Download Lecture 2 Turunen 14.9. - MyCourses

8/29/2016
PowerPoint® Lecture
Presentations prepared by
Mindy Miller-Kittrell,
North Carolina State University
Modified by Ossi Turunen, Aalto University
CHAPTER
7
Microbial
Genetics
© 2015 Pearson Education, Ltd.
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.
Hydrogen bond
Sugar
Thymine (T)
nucleoside
Adenine (A)
nucleoside
A–T base pair (DNA)
5′ end
3′ end
Uracil (U)
nucleoside
Adenine (A)
nucleoside
A–U base pair (RNA)
Cytosine (G)
nucleoside
Guanine (G)
nucleoside
G–C base pair (DNA and RNA)
5′ end
3′ end
A
T
G
C
A
T
G
Guanine
3′ end
5′ end
Double-stranded DNA
Cytosine
Adenine
3′ end
Thymine
5 end′
Thymine nucleoside
Thymine nucleotide
© 2015 Pearson Education, Ltd.
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.
Nucleoid
Bacterium
Chromosome
Plasmid
© 2015 Pearson Education, Ltd.
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)
© 2015 Pearson Education, Ltd.
Figure 7.3 Eukaryotic nuclear chromosomal packaging.
10 nm
Nucleosome
Active
(loosely packed)
Histones
Linker
DNA
Inactive
(tightly
packed)
DNA
10 nm
30 nm
Nucleosomes
Chromatin fiber
700 nm
Euchromatin and
heterochromatin
1400 nm
Highly condensed,
duplicated
chromosome of
dividing nucleus
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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
• Code only for about 5% of RNA and proteins
• Some fungi, algae, and protozoa carry plasmids
© 2015 Pearson Education, Ltd.
© 2015 Pearson Education, Ltd.
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The Structure and Replication of Genomes
• DNA Replication
• Key to replication is complementary structure of the two
strands
• Replication is semiconservative
• New DNA composed of one original and one daughter
strand
• Anabolic polymerization process that requires
monomers and energy
• Triphosphate deoxyribonucleotides serve both functions
© 2015 Pearson Education, Ltd.
DNA Replication
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Figure 7.4 Semiconservative model of DNA replication.
Original
DNA
First
replication
Second
replication
Original strand
New strands
© 2015 Pearson Education, Ltd.
Figure 7.5 The dual role of triphosphate deoxyribonucleotides as building blocks and energy sources in DNA synthesis.
Guanosine triphosphate deoxyribonucleotide (dGTP)
Guanine nucleotide (dGMP)
High-energy
bond
Guanine base Deoxyribose
Guanosine (nucleoside)
OH
Diphosphate released,
energy used for synthesis
Existing DNA strand
Triphosphate
nucleotide
Longer DNA strand
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The Structure and Replication of Genomes
• DNA Replication
• Initial processes in bacterial DNA replication
• Replication begins at the origin
• DNA polymerase replicates DNA only 5′ to 3′
• Because strands are antiparallel, new strands are
synthesized differently
• Leading strand synthesized continuously
• Lagging strand synthesized discontinuously
© 2015 Pearson Education, Ltd.
Figure 7.6a DNA replication.
Chromosomal proteins
(histones in eukaryotes and
archaea) removed
DNA polymerase III
3′
Replication fork
5′
DNA helicase
Stabilizing proteins
Initial processes
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Figure 7.6b-c DNA replication.
Primase
3
1
3′
Replication fork
5′
2
Leading strand
P+P
Triphosphate
nucleotide
RNA primer
Synthesis of leading strand
Replication fork
Triphosphate
nucleotide
RNA
primer
Okazaki
fragment
6
7
Lagging
strand
3′
5′
8
Primase
9
10
DNA ligase
DNA polymerase III DNA polymerase I
Synthesis of lagging strand
© 2015 Pearson Education, Ltd.
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
© 2015 Pearson Education, Ltd.
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Figure 7.7 The bidirectionality of DNA replication in prokaryotes.
Parental
strand
Origin
Replication forks
Daughter
strand
Replication
proceeds in
both directions
Termination
of replication
© 2015 Pearson Education, Ltd.
The Structure and Replication of Genomes
• DNA Replication
• Replication of eukaryotic DNA
• Similar to bacterial replication
• Some differences
• Uses four DNA polymerases
• Thousands of replication origins
• Shorter Okazaki fragments
• Plant and animal cells methylate only cytosine bases
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The Structure and Replication of Genomes
• Tell Me Why
• DNA replication requires a large amount of energy, yet
none of a cell's ATP energy supply is used. Why isn't it?
© 2015 Pearson Education, Ltd.
Gene Function
• 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
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Gene Function
• The Transfer of Genetic Information
• Transcription
• Information in DNA is copied as RNA
• Translation
• Polypeptides are synthesized from RNA
• Central dogma of genetics
• DNA is transcribed to RNA
• RNA is translated to form polypeptides
© 2015 Pearson Education, Ltd.
Figure 7.8 The central dogma of genetics.
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Gene Function
• The Events in Transcription
• Five types of RNA transcribed from DNA
• RNA primers
• mRNA
• rRNA
• tRNA
• Regulatory RNA
• Occur in nucleoid of prokaryotes
• Three steps
• Initiation
• Elongation
• Termination
© 2015 Pearson Education, Ltd.
Figure 7.9a The events in the transcription of RNA in prokaryotes.
1a RNA polymerase attaches
nonspecifically to DNA and
RNA polymerase
travels down its length until 5′
it recognizes a promoter
3′
sequence. Sigma factor
Sigma factor
Promoter
enhances promoter
recognition in bacteria.
Attachment of RNA polymerase
1b Upon recognition of the
3′
5′
DNA
Terminator
"Bubble"
promoter, RNA polymerase
5′
unzips the DNA molecule
beginning at the promoter. 3′
3′
5′
Unzipping of DNA, movement of RNA polymerase
Template
DNA strand
Initiation of transcription
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Figure 7.9b The events in the transcription of RNA in prokaryotes.
"Bubble"
2 Triphosphate ribonucleotides
align with their DNA
5′
complements and RNA
3′
polymerase links them
together, synthesizing RNA.
Growing RNA molecule
No primer is needed. The
(transcript)
triphosphate ribonucleotides
also provide the energy
required for RNA synthesis.
3′
3′
5′
5′
3′
5′
G
C
5′
3′
Elongation of the RNA transcript
Template
DNA
strand
© 2015 Pearson Education, Ltd.
Figure 7.10 Concurrent RNA transcription.
RNA polymerases
Promoter
5′
3′
5′
5′
3′
3′
3′
3′
3′
5′
Sigma factor
3′
3′
5′
Template DNA
strand
RNA
5′
5′
5′
© 2015 Pearson Education, Ltd.
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Figure 7.9c The events in the transcription of RNA in prokaryotes.
3′
5′
5′
3′
5′
3′
Terminator
RNA transcript
released
3a Self-termination: transcription of GC-rich terminator
region produces a hairpin loop, which creates tension,
loosening the grip of the polymerase
on the DNA.
3b 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
GC-rich
hairpin
loop
Terminator
Terminator
C
3′
Template
strand
Termination of transcription: release of RNA polymerase
© 2015 Pearson Education, Ltd.
Gene Function
• The Events in Transcription
• Transcriptional differences in eukaryotes
• RNA transcription occurs in the nucleus
• Transcription also occurs in mitochondria and chloroplasts
• Three types of nuclear RNA polymerases
• Numerous transcription factors
• mRNA is processed before translation
• Capping
• Polyadenylation
• Splicing
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Figure 7.11 Processing eukaryotic mRNA.
Exons (polypeptide coding regions)
5′ Template
DNA strand
3′
Introns (noncoding regions)
Transcription
Exon 2
Exon 1
Exon 3
Pre-mRNA
A AAAAAAAAAAAAA
5′ cap Intron 1
Intron 2
Intron 1
Intron 3 Poly-A tail
Processing
Spliceosomes
5′
Exon 1
Exon 2
A AAAAAAAAAAAAA
3′ mRNA splicing
Exon 3
5′
A AAAAAAAAAAAAA
mRNA (codes for
3′ one polypeptide)
Nuclear envelope
Nucleoplasm
Nuclear pore
Cytosol
mRNA
© 2015 Pearson Education, Ltd.
Gene Function
• Translation
• Process in which ribosomes use genetic information of
nucleotide sequences to synthesize polypeptides
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Figure 7.12 The genetic code.
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Gene Function
• Translation
• Participants in translation
• Messenger RNA
• Transfer RNA
• Ribosomes and ribosomal RNA
© 2015 Pearson Education, Ltd.
Figure 7.13 A single prokaryotic mRNA can code for several polypeptides.
3′
Gene 2
Gene 1
Promoter
Gene 3
Terminator
Transcription
5′
Start
codon
AUG
UAA
Ribosome
binding
site (RBS)
Start
codon
AUG
Stop RBS
codon
UAG
Start
codon
AUG
Stop RBS
codon
UAA
5′ Template
DNA strand
3′ mRNA
Untranslated
Stop
codon mRNA
Translation
Polypeptide 1
Polypeptide 2
Polypeptide 3
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Figure 7.14 Transfer RNA.
OH
Acceptor
stem
3′
5′
5′
Hydrogen
bonds
3′
tRNA icon
Hairpin
loops
Anticodon
Anticodon
© 2015 Pearson Education, Ltd.
Figure 7.15 Ribosomal structures.
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Figure 7.16 Assembled ribosome and its tRNA-binding sites.
Large
subunit
Large
subunit
mRNA
Nucleotide
bases
5′
E P A
site sitesite
tRNAbinding
sites
3′
Small
subunit
Small
subunit
mRNA
Prokaryotic ribosome
(angled view) attached
to mRNA
Prokaryotic ribosome
(schematic view) showing
tRNA-binding sites
© 2015 Pearson Education, Ltd.
Gene Function
• Translation
• Events in translation
• Three stages of translation
• Initiation
• Elongation
• Termination
• All stages require additional protein factors
• Initiation and elongation require energy (GTP)
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Figure 7.17 The initiation of translation in prokaryotes.
1
fMet
2
Initiator
tRNA
3
GTP
Large
ribosomal
subunit
fMet
fMet
tRNA
GDP + P
fMet
Anticodon
mRNA
5′
E
Start codon
A U G U U U A C G
P
3′
A
U A C
A U G U U U A C G
P
Small
ribosomal
subunit
A
U A C
A U G U U U A C G
P
A
Initiation complex
© 2015 Pearson Education, Ltd.
Figure 7.18 The elongation stage of translation.
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Figure 7.19 In prokaryotes a polyribosome—one mRNA and many ribosomes and polypeptides.
Polypeptides
mRNA Ribosomes
mRNA Ribosomes
Polypeptides
Direction of
transcription
© 2015 Pearson Education, Ltd.
Gene Function
• Translation
• Events in translation
• Termination
• Release factors recognize stop codons
• Modify ribosome to activate ribozymes
• Ribosome dissociates into subunits
• Polypeptides released at termination may function
alone or together
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Gene Function
• Translation
• Translation differences in eukaryotes
• Initiation occurs when ribosomal subunit binds to 5′
guanine cap
• First amino acid is methionine rather than f-methionine
• Ribosomes can synthesize polypeptides into the cavity of
the rough endoplasmic reticulum
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Protein Synthesis
© 2015 Pearson Education, Ltd.
Gene Function
• Regulation of Genetic Expression
• Most genes are expressed at all times (constitutive)
• Other genes are transcribed and translated when cells
need them (inductive)
• Allows cell to conserve energy
• Quorum sensing regulates production of some proteins
• Quorum sensing is a system, in which stimuli and
response correlate to population density
• Detection of secreted quorum-sensing molecules can
signal bacteria to synthesize a certain protein
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Gene Function
• Regulation of Genetic Expression
• Regulation of polypeptide synthesis
• Typically halts transcription
• Can stop translation directly
© 2015 Pearson Education, Ltd.
Gene Function
• Regulation of Genetic Expression
• Nature of prokaryotic operons
• An operon consists of a promoter and a series of genes
• Controlled by a regulatory element called an operator
• Typically polycistronic (code for several polypeptides)
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Figure 7.20 An operon.
Operon
Regulatory gene
Promoter Operator Structural genes
3′
1
2
3
4
5′ Template DNA strand
© 2015 Pearson Education, Ltd.
Gene Function
• Regulation of Genetic Expression
• Induction and Repression in prokaryotic operons
• Inducible operons must be activated by inducers
• Lactose operon
• Repressible operons are transcribed continually until
deactivated by repressors
• Tryptophan operon
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Figure 7.21 The lac operon, an inducible operon.
lac operon
Promoter and
regulatory gene
Operator
(blocked)
Promoter
1
3′
2
3
Lactose catabolism genes
Continual transcription
5′ Template DNA
strand
RNA
2
polymerase
cannot
bind
Repressor mRNA
Continual translation
1
Repressor
lac operon repressed
RNA polymerase
1
3′
Repressor
cannot bind
Repressor mRNA
Transcription
proceeds
2
3
5′
Template DNA
strand
4
mRNA for
lactose catabolism
5′
Repressor
3 Inactivated
repressor
Inducer (allolactose
from lactose)
lac operon induced
© 2015 Pearson Education, Ltd.
Figure 7.22 CAP-cAMP enhances lac transcription.
cAMP
bound to
CAP
RNA
polymerase
Binding of RNA polymerase to the
promoter is enhanced by the cAMPbound catabolite activator protein
(CAP = cAMP receptor protein)
Transcription proceeds
CAP
binding
site
Promoter
Operator
lac genes
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Lac operon genes
lacZ: β-galactosidase, cleaves lactose into glucose and galactose
lacY: lactose permease in the cytoplasmic membrane to enable
transport of lactose into the cell
lacA: galactoside O-acetyltransferase, an enzyme that transfers an
acetyl group from acetyl-CoA to β-galactosides
© 2015 Pearson Education, Ltd.
Figure 7.23 The trp operon, a repressible operon.
trp operon with five genes
Regulatory gene
Promoter Operator
3′
1
2
3
4
5
5′ Template DNA strand
Transcription
3′ mRNA
5′
5′
3′
mRNA coding
multiple polypeptides
Enzymes of tryptophan biosynthetic pathway
Inactive repressor
trp operon active
Trp Tryptophan
Movement of RNA
polymerase ceases
1
3′
Inactive
repressor
Trp
2
3
4
5
5′
Operator
blocked
Trp
Trp
Trp
Trp
Tryptophan
(corepressor)
trp operon repressed
Activated
repressor
trp operon codes for the components for
production of tryptophan
© 2015 Pearson Education, Ltd.
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© 2015 Pearson Education, Ltd.
Gene Function
• Regulation of Genetic Expression
• RNA molecules can control translation
• Regulatory RNAs can regulate translation of polypeptides
• microRNAs
• About 22 ntd long pieces of RNA
• Produced by eukaryotic cells
• Bind regulatory proteins to form miRNA-induced
silencing complex (miRISC)
• Bind complementary mRNA and inhibit its translation
• Regulates several cellular processes
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How miRNAs function?
- Wikipedia
© 2015 Pearson Education, Ltd.
Gene Function
• Regulation of Genetic Expression
• RNA molecules can control translation
• Regulatory RNAs can regulate translation of polypeptides
• Short interference RNA (siRNA)
• RNA molecule complementary to a portion of
mRNA, tRNA, or DNA
• Binds RISC proteins to form siRISC
• siRISC binds and cuts the target nucleic acid
• Riboswitch
• A regulatory segment of a mRNA that binds a
small molecule and then changes shape to help
regulate translation
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Gene Function
• Tell Me Why
• In bacteria, polypeptide translation can begin even
before mRNA transcription is complete. Why can't this
happen in eukaryotes?
© 2015 Pearson Education, Ltd.
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
• Types of Mutations
• Point mutations
• One base pair is affected
• Substitutions and frameshift mutations
• Frameshift mutations
• Nucleotide triplets after the mutation are displaced
• Creates new sequence of codons
• Insertions and deletions of several amino acids
© 2015 Pearson Education, Ltd.
Figure 7.24 The effects of the various types of point mutations.
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© 2015 Pearson Education, Ltd.
Mutations of Genes
• Mutagens
• Radiation
• Ionizing radiation
• Nonionizing radiation
• Chemical mutagens
• Nucleotide analogs
• Disrupt DNA and RNA replication
• Nucleotide-altering chemicals
• Alter the structure of nucleotides
• Result in base-pair substitutions and missense
mutations
• Frameshift mutagens
• Result in nonsense mutations
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Figure 7.25 A pyrimidine (in this case, thymine) dimer.
Ultraviolet light
Ultraviolet light induces the
formation of covalent
linkages by reactions
localized on the C=C double
bonds
Thymine dimer
GC T
G T=T G
GTA
CGACAACCAT
© 2015 Pearson Education, Ltd.
Figure 7.26 The structure and effects of a nucleotide analog.
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Figure 7.27 The action of a frameshift mutagen.
© 2015 Pearson Education, Ltd.
Mutations of Genes
• Frequency of Mutation
• Mutations are rare events
• Otherwise, organisms could not effectively reproduce
• About 1 of every 10 million genes contains an error
• Mutagens increase the mutation rate by a factor of 10 to
1000 times
• Many mutations stop transcription or code for
nonfunctional proteins
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Figure 7.28a-b DNA repair mechanisms.
Visible light
Thymine dimer
G AC
T=T
A
G AC T T A
C T G A A T
C T G A A T
Light-activated
repair enzyme
Light repair
Cut
G AC
T=T
A C
Repair
enzyme
C T G A A T G
C
DNA polymerase I
and ligase repair
GAC T TAC
the gap
C T G A A T G
C T G A A T G
G
AC
T=T
A
Dark repair
© 2015 Pearson Education, Ltd.
Figure 7.28c-d DNA repair mechanisms.
Base excision repair enzymes
remove incorrect nucleotide
DNA polymerase I
and ligase repair gap
G GC T T AGC G T
G GC T T A
C G T
G GC T T A TC G T
C CG A A T AG C A
C CG A A T AG C A
C CG A A T AG C A
Base-excision repair
Mismatch repair enzyme
removes incorrect segment
Mutated DNA
(incorrect nucleotide pair)
DNA polymerase III correctly
repairs the gap
C T T A GC G T
GC G T
C C T A GC G T
G G A T CG C A
G G A T CG C A
G G A T CG C A
Mismatch repair
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Mutations of Genes
• Identifying Mutants, Mutagens, and Carcinogens
• Mutants
• Descendants of a cell that does not repair a mutation
• Wild types
• Cells normally found in nature
• Methods to recognize mutants
• Positive selection
• Negative (indirect) selection
• Ames test
© 2015 Pearson Education, Ltd.
Figure 7.29 Positive selection of mutants.
Penicillinresistant cell
Medium with penicillin
(only penicillin-resistant
cell grows into colony)
Penicillinsensitive cells
Medium without
penicillin (both
types of cells form
colonies)
Mutagen
induces
mutations
Penicillinresistant
mutants
indistinguishable
from nonmutants
Medium with penicillin
Medium without
penicillin
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Figure 7.30 The use of negative (indirect) selection to isolate a tryptophan auxotroph.
1 Inoculate bacteria onto
complete medium
containing tryptophan.
Mutagen
Bacterial
suspension
X
Incubation
Bacterial colonies
2 grow. A few may be
tryptophan
auxotrophs. Most are
wild type.
X
3 Stamp sterile velvet onto
plate, picking up cells
from each colony.
Sterile velvet
surface
X
Bacteria
4 Stamp replica plates
with velvet.
X
X
Complete medium
containing
tryptophan
Medium lacking
tryptophan
Incubation
5 Identify auxotroph
as colony growing on
complete medium but
not on lacking medium.
X
X
All colonies grow.
Tryptophan auxotroph
cannot grow.
6 Inoculate auxotroph
colony into complete
medium.
© 2015 Pearson Education, Ltd.
Figure 7.31 The Ames test.
Experimental
tube
Liver
extract
Control
tube
Suspected
mutagen
Liver
extract
Culture of his– Salmonella
Medium
lacking
histidine
Incubation
© 2015 Pearson Education, Ltd.
Colony of revertant
(his+) Salmonella
No growth
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Genetic Recombination and Transfer
• Exchange of nucleotide sequences often occurs
between homologous sequences
• Recombinants
• Cells with DNA molecules that contain new nucleotide
sequences
© 2015 Pearson Education, Ltd.
Figure 7.32 Genetic recombination.
Homologous
sequences
3′
DNA A
5′
3′ DNA B
5′
Enzyme nicks one strand of
DNA at homologous sequence.
A
B
Recombination enzyme
inserts the cut strand
into second molecule,
which is nicked in the
process.
Ligase anneals nicked ends
in new combinations.
Molecules resolve
into recombinants.
Recombinant A
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Recombinant B
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Genetic Recombination and Transfer
• Horizontal Gene Transfer Among Prokaryotes
• Vertical gene transfer
• Passing of genes to the next generation
• Horizontal gene transfer
• Donor cell contributes part of genome to recipient cell
• Three types
• Transformation
• Transduction
• Bacterial conjugation
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Genetic Recombination and Transfer
• Horizontal Gene Transfer Among Prokaryotes
• Transformation
• Recipient cell takes up DNA from the environment
• Provided evidence 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
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Figure 7.33 Transformation of Streptococcus pneumoniae.
Observations of Streptococcus pneumoniae
Griffith's experiment:
Living
strain R
Live cells
Injection
Capsule
+
In vitro transformation
Heat-treated
dead cells
of strain S
Mouse dies
Injection
DNA broken
into pieces
Heat-treated
dead cells of
strain S
DNA fragment
from strain S
Living strain R
Heat-treated
dead cells of
strain S
Injection
Mouse dies
Some cells take
up DNA from the
environment and
incorporate it into
their chromosomes
Mouse lives
Culture of
Streptococcus
from dead
mouse
Strain R live cells
(no capsule)
Injection
Transformed cells
acquire ability to
synthesize capsules
Living cells
with capsule
(strain S)
Mouse lives
© 2015 Pearson Education, Ltd.
Genetic Recombination and Transfer
• Horizontal Gene Transfer Among Prokaryotes
• Transduction
• Transfer of DNA from one cell to another via replicating
virus
• Virus must be able to infect both donor and recipient cells
• Virus that infects bacteria called a bacteriophage (phage)
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Figure 7.34 Transduction.
Bacteriophage
Host bacterial cell
(donor cell)
Bacterial chromosome
1 Phage injects its DNA.
2 Phage enzymes
degrade host DNA.
Phage
DNA
Phage with donor DNA
(transducing phage)
3 Cell synthesizes new
phages that incorporate
phage DNA and, mistakenly,
some host DNA.
Transducing phage
Recipient host cell
4 Transducing phage
injects donor DNA.
Transduced cell
Inserted
DNA
5 Donor DNA is incorporated
into recipient's chromosome
by recombination.
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Genetic Recombination and Transfer
• Horizontal Gene Transfer Among Prokaryotes
• Transduction
• Generalized transduction
• Transducing phage carries random DNA segment
from donor to recipient
• Specialized transduction
• Only certain donor DNA sequences are transferred
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Genetic Recombination and Transfer
• Horizontal Gene Transfer Among Prokaryotes
• Conjugation
• Genetic transfer requires physical contact between the
donor and recipient cell
• Donor cell remains alive
• Mediated by conjugation (sex) pili
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Figure 7.35 Bacterial conjugation.
F-plasmid is an episome, a plasmid
that can integrate itself into the
bacterial chromosome by
homologous recombination
F plasmid
Origin of
transfer
Chromosome
Pilus
1 Donor cell attaches to a recipient cell with
its pilus.
F+ cell
_
F cell
2 Pilus may draw cells together.
3 One strand of F plasmid DNA transfers
to the recipient.
Pilus
4 The recipient synthesizes a complementary
strand to become an F+ cell with a pilus; the
donor synthesizes a complementary strand,
restoring its complete plasmid.
F+ cell
F+ cell
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Figure 7.36 Conjugation involving an Hfr cell.
Donor chromosome
High-frequency recombination cell = Hfr cell
Pilus
F+ cell
1 F plasmid integrates
into chromosome by
recombination.
Hfr cell
Pilus
F+ cell (Hfr)
F plasmid
2 Cells join via a pilus.
–
F recipient
Donor DNA Part of F plasmid
3 Portion of F plasmid partially
moves into recipient cell
trailing a strand of donor's
DNA.
Incomplete F plasmid;
cell remains F−
4 Conjugation ends with pieces
of F plasmid and donor DNA
in recipient cell; cells synthesize
complementary DNA strands.
5 Donor DNA and recipient
DNA recombine, making a
recombinant F –cell.
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Recombinant cell (still F− )
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Genetic Recombination and Transfer
• Transposons and Transposition
• Transposons
• Segments of DNA that move from one location to another
in the same or different molecule
• Result is a kind of frameshift insertion (transpositions)
• Transposons all contain palindromic sequences at each
end
© 2015 Pearson Education, Ltd.
Figure 7.37 Transposition.
Transposon
Plasmid with
transposon
DNA
Jumping transposons. Transposons
move from one place to another
on a DNA molecule.
Replicating transposons. Transposons
may replicate while moving, resulting in
more transposons in the cell.
Transposons can move onto plasmids.
Transposons moving onto plasmids can
be transferred to another cell.
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Genetic Recombination and Transfer
• Transposons and Transposition
• Simplest transposons
• Insertion sequences
• Have no more than two inverted repeats and a gene for
transposase
• Complex transposons
• Contain one or more genes not connected with
transposition
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Figure 7.38 Transposons.
Transposon: Insertion sequence IS1
A CT T AC T GA T
T GA A T GAC T A
A T CAG T AAG T
T AG T CA T T CA
Inverted repeat (IR) Transposase gene Inverted repeat (IR)
DNA
molecule
Target site
IS1
Target
IS1
Transposase
Target site Copy Copy of
of IS1 target site
Original
IS1
Complex transposon
IR
IS1
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IR Kanamycin- IR
resistance
gene
IR
IS1
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