Fig. 19-1

0.5 µm

Chapter 19Chapter 19

Fig. 19-2RESULTS

1 2 3Extracted sapfrom tobaccoplant withtobaccomosaic disease

Passed sapthrough aporcelain filter knownto trapbacteria

Rubbed filteredsap on healthytobacco plants

4 Healthy plantsbecame infected

•In 1935, Wendell Stanley confirmed this hypothesis

by crystallizing the infectious particle,

now known as tobacco mosaic virus (TMV)

• Viruses : nucleic acid + a protein coat and, in some cases +a membranous envelope

• Viral genomes may consist of either– Double- or single-stranded DNA, or– Double- or single-stranded RNA

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

Structure of Viruses

Fig. 19-3

RNA

Capsomere

Capsomereof capsid

DNA

Glycoprotein

18 250 nm 70–90 nm (diameter)

Glycoproteins

80–200 nm (diameter) 80 225 nm

Membranousenvelope RNA

Capsid

HeadDNA

Tailsheath

Tailfiber

50 nm50 nm50 nm20 nm

(a) Tobacco mosaic virus

(b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4

viral envelopes

• Some viruses have membranous envelopes that help them infect hosts

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

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

Bacteriophages

• Bacteriophages, also called phages, 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 tail piece attaches the phage to the host and injects the phage DNA inside

The Lytic Cycle

• The lytic cycle 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 virulent phage

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

The lysogenic cycle

• The lysogenic cycle replicates the phage genome without destroying the host

• The viral DNA molecule is incorporated into the host cell’s chromosome

• This integrated viral DNA is known as a prophage

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

Temperate phages

• An environmental signal can trigger the virus genome to exit the bacterial chromosome and switch to the lytic mode

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

Transcriptionand manufactureof capsid proteins

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

Entry anduncoating

Fig. 19-4VIRUS1

2

3

DNA

Capsid

4

Replication

HOST CELL

Viral DNA

mRNA

Capsidproteins

Viral DNA

Fig. 19-5-1

Attachment1

Fig. 19-5-2

Entry of phageDNA anddegradation ofhost DNA

Attachment1

2

Fig. 19-5-3

Synthesis of viralgenomes andproteins

Entry of phageDNA anddegradation ofhost DNA

Attachment1

2

3

Fig. 19-5-4

Phage assembly

Assembly

Synthesis of viralgenomes andproteins

Entry of phageDNA anddegradation ofhost DNA

Attachment1

2

4

Head Tail Tail fibers

3

Fig. 19-5-5

Phage assembly

Head Tail Tail fibers

Assembly

Release

Synthesis of viralgenomes andproteins

Entry of phageDNA anddegradation ofhost DNA

Attachment1

2

4

5

3

Fig. 19-6

PhageDNA

Phage

The phage injects its DNA.

Bacterialchromosome

Phage DNAcircularizes.

Daughter cellwith prophage

Occasionally, a prophageexits the bacterialchromosome,initiating a lytic cycle.

Cell divisionsproducepopulation ofbacteria infectedwith the prophage.

The cell lyses, releasing phages.

Lytic cycle

Lytic cycleis induced or Lysogenic cycle

is entered

Lysogenic cycle

Prophage

The bacterium reproduces,copying the prophage andtransmitting it to daughter cells.

Phage DNA integrates intothe bacterial chromosome,becoming a prophage.

New phage DNA and proteinsare synthesized andassembled into phages.

Reproductive Cycles of Animal Viruses

• There are two key variables used to classify viruses that infect animals:– DNA or RNA?– Single-stranded or double-stranded?

Table 19-1

Table 19-1a

Table 19-1b

Fig. 19-7

Capsid

RNA

Envelope (withglycoproteins)

Capsid and viral genomeenter the cell

HOST CELL

Viral genome (RNA)

Template

mRNA

ER

Glyco-proteins

Capsidproteins Copy of

genome (RNA)

New virus

RNA as Viral Genetic Material

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

• Retroviruses use reverse transcriptase to copy their RNA genome into DNA

• HIV (human immunodeficiency virus) is the retrovirus that causes AIDS (acquired immunodeficiency syndrome)

Fig. 19-8Glycoprotein Viral envelope

Capsid

RNA (twoidenticalstrands)Reverse

transcriptase HIV

HIVMembrane ofwhite blood cell

HIV entering a cell

0.25 µm

Viral RNA

RNA-DNAhybrid

HOST CELL

Reversetranscriptase

DNA

NUCLEUS

Provirus

ChromosomalDNA

RNA genomefor the next viralgeneration

mRNA

New virusNew HIV leaving a cell

Fig. 19-8aGlycoprotein

Reversetranscriptase HIV

RNA (twoidenticalstrands)

Capsid

Viral envelope

HOST CELL

Reversetranscriptase

Viral RNA

RNA-DNAhybrid

DNA

NUCLEUS

Provirus

ChromosomalDNA

RNA genomefor thenext viralgeneration

mRNA

New virus

Fig. 19-8b

HIVMembrane ofwhite blood cell

HIV entering a cell

0.25 µm

New HIV leaving a cell

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

Fig. 19-9

(a) The 1918 flu pandemic

(b) Influenza A H5N1 virus

(c) Vaccinating ducks

0.5 µm

Fig. 19-10

Fig. 19-11

Prion

Normalprotein

Originalprion

Newprion

Aggregatesof prions

Fig. 19-UN1

PhageDNA

Bacterialchromosome

The phage attaches to ahost cell and injects its DNA

Prophage

Lysogenic cycle• Temperate phage only• Genome integrates into bacterial chromosome as prophage, which (1) is replicated and passed on to daughter cells and (2) can be induced to leave the chromosome and initiate a lytic cycle

Lytic cycle• Virulent or temperate phage• Destruction of host DNA• Production of new phages• Lysis of host cell causes release of progeny phages

Fig. 19-UN2

Time Time

A B

Nu

mb

er

of

ba

cte

ria

Nu

mb

er

of

vir

use

s

Fig. 19-UN3