcript
5′
3′ 5′
3′
Terminator 3′ 5′
RNA transcript
released
Self-termination: transcription of GC-rich terminator
region produces a hairpin loop, which creates tension,
loosening the grip of the polymerase
on the DNA.
Rho-dependant termination: Rho pushes between polymerase
and DNA. This causes release of polymerase, RNA transcript,
and Rho.
RNA polymerase
Rho termination
protein
Rho protein moves
along RNA
Terminator 3′
Template
strand
Terminator
Termination of transcription: release of RNA polymerase
GC-rich
hairpin
loop
3′
5′
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Figure 7.11 Processing eukaryotic mRNA.
Exons (polypeptide coding regions)
Template
DNA strand
Introns (noncoding regions)
Transcription
Exon 1 Exon 2 Exon 3 Exon 4 Pre-mRNA
Poly-A tail Intron 3 Intron 2 Intron 1 cap 5′
Processing
Spliceosomes
mRNA
splicing
3′
Exon 4
Exon 3 Exon 1
Exon 2
5′
5′
Intron 1
mRNA (codes for
one polypeptide) 3′
Nuclear envelope
Nuclear pore Nucleoplasm
Cytosol
mRNA
5′ 3′
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• Translation
• Events in Translation
• Three stages of translation:
• Initiation
• Elongation
• Termination
• All stages require additional protein factors
• Initiation and elongation require energy (GTP)
Gene Function
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Translation: The Process
Translation: The Process PLAY
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Gene Function
• Regulation of Genetic Expression
• Most genes are expressed at all times
• Other genes transcribed and translated when cells need
them
• Allows cell to conserve energy
• Regulation of polypeptide synthesis:
• Typically halts transcription
• Can stop translation directly
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Mutations of Genes
• Mutation
• Change in the nucleotide base sequence of a genome
• Rare event
• Almost always deleterious
• Rarely leads to a protein that improves ability of
organism to survive
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Figure 7.24 The effects of the various types of point mutations.
Normal DNA
Normal mRNA
Normal
polypeptide
mRNA
Template DNA strand
Mutated template
DNA strand
Mutated mRNA
Mutated template
DNA strand
Mutated mRNA
Mutated template
DNA strand
Mutated mRNA
STOP
CODON
Insertion
Mutated template
DNA strand
Mutated mRNA
Mutated template
DNA strand
Mutated mRNA
Phe Leu His Val
Phe
Phe
Phe Phe Ile Cys Thr Tyr Ala Arg
Arg Tyr Gly
Phe Tyr Ala Arg
Normal
Silent mutation No change in amino acid
sequence of polypeptide
Missense mutation Slightly different amino
acid sequence
Frameshift mutations:
Frameshift insertion Major difference in
amino acid sequence
Major difference in
amino acid sequence
Frameshift deletion
Nonsense mutation Polypeptide synthesis
ceases
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Mutations of Genes
• Mutagens
• Radiation
• Ionizing radiation
• Nonionizing radiation
• Chemical mutagens
• Nucleotide analogs
• Disrupt DNA and RNA replication
• Nucleotide-altering chemicals
• Result in base-pair substitutions and missense
mutations
• Frameshift mutagens
• Result in nonsense mutations
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Mutations of Genes
• Frequency of Mutation
• Mutations are rare events
• Otherwise organisms could not effectively reproduce
• Mutagens increase the mutation rate by a factor of 10 to
1000 times
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• Vertical gene transfer
• Organisms replicate their genomes and provide
copies to descendants
• Horizontal Gene Transfer Among Prokaryotes
• Horizontal gene transfer
• Donor cell contributes part of genome to recipient cell
• Three types:
• Transformation
• Transduction
• Bacterial conjugation
Genetic Recombination and Transfer
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Figure 7.33 Transformation of Streptococcus pneumoniae.
Observations of Streptococcus pneumoniae
Live cells Injection
Capsule
Heat-treated dead cells of strain S Injection
Strain R live cells (no capsule)
Injection
Mouse dies
Mouse lives
Mouse lives
Griffith's experiment:
Living
strain R
Heat-treated dead cells of strain S
Injection
Mouse dies
Culture of Streptococcus from dead mouse
Living cells with capsule (strain S)
+
In vitro transformation
Heat-treated dead cells of strain S
DNA broken into pieces
DNA fragment from strain S
Living strain R
Some cells take up DNA from the environment and incorporate it into their chromosomes
Transformed cells acquire ability to synthesize capsules
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Figure 7.34 Transduction.
Cell 1
Bacteriophage
Host bacterial cell
(donor cell)
Bacterial chromosome
Phage injects its DNA.
Phage enzymes
degrade host DNA.
Cell synthesizes new
phages that incorporate
phage DNA and, mistakenly,
some host DNA.
Phage
DNA Phage with donor cell's
DNA (transducing phage)
Transducing phage
Recipient host cell
Cell 2
Transduced cell
Transducing phage
injects donor DNA.
Donor DNA is incorporated
into recipient's chromosome
by recombination.
Inserted DNA
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Figure 7.35 Bacterial conjugation.
Pilus
Cell 2 Cell 1
F plasmid Origin of
transfer Chromosome
Donor cell attaches to a recipient cell with
its pilus.
Pilus draws cells together.
One strand of F plasmid DNA transfers
to the recipient.
The recipient synthesizes a complementary
strand to become an F+ cell with a pilus; the
donor synthesizes a complementary strand,
restoring its complete plasmid.
Now F+ cell F+ cell
Origin of
transfer
F+ cell F– cell
Pilus
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Deadly Horizontal Gene Transfer
• Patient Details?
• What infection? MDR?
• Define healthcare-associated infection
• Source of the infection?
• Three ways by which Enterococcus faecium might
have acquired genes for drug resistance.
• How can hospital personnel prevent the spread of
resistant E. faecium throughout the hospital?
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