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Chapter 18. The Genetics of Viruses and Bacteria. Overview: Microbial Model Systems. __________ called __________ can infect and set in motion a genetic takeover of __________, such as Escherichia coli

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Chapter 18

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Chapter 18

Chapter 18

The Genetics of Virusesand Bacteria


Overview microbial model systems

Overview: Microbial Model Systems

  • __________ called __________ can infect and set in motion a genetic takeover of __________, such as Escherichia coli

  • E. coli and its viruses are called __________ because of their frequent use by researchers in studies that reveal broad biological principles

  • Beyond their value as model systems, viruses and bacteria have unique genetic mechanisms that are interesting in their own right


Chapter 18

  • Bacteria are prokaryotes with cells much smaller and more simply organized than those of eukaryotes

  • Viruses are smaller and simpler than bacteria


Le 18 2

LE 18-2

Virus

Bacterium

Animal

cell

Animal cell nucleus

0.25 µm


Structure of viruses

Structure of Viruses

  • Viruses are not cells

  • Viruses are very small infectious particles consisting of __________ enclosed in a __________ and, in some cases, a _______________


Viral genomes

Viral Genomes

  • Viral genomes may consist of

    • Double- or single-stranded _____

    • Double- or single-stranded _____

  • Depending on its type of __________, a virus is called a DNA virus or an RNA virus


Capsids and envelopes

Capsids and Envelopes

  • A __________ is the protein shell that encloses the viral genome

  • A capsid can have various structures


Le 18 4a

LE 18-4a

Capsomere

of capsid

RNA

18  250 mm

20 nm

Tobacco mosaic virus


Le 18 4b

LE 18-4b

Capsomere

DNA

Glycoprotein

70–90 nm (diameter)

50 nm

Adenoviruses


Chapter 18

  • Some viruses have structures have membranous envelopes that help them infect hosts

  • These viral envelopes surround the capsids of influenza viruses and many other viruses found in animals

  • __________, which are derived from the host cell’s membrane, contain a combination of viral and host cell molecules


Le 18 4c

LE 18-4c

Membranous

envelope

Capsid

RNA

Glycoprotein

80–200 nm (diameter)

50 nm

Influenza viruses


Chapter 18

  • Bacteriophages, also called __________, are viruses that infect bacteria

  • They have the most complex capsids found among viruses

  • Phages have an elongated capsid head that encloses their DNA

  • A protein tailpiece attaches the phage to the host and injects the phage DNA inside


Le 18 4d

LE 18-4d

Head

DNA

Tail

sheath

Tail

fiber

80  225 nm

50 nm

Bacteriophage T4


General features of viral reproductive cycles

General Features of Viral Reproductive Cycles

  • Viruses are ____________________, which means they can reproduce only within a host cell

  • Each virus has a __________, a limited number of host cells that it can infect

  • Viruses use enzymes, ribosomes, and small host molecules to synthesize progeny viruses

Animation: Simplified Viral Reproductive Cycle


Le 18 5

LE 18-5

VIRUS

Entry into cell and

uncoating of DNA

DNA

Capsid

Transcription

Replication

HOST CELL

Viral DNA

mRNA

Viral DNA

Capsid

proteins

Self-assembly of

new virus particles

and their exit from cell


Reproductive cycles of phages

Reproductive Cycles of Phages

  • Phages are the best understood of all viruses

  • Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle


The lytic cycle

The Lytic Cycle

  • The __________ is a phage reproductive cycle that culminates in the death of the host cell

  • The lytic cycle produces new phages and digests the host’s cell wall, releasing the progeny viruses

  • A phage that reproduces only by the lytic cycle is called a __________ phage

  • Bacteria have defenses against phages, including _______________ that recognize and cut up certain phage DNA


Le 18 6

LE 18-6

Attachment

Entry of phage DNA

and degradation of

host DNA

Phage assembly

Release

Head

Tail fibers

Tails

Synthesis of viral

genomes and proteins

Assembly


The lysogenic cycle

The Lysogenic Cycle

  • The __________ replicates the phage genome without destroying the host

  • The viral DNA molecule is __________ by genetic recombination into the host cell’s chromosome

  • This integrated viral DNA is known as a __________

  • Every time the host divides, it copies the phage DNA and passes the copies to daughter cells

  • Phages that use both the lytic and lysogenic cycles are called __________ phages


Le 18 7

LE 18-7

Phage

DNA

The phage attaches to a

host cell and injects its DNA.

Daughter cell

with prophage

Many cell divisions

produce a large

population of

bacteria infected with

the prophage.

Phage DNA

circularizes

Phage

Bacterial

chromosome

Occasionally, a prophage

exits the bacterial chromosome,

initiating a lytic cycle.

Lytic cycle

Lysogenic cycle

The bacterium reproduces

normally, copying the prophage

and transmitting it to daughter cells.

Certain factors

determine whether

The cell lyses, releasing phages.

Lytic cycle

is induced

Lysogenic cycle

is entered

or

Prophage

Phage DNA integrates into the

bacterial chromosomes, becoming a

prophage.

New phage DNA and proteins are

synthesized and assembled into phages.


Reproductive cycles of animal viruses

Reproductive Cycles of Animal Viruses

  • Two key variables in classifying viruses that infect animals:

    • DNA or RNA?

    • Single-stranded or double-stranded?


Viral envelopes

Viral Envelopes

  • Many viruses that infect animals have a membranous envelope

  • Viral glycoproteins on the envelope bind to specific receptor molecules on the surface of a host cell


Le 18 8

LE 18-8

Capsid

Capsid and viral genome

enter cell

RNA

HOST CELL

Envelope (with

glycoproteins)

Viral genome (RNA)

Template

mRNA

Capsid

proteins

ER

Glyco-

proteins

Copy of

genome (RNA)

New virus


Rna as viral genetic material

RNA as Viral Genetic Material

  • The broadest variety of RNA genomes is found in viruses that infect animals

  • Retroviruses use _______________ to copy their RNA genome into DNA

  • HIV is the retrovirus that causes AIDS


Le 18 9

LE 18-9

Viral envelope

Glycoprotein

Capsid

RNA

(two identical

strands)

Reverse

transcriptase


Chapter 18

  • The viral DNA that is integrated into the host genome is called a __________

  • Unlike a prophage, a provirus remains a __________ resident of the host cell

  • The host’s RNA polymerase transcribes the proviral DNA into RNA molecules

  • The RNA molecules function both as mRNA for synthesis of viral proteins and as genomes for new virus particles released from the cell


Le 18 10

LE 18-10

Membrane of

white blood cell

HIV

HOST CELL

Reverse

transcription

Viral RNA

RNA-DNA

hybrid

0.25 µm

HIV entering a cell

DNA

NUCLEUS

Provirus

Chromosomal

DNA

RNA genome

for the

next viral

generation

mRNA

New HIV leaving a cell


Viral diseases in animals

Viral Diseases in Animals

  • Viruses may damage or kill cells by causing the release of hydrolytic enzymes from lysosomes

  • Some viruses cause infected cells to produce toxins that lead to disease symptoms


Chapter 18

  • __________ are harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogen

  • Vaccines can prevent certain viral illnesses


Viroids and prions the simplest infectious agents

Viroids and Prions: The Simplest Infectious Agents

  • Viroids are circular RNA molecules that infect plants and disrupt their growth

  • __________ are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals

  • Prions propagate by converting normal proteins into the prion version


Le 18 13

LE 18-13

Original

prion

Prion

Many prions

New

prion

Normal

protein


Chapter 18

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”


The bacterial genome and its replication

The Bacterial Genome and Its Replication

  • The bacterial chromosome is usually a circular DNA molecule with few associated proteins

  • Many bacteria also have __________, 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


Le 18 14

LE 18-14

Replication fork

Origin of

replication

Termination

of replication


Mutation and genetic recombination as sources of genetic variation

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


Mechanisms of gene transfer and genetic recombination in bacteria

Mechanisms of Gene Transfer and Genetic Recombination in Bacteria

  • Three processes bring bacterial DNA from different individuals together:

    • Transformation

    • Transduction

    • Conjugation


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


Transduction

Transduction

  • In the process known as __________, phages carry bacterial genes from one host cell to another


Le 18 16

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 and Plasmids

  • __________ 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


Chapter 18

  • “Maleness,” the ability to form a sex pilus and donate DNA, results from an _______________ as part of the chromosome or as a plasmid

  • Plasmids, including the __________, are small, circular, self-replicating DNA molecules


Le 18 17

LE 18-17

Sex pilus

5 µm


The f plasmid and conjugation

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 _____ cell

  • The F factor of an Hfr cell brings some chromosomal DNA along when transferred to an F– cell


Le 18 18 1

LE 18-18_1

F plasmid

Bacterial chromosome

F+ cell

F+ cell

Mating

bridge

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

LE 18-18_2

F plasmid

Bacterial chromosome

F+ cell

F+ cell

Mating

bridge

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

LE 18-18_3

F plasmid

Bacterial chromosome

F+ cell

F+ cell

Mating

bridge

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

LE 18-18_4

F plasmid

Bacterial chromosome

F+ cell

F+ cell

Mating

bridge

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

partial

diploid

Recombinant F–

bacterium

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 and Antibiotic Resistance

  • __________ 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


Transposition of genetic elements

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


Insertion sequences

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


Le 18 19a

LE 18-19a

Insertion sequence

Inverted

repeat

Inverted

repeat

Transposase gene


Transposons

Transposons

  • Transposable elements called __________ 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


Le 18 19b

LE 18-19b

Transposon

Insertion

sequence

Insertion

sequence

Antibiotic

resistance gene

Transposase gene

Inverted repeat


Chapter 18

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:

    • _______________ of metabolic enzymes

    • _______________ that encode metabolic enzymes


Le 18 20

LE 18-20

Regulation of enzyme

production

Regulation of enzyme

activity

Precursor

Feedback

inhibition

Enzyme 1

Gene 1

Gene 2

Enzyme 2

Regulation

of gene

expression

Gene 3

Enzyme 3

Enzyme 4

Gene 4

Gene 5

Enzyme 5

Tryptophan


Operons the basic concept

Operons: The Basic Concept

  • In bacteria, genes are often clustered into __________, 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


Le 18 21a

LE 18-21a

trp operon

Promoter

Promoter

Genes of operon

DNA

trpB

trpA

trpE

trpC

trpD

trpR

Operator

Stop codon

RNA

polymerase

Regulatory

gene

Start codon

mRNA 5¢

mRNA

D

B

E

C

A

Inactive

repressor

Protein

Polypeptides that make up

enzymes for tryptophan synthesis

Tryptophan absent, repressor inactive, operon on


Le 18 21b 1

LE 18-21b_1

DNA

mRNA

Active

repressor

Protein

Tryptophan

(corepressor)

Tryptophan present, repressor active, operon off


Le 18 21b 2

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

Repressible and Inducible Operons: Two Types of Negative Gene Regulation

  • A __________ operon is one that is usually _____; binding of a repressor to the operator shuts off transcription

  • The __________ is a repressible operon

  • An __________ operon is one that is usually _____; a molecule called an inducer inactivates the repressor and turns on transcription

  • The classic example of an inducible operon is the __________, which contains genes coding for enzymes in hydrolysis and metabolism of lactose


Le 18 22a

LE 18-22a

Promoter

Regulatory

gene

Operator

lacl

lacZ

DNA

No

RNA

made

mRNA

RNA

polymerase

Active

repressor

Protein

Lactose absent, repressor active, operon off


Le 18 22b

LE 18-22b

lac operon

DNA

lacl

lacY

lacA

lacZ

RNA

polymerase

mRNA

mRNA 5¢

Transacetylase

Permease

-Galactosidase

Protein

Inactive

repressor

Allolactose

(inducer)

Lactose present, repressor inactive, operon on


Chapter 18

  • Inducible enzymes usually function in catabolic pathways

  • Repressible enzymes usually function in anabolic pathways

  • Regulation of the trp and lac operons involves __________ control of genes because operons are switched off by the active form of the repressor


Positive gene regulation

Positive Gene Regulation

  • Some operons are also subject to __________ 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


Le 18 23a

LE 18-23a

Promoter

DNA

lacl

lacZ

RNA

polymerase

can bind

and transcribe

Operator

CAP-binding site

Active

CAP

cAMP

Inactive lac

repressor

Inactive

CAP

Lactose present, glucose scarce (cAMP level high): abundant lac

mRNA synthesized


Le 18 23b

LE 18-23b

Promoter

DNA

lacl

lacZ

CAP-binding site

Operator

RNA

polymerase

can’t bind

Inactive

CAP

Inactive lac

repressor

Lactose present, glucose present (cAMP level low): little lac

mRNA synthesized


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