Transcript Operons

• Vaccines are harmless derivatives of pathogenic
microbes that stimulate the immune system to
mount defenses against the actual pathogen
• Vaccines can prevent certain viral illnesses
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Emerging Viruses
• Emerging viruses are those that appear suddenly
or suddenly come to the attention of scientists
• Severe acute respiratory syndrome (SARS)
recently appeared in China
• Outbreaks of “new” viral diseases in humans are
usually caused by existing viruses that expand
their host territory
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-11
Young ballet students in Hong
Kong wear face masks to
protect themselves from the
virus causing SARS.
The SARS-causing agent is a
coronavirus like this one
(colorized TEM), so named for
the “corona” of glyco-protein
spikes protruding form the
envelope.
Viral Diseases in Plants
• More than 2,000 types of viral diseases of plants
are known
• Some symptoms are spots on leaves and fruits,
stunted growth, and damaged flowers or roots
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Plant viruses spread disease in two major modes:
– Horizontal transmission, entering through
damaged cell walls
– Vertical transmission, inheriting the virus from
a parent
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Viroids and Prions: The Simplest Infectious
Agents
• Viroids are circular RNA molecules that infect
plants and disrupt their growth
• Prions are slow-acting, virtually indestructible
infectious proteins that cause brain diseases in
mammals
• Prions propagate by converting normal proteins
into the prion version
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-13
Prion
Original
prion
Many prions
Normal
protein
New
prion
Concept 18.3: Rapid reproduction, mutation, and genetic
recombination contribute to the genetic diversity of bacteria
• Bacteria allow researchers to investigate
molecular genetics in the simplest true organisms
• The well-studied intestinal bacterium Escherichia
coli (E. coli) is “the laboratory rat of molecular
biology”
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Bacterial Genome and Its Replication
• The bacterial chromosome is usually a circular
DNA molecule with few associated proteins
• Many bacteria also have plasmids, smaller circular
DNA molecules that can replicate independently of
the chromosome
• Bacterial cells divide by binary fission, which is
preceded by replication of the chromosome
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-14
Replication fork
Origin of
replication
Termination
of replication
Mutation and Genetic Recombination as Sources
of Genetic Variation
• Since bacteria can reproduce rapidly, new
mutations quickly increase genetic diversity
• More genetic diversity arises by recombination of
DNA from two different bacterial cells
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-15
Mixture
Mutant
strain
arg+ trp–
Mutant
strain
arg– trp+
Mixture
Mutant
strain
arg+ trp–
Mutant
strain
arg– trp+
No
colonies
(control)
Colonies
grew
No
colonies
(control)
Mechanisms of Gene Transfer and Genetic
Recombination in Bacteria
• Three processes bring bacterial DNA from
different individuals together:
– Transformation
– Transduction
– Conjugation
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Transformation
• Transformation is the alteration of a bacterial cell’s
genotype and phenotype by the uptake of naked,
foreign DNA from the surrounding environment
• For example, harmless Streptococcus
pneumoniae bacteria can be transformed to
pneumonia-causing cells
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Transduction
• In the process known as transduction, phages
carry bacterial genes from one host cell to another
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-16
Phage DNA
A+ B+
A+ B+
Donor
cell
A+
Crossing
over
A+
A– B–
Recipient
cell
A+ B–
Recombinant cell
Conjugation and Plasmids
• Conjugation is the direct transfer of genetic
material between bacterial cells that are
temporarily joined
• The transfer is one-way: One cell (“male”)
donates DNA, and its “mate” (“female”) receives
the genes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• “Maleness,” the ability to form a sex pilus and
donate DNA, results from an F (for fertility) factor
as part of the chromosome or as a plasmid
• Plasmids, including the F plasmid, are small,
circular, self-replicating DNA molecules
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-17
Sex pilus
5 µm
The F Plasmid and Conjugation
• Cells containing the F plasmid, designated F+
cells, function as DNA donors during conjugation
• F+ cells transfer DNA to an F recipient cell
• Chromosomal genes can be transferred during
conjugation when the donor cell’s F factor is
integrated into the chromosome
• A cell with a built-in F factor is called an Hfr cell
• The F factor of an Hfr cell brings some
chromosomal DNA along when transferred to an
F– cell
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-18_1
F plasmid
F+ cell
Mating
bridge
F– cell
Bacterial chromosome
F+ cell
F+ cell
Bacterial
chromosome
Conjunction and transfer of an F plasmid from and F+ donor to an F– recipient
LE 18-18_2
F plasmid
Bacterial chromosome
F+ cell
Mating
bridge
F– cell
F+ cell
F+ cell
Bacterial
chromosome
Conjunction and transfer of an F plasmid from and F+ donor to an F– recipient
Hfr cell
F+ cell
F factor
LE 18-18_3
F plasmid
Bacterial chromosome
F+ cell
Mating
bridge
F– cell
F+ cell
F+ cell
Bacterial
chromosome
Conjunction and transfer of an F plasmid from and F+ donor to an F– recipient
Hfr cell
F+ cell
F factor
Hfr cell
F– cell
LE 18-18_4
F plasmid
Bacterial chromosome
F+ cell
Mating
bridge
F– cell
F+ cell
F+ cell
Bacterial
chromosome
Conjunction and transfer of an F plasmid from and F+ donor to an F– recipient
Hfr cell
F+ cell
F factor
Hfr cell
F– cell
Temporary
Recombinant F–
partial
bacterium
diploid
Conjugation and transfer of part of the bacterial chromosome from an
Hfr donor to an F– recipient, resulting in recombiination
R plasmids and Antibiotic Resistance
• R plasmids confer resistance to various antibiotics
• When a bacterial population is exposed to an
antibiotic, individuals with the R plasmid will
survive and increase in the overall population
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Transposition of Genetic Elements
• The DNA of a cell can also undergo recombination
due to movement of transposable elements within
the cell’s genome
• Transposable elements, often called “jumping
genes,” contribute to genetic shuffling in bacteria
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Insertion Sequences
• The simplest transposable elements, called
insertion sequences, exist only in bacteria
• An insertion sequence has a single gene for
transposase, an enzyme catalyzing movement of
the insertion sequence from one site to another
within the genome
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-19a
Insertion sequence
5
3
3
5
Inverted
repeat
Transposase gene
Inverted
repeat
Transposons
• Transposable elements called transposons are
longer and more complex than insertion
sequences
• In addition to DNA required for transposition,
transposons have extra genes that “go along for
the ride,” such as genes for antibiotic resistance
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-19b
Transposon
Insertion
sequence
Antibiotic
resistance gene
Insertion
sequence
5
3
3
5
Inverted repeat
Transposase gene
Concept 18.4: Individual bacteria respond to
environmental change by regulating their gene expression
• A bacterium can tune its metabolism to the
changing environment and food sources
• This metabolic control occurs on two levels:
– Adjusting activity of metabolic enzymes
– Regulating genes that encode metabolic
enzymes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-20
Regulation of enzyme
activity
Precursor
Regulation of enzyme
production
Feedback
inhibition
Enzyme 1
Gene 1
Enzyme 2
Gene 2
Regulation
of gene
expression
Enzyme 3
Gene 3
Enzyme 4
Gene 4
Enzyme 5
Tryptophan
Gene 5
Operons: The Basic Concept
• In bacteria, genes are often clustered into
operons, composed of
– An operator, an “on-off” switch
– A promoter
– Genes for metabolic enzymes
• An operon can be switched off by a protein called
a repressor
• A corepressor is a small molecule that cooperates
with a repressor to switch an operon off
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-21a
trp operon
Promoter
Promoter
Genes of operon
DNA
Regulatory
gene
mRNA
trpE
trpR
3
trpC
trpB
trpA
C
B
A
Operator
Start codon Stop codon
RNA
polymerase
mRNA 5
5
E
Protein
trpD
Inactive
repressor
D
Polypeptides that make up
enzymes for tryptophan synthesis
Tryptophan absent, repressor inactive, operon on
LE 18-21b_1
DNA
mRNA
Active
repressor
Protein
Tryptophan
(corepressor)
Tryptophan present, repressor active, operon off
LE 18-21b_2
DNA
No RNA made
mRNA
Active
repressor
Protein
Tryptophan
(corepressor)
Tryptophan present, repressor active, operon off
Repressible and Inducible Operons: Two Types of
Negative Gene Regulation
• A repressible operon is one that is usually on;
binding of a repressor to the operator shuts off
transcription
• The trp operon is a repressible operon
• An inducible operon is one that is usually off; a
molecule called an inducer inactivates the
repressor and turns on transcription
• The classic example of an inducible operon is the
lac operon, which contains genes coding for
enzymes in hydrolysis and metabolism of lactose
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-22a
Promoter
Regulatory
gene
Operator
lacl
DNA
lacZ
No
RNA
made
3
mRNA
5
Protein
RNA
polymerase
Active
repressor
Lactose absent, repressor active, operon off
LE 18-22b
lac operon
DNA
lacZ
lacl
3
mRNA
5
lacA
Permease
Transacetylase
RNA
polymerase
mRNA 5
-Galactosidase
Protein
Allolactose
(inducer)
lacY
Inactive
repressor
Lactose present, repressor inactive, operon on
• Inducible enzymes usually function in catabolic
pathways
• Repressible enzymes usually function in anabolic
pathways
• Regulation of the trp and lac operons involves
negative control of genes because operons are
switched off by the active form of the repressor
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Positive Gene Regulation
• Some operons are also subject to positive control
through a stimulatory activator protein, such as
catabolite activator protein (CAP)
• When glucose (a preferred food source of E. coli )
is scarce, the lac operon is activated by the
binding of CAP
• When glucose levels increase, CAP detaches from
the lac operon, turning it off
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-23a
Promoter
DNA
lacl
lacZ
CAP-binding site
Active
CAP
cAMP
Inactive
CAP
RNA
Operator
polymerase
can bind
and transcribe
Inactive lac
repressor
Lactose present, glucose scarce (cAMP level high): abundant lac
mRNA synthesized
LE 18-23b
Promoter
DNA
lacl
CAP-binding site
Inactive
CAP
lacZ
Operator
RNA
polymerase
can’t bind
Inactive lac
repressor
Lactose present, glucose present (cAMP level low): little lac
mRNA synthesized