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Chapter 18. Viruses and Gene regulation. Concept 18.1: A virus has a genome but can reproduce only within a host cell Scientists were able to detect viruses indirectly Long before they were actually able to see them. Figure 18.3. The Discovery of Viruses: Scientific Inquiry.

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chapter 18

Chapter 18

Viruses and Gene regulation

slide2
Concept 18.1: A virus has a genome but can reproduce only within a host cell
  • Scientists were able to detect viruses indirectly
    • Long before they were actually able to see them
the discovery of viruses scientific inquiry

Figure 18.3

The Discovery of Viruses: Scientific Inquiry
  • Tobacco mosaic disease
    • Stunts the growth of tobacco plants and gives their leaves a mosaic coloration
slide4
In the late 1800s
    • Researchers hypothesized that a particle smaller than bacteria caused tobacco mosaic disease
  • In 1935, Wendell Stanley
    • Confirmed this hypothesis when he crystallized the infectious particle, now known as tobacco mosaic virus (TMV)
structure of viruses
Structure of Viruses
  • Viruses
    • Are very small infectious particles consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope
viral genomes
Viral Genomes
  • Viral genomes may consist of
    • Double- or single-stranded DNA
    • Double- or single-stranded RNA
capsids and envelopes

Capsomereof capsid

RNA

DNA

Capsomere

Glycoprotein

18  250 mm

70–90 nm (diameter)

20 nm

50 nm

(b) Adenoviruses

Figure 18.4a, b

(a) Tobacco mosaic virus

Capsids and Envelopes
  • A capsid
    • Is the protein shell that encloses the viral genome
    • Can have various structures
slide8

Membranousenvelope

Capsid

RNA

Glycoprotein

80–200 nm (diameter)

50 nm

(c) Influenza viruses

Figure 18.4c

  • Some viruses have envelopes
    • Which are membranous coverings derived from the membrane of the host cell
slide9

Head

DNA

Tail sheath

Tail fiber

80  225 nm

50 nm

(d) Bacteriophage T4

Figure 18.4d

  • Bacteriophages, also called phages
    • Have the most complex capsids found among viruses
general features of viral reproductive cycles
General Features of Viral Reproductive Cycles
  • Viruses are obligate intracellular parasites
    • They can reproduce only within a host cell
  • Each virus has a host range
    • A limited number of host cells that it can infect
slide11

VIRUS

DNA

Entry into cell and

uncoating of DNA

Capsid

Transcription

Replication

HOST CELL

Viral DNA

mRNA

Viral DNA

Capsid proteins

Self-assembly of new virus particles and their exit from cell

Figure 18.5

  • Viruses use enzymes, ribosomes, and small molecules of host cells
    • To synthesize progeny viruses
reproductive cycles of phages
Reproductive Cycles of Phages
  • Phages
    • Are the best understood of all viruses
    • Go through two alternative reproductive mechanisms: the lytic cycle and the lysogenic cycle
the lytic cycle
The Lytic Cycle
  • The lytic cycle
    • Is a phage reproductive cycle that culminates in the death of the host
    • Produces new phages and digests the host’s cell wall, releasing the progeny viruses
slide14

1

Attachment. The T4 phage usesits tail fibers to bind to specificreceptor sites on the outer surface of an E. coli cell.

2

Entry of phage DNA and degradation of host DNA.The sheath of the tail contracts,injecting the phage DNA intothe cell and leaving an emptycapsid outside. The cell’sDNA is hydrolyzed.

5

Release. The phage directs productionof an enzyme that damages the bacterialcell wall, allowing fluid to enter. The cellswells and finally bursts, releasing 100 to 200 phage particles.

Phage assembly

3

Synthesis of viral genomes and proteins. The phage DNAdirects production of phageproteins and copies of the phagegenome by host enzymes, usingcomponents within the cell.

4

Assembly. Three separate sets of proteinsself-assemble to form phage heads, tails,and tail fibers. The phage genome ispackaged inside the capsid as the head forms.

Head

Tail fibers

Figure 18.6

Tails

  • The lytic cycle of phage T4, a virulent phage
the lysogenic cycle
The Lysogenic Cycle
  • The lysogenic cycle
    • Replicates the phage genome without destroying the host
  • Temperate phages
    • Are capable of using both the lytic and lysogenic cycles of reproduction
slide16

Phage

DNA

The phage attaches to a

host cell and injects its DNA.

Many cell divisions produce a large population of bacteria infected with the prophage.

Phage DNA

circularizes

Phage

Occasionally, a prophage exits the bacterial chromosome,

initiating a lytic cycle.

Bacterial

chromosome

Lytic cycle

Lysogenic cycle

Certain factors

determine whether

The bacterium reproduces

normally, copying the prophage

and transmitting it to daughter cells.

The cell lyses, releasing phages.

Prophage

Lytic cycle

is induced

Lysogenic cycle

is entered

or

New phage DNA and proteins are synthesized and assembled into phages.

Phage DNA integrates into the bacterial chromosome,becoming a prophage.

Figure 18.7

  • The lytic and lysogenic cycles of phage , a temperate phage
reproductive cycles of animal viruses
Reproductive Cycles of Animal Viruses
  • The nature of the genome
    • Is the basis for the common classification of animal viruses
slide18

Table 18.1

  • Classes of animal viruses
viral envelopes
Viral Envelopes
  • Many animal viruses
    • Have a membranous envelope
  • Viral glycoproteins on the envelope
    • Bind to specific receptor molecules on the surface of a host cell
slide20

Glycoproteins on the viral envelope bind to specific receptor molecules(not shown) on the host cell, promoting viral entry into the cell.

Capsid

RNA

2

1

Envelope (with

glycoproteins)

Capsid and viral genome

enter cell

3

HOST CELL

The viral genome (red)

functions as a template forsynthesis of complementary

RNA strands (pink) by a viral

enzyme.

Viral genome (RNA)

Template

5

mRNA

Complementary RNA

strands also function as mRNA,

which is translated into both

capsid proteins (in the cytosol)and glycoproteins for the viral

envelope (in the ER).

New copies of viral

genome RNA are made

using complementary RNA

strands as templates.

4

Capsid

proteins

ER

Copy of

genome (RNA)

Glyco-

proteins

6

Vesicles transport

envelope glycoproteins to

the plasma membrane.

7

New virus

8

A capsid assembles

around each viral

genome molecule.

Figure 18.8

  • The reproductive cycle of an enveloped RNA virus
rna as viral genetic material
RNA as Viral Genetic Material
  • The broadest variety of RNA genomes
    • Is found among the viruses that infect animals
slide22

Glycoprotein

Viral envelope

Capsid

RNA(two identicalstrands)

Reversetranscriptase

Figure 18.9

  • Retroviruses, such as HIV, use the enzyme reverse transcriptase
    • To copy their RNA genome into DNA, which can then be integrated into the host genome as a provirus
slide23

1

Reverse transcriptase

catalyzes the synthesis of a

DNA strand complementary

to the viral RNA.

2

The virus fuses with the

cell’s plasma membrane.

The capsid proteins are

removed, releasing the viral proteins and RNA.

Membrane of white blood cell

HIV

Reverse transcriptase

catalyzes the synthesis ofa second DNA strand

complementary to the first.

3

Reverse transcriptase

HOST CELL

Viral RNA

4

The double-stranded DNA is incorporated

as a provirus into the cell’s DNA.

RNA-DNAhybrid

0.25 µm

HIV entering a cell

DNA

ChromosomalDNA

NUCLEUS

Provirus

5

Proviral genes are transcribed into RNA molecules, which serve as genomes for the next viral generation and as mRNAs for translation into viral proteins.

RNA genomefor the nextviral generation

mRNA

6

The viral proteins include capsid proteins and reverse transcriptase (made in the cytosol) and envelope glycoproteins (made in the ER).

7

Capsids are

assembled around

viral genomes and

reverse transcriptase

molecules.

Vesicles transport the

glycoproteins from the ER to

the cell’s plasma membrane.

8

9

New viruses bud

off from the host cell.

Figure 18.10

New HIV leaving a cell

  • The reproductive cycle of HIV, a retrovirus
evolution of viruses
Evolution of Viruses
  • Viruses do not really fit our definition of living organisms
  • Since viruses can reproduce only within cells
    • They probably evolved after the first cells appeared, perhaps packaged as fragments of cellular nucleic acid
slide25
Concept 18.2: Viruses, viroids, and prions are formidable pathogens in animals and plants
  • Diseases caused by viral infections
    • Affect humans, agricultural crops, and livestock worldwide
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
slide27
Vaccines
    • Are harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogen
    • Can prevent certain viral illnesses
emerging viruses
Emerging Viruses
  • Emerging viruses
    • Are those that appear suddenly or suddenly come to the attention of medical scientists
slide29

(b) The SARS-causing agent is a coronavirus like this one (colorized TEM), so named for the “corona” of glycoprotein spikes protruding from the envelope.

(a) Young ballet students in Hong Kong wear face masks to protect themselves from the virus causing SARS.

Figure 18.11 A, B

  • Severe acute respiratory syndrome (SARS)
    • Recently appeared in China
slide30
Outbreaks of “new” viral diseases in humans
    • Are usually caused by existing viruses that expand their host territory
viral diseases in plants

Figure 18.12

Viral Diseases in Plants
  • More than 2,000 types of viral diseases of plants are known
  • Common symptoms of viral infection include
    • Spots on leaves and fruits, stunted growth, and damaged flowers or roots
slide32
Plant viruses spread disease in two major modes
    • Horizontal transmission, entering through damaged cell walls
    • Vertical transmission, inheriting the virus from a parent
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
slide34

Originalprion

Prion

Many prions

Normalprotein

Newprion

Figure 18.13

  • Prions
    • Are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals
    • Propagate by converting normal proteins into the prion version
slide35

(a) Regulation of enzyme activity

(b) Regulation of enzyme production

Precursor

Feedback

inhibition

Enzyme 1

Gene 1

Regulation

of gene

expression

Enzyme 2

Gene 2

Gene 3

Enzyme 3

Gene 4

Enzyme 4

Gene 5

Enzyme 5

Tryptophan

Figure 18.20a, b

  • This metabolic control occurs on two levels
    • Adjusting the activity of metabolic enzymes already present
    • Regulating the genes encoding the metabolic enzymes
operons the basic concept
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
slide37
An operon
    • Is usually turned “on”
    • Can be switched off by a protein called a repressor
slide38

trp operon

Promoter

Promoter

Genes of operon

RNA polymerase

Start codon Stop codon

trpR

trpD

trpC

trpB

trpE

trpA

DNA

Operator

Regulatory

gene

3

mRNA 5

mRNA

5

C

E

D

B

A

Polypeptides that make up

enzymes for tryptophan synthesis

Inactiverepressor

Protein

(a) Tryptophan absent, repressor inactive, operon on. RNA polymerase attaches to the DNA at the promoter and transcribes the operon’s genes.

Figure 18.21a

  • The trp operon: regulated synthesis of repressible enzymes
slide39

DNA

No RNA made

mRNA

Active

repressor

Protein

Tryptophan

(corepressor)

(b)

Tryptophan present, repressor active, operon off. As tryptophan

accumulates, it inhibits its own production by activating the repressor protein.

Figure 18.21b

repressible and inducible operons two types of negative gene regulation
Repressible and Inducible Operons: Two Types of Negative Gene Regulation
  • In a repressible operon
    • Binding of a specific repressor protein to the operator shuts off transcription
  • In an inducible operon
    • Binding of an inducer to an innately inactive repressor inactivates the repressor and turns on transcription
slide41

Promoter

Regulatorygene

Operator

DNA

lacl

lacZ

NoRNAmade

3

RNApolymerase

mRNA

5

Activerepressor

Protein

(a)

Lactose absent, repressor active, operon off. The lac repressor is innately active, and inthe absence of lactose it switches off the operon by binding to the operator.

Figure 18.22a

  • The lac operon: regulated synthesis of inducible enzymes
slide42

lac operon

DNA

lacl

lacz

lacY

lacA

RNApolymerase

3

mRNA 5

mRNA 5'

mRNA

5

-Galactosidase

Permease

Transacetylase

Protein

Inactiverepressor

Allolactose(inducer)

(b)

Lactose present, repressor inactive, operon on. Allolactose, an isomer of lactose, derepresses the operon by inactivating the repressor. In this way, the enzymes for lactose utilization are induced.

Figure 18.22b

slide43
Inducible enzymes
    • Usually function in catabolic pathways
  • Repressible enzymes
    • Usually function in anabolic pathways
slide44
Regulation of both the trp and lac operons
    • Involves the negative control of genes, because the operons are switched off by the active form of the repressor protein
positive gene regulation
Positive Gene Regulation
  • Some operons are also subject to positive control
    • Via a stimulatory activator protein, such as catabolite activator protein (CAP)
slide46

Operator

RNA

polymerase

can bindand transcribe

Promoter

DNA

lacl

lacZ

CAP-binding site

ActiveCAP

cAMP

Inactive lac

repressor

InactiveCAP

(a)

Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized.If glucose is scarce, the high level of cAMP activates CAP, and the lac operon produces large amounts of mRNA for the lactose pathway.

Figure 18.23a

  • In E. coli, when glucose, a preferred food source, is scarce
    • The lac operon is activated by the binding of a regulatory protein, catabolite activator protein (CAP)
slide47

Promoter

Operator

DNA

lacl

lacZ

CAP-binding site

RNA

polymerase

can’t bind

InactiveCAP

Inactive lac

repressor

Lactose present, glucose present (cAMP level low): little lac mRNA synthesized.When glucose is present, cAMP is scarce, and CAP is unable to stimulate transcription.

(b)

Figure 18.23b

  • When glucose levels in an E. coli cell increase
    • CAP detaches from the lac operon, turning it off