Transcript Chap 7 Microbial Genetics Fall 2012

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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Figure 7.1 The structure of nucleic acids-overview
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The Structure and Replication of Genomes
• 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 circular molecule of
DNA in nucleoid
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Figure 7.2 Bacterial genome-overview
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The Structure and Replication of Genomes
• 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
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The Structure and Replication of Genomes
• The Structure of Eukaryotic Genomes
– Nuclear chromosomes
– Typically have more than one chromosome
per cell
– Chromosomes are linear and sequestered
within nucleus
– Eukaryotic cells are often diploid (two
chromosome copies)
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The Structure and Replication of Genomes
• The Structure of Eukaryotic Genomes
– Extranuclear DNA of eukaryotes
– DNA molecules of mitochondria and
chloroplasts
– Resemble chromosomes of prokaryotes
– Only code for about 5% of RNA and
proteins
– Some fungi and protozoa carry plasmids
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The Structure and Replication of Genomes
• DNA Replication
– Anabolic polymerization process that requires
monomers and energy
– Triphosphate deoxyribonucleotides serve both
functions
– Key to replication is complementary structure of
the two strands
– Replication is semiconservative
– New DNA composed of one original and one
daughter strand
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The Structure and Replication of Genomes
ANIMATION DNA Replication: Overview
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The Structure and Replication of Genomes
ANIMATION DNA Replication: Synthesis
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The Structure and Replication of Genomes
• DNA Replication
– Other characteristics of bacterial DNA replication
– Bidirectional
– DNA is methylated
– Control of genetic expression
– Initiation of DNA replication
– Protection against viral infection
– Repair of DNA
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Gene Function
• The Relationship Between Genotype and
Phenotype
– Genotype
– Set of genes in the genome
– Phenotype
– Physical features and functional traits of the
organism
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Gene Function
• 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
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Figure 7.8 The central dogma of genetics
5´
3´
DNA
(genotype)
5´
3´
Transcription
5´
3´
mRNA
Translation
by ribosomes
NH2
Methionine
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Arginine
Tyrosine
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Phenotype
Leucine
Polypeptide
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Gene Function
ANIMATION Transcription: The Process
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Gene Function
• Translation
– Process where ribosomes use genetic information
of nucleotide sequences to synthesize polypeptides
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Figure 7.12 The genetic code
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Figure 7.13 Prokaryotic mRNA
Promoter
Gene 1
Gene 2
Gene 3
Terminator
5´ Template
DNA strand
3´
Transcription
5´
Start
codon
AUG
UAA
Ribosome
binding
site (RBS)
Start
codon
AUG
Stop RBS
codon
UAG
Start
codon
AUG
Stop RBS
codon
UAA
3´ mRNA
Stop
codon
Untranslated
mRNA
Translation
Polypeptide 1
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Polypeptide 2
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Polypeptide 3
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Figure 7.14 Transfer RNA-overview
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Figure 7.16 Transfer RNA binding sites in a ribosome
Large
subunit
Large
subunit
Nucleotide
bases
mRNA
5´
Small
subunit
P
A
site site site
3´
mRNA
Prokaryotic ribosome
(angled view) attached
to mRNA
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E
tRNAbinding
sites
Small
subunit
Prokaryotic ribosome
(schematic view) showing
tRNA-binding sites
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Figure 7.17 The initiation of translation in prokaryotes
Initiator
tRNA
Large
ribosomal
subunit
tRNAfMet
Anticodon
mRNA
E
Start codon
5´
3´
P
A
P
Small
ribosomal
subunit
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A
P
A
Initiation complex
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Figure 7.18 The elongation stage of translation-overview
E
Peptide bond
E
5´
3´
P
5´
A
3´
P
A
Movement of ribosome
one codon toward 3´ end
5´
3´
E
P
A
P
A
P
A
E
5´
3´
E
5´
3´
Two more cycles
Growing
polypeptide
E
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5´
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3´
P
A
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Gene Function
• Regulation of Genetic Expression
– 75% of genes are expressed at all times
– Other genes transcribed and translated when
cells need them
– Allows cell to conserve energy
– Regulation of protein 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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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
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– 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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Genetic Transfer
• Horizontal Gene Transfer Among
Prokaryotes
– Horizontal gene transfer
– Donor cell contributes part of genome to
recipient cell
– Three types
– Transformation
– Transduction
– Bacterial conjugation
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Figure 7.33 Transformation in Streptococcus pneumoniae-overview
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Figure 7.34 Transduction-overview
Bacteriophage
Host bacterial cell
(donor cell)
Bacterial chromosome
Phage injects its DNA.
Phage enzymes
degrade host DNA.
Phage
DNA
Phage with donor DNA
(transducing phage)
Cell synthesizes new
phages that incorporate
phage DNA and, mistakenly,
some host DNA.
Transducing phage
Recipient host cell
Transducing phage
injects donor DNA.
Transduced cell
Inserted
DNA
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Donor DNA is incorporated
into recipient’s chromosome
by recombination.
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Figure 7.35 Bacterial conjugation-overview
F plasmid
Origin of Conjugation pilus
transfer
Chromosome
Donor cell attaches to a recipient cell with
its pilus.
F+ cell
F– cell
Pilus may draw cells together.
One strand of F plasmid DNA transfers
to the recipient.
Pilus
The recipient synthesizes a complementary
strand to become an F+ cell with a pilus; the
donor synthesizes a complementary strand,
restoring its complete plasmid.
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F+ cell
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F+ cell
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Figure 7.36 Conjugation involving an Hfr cell-overview
Donor chromosome
Pilus
F+ cell
F plasmid integrates
into chromosome by
recombination.
Hfr cell
Pilus
F+ cell (Hfr)
F plasmid
Cells join via a
conjugation pilus.
F– recipient
Donor DNA Part of F plasmid
Portion of F plasmid partially
moves into recipient cell
trailing a strand of donor’s
DNA.
Incomplete F plasmid;
cell remains F–
Conjugation ends with pieces
of F plasmid and donor DNA
in recipient cell; cells synthesize
complementary DNA strands.
Donor DNA and recipient
DNA recombine, making a
recombinant F– cell.
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Recombinant cell (still F–)
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