VIRUSES Chapter 19

What is a virus?

• A virus is a submicroscopic infectious particle composed of a protein coat (capsid) and a nucleic acid core (either DNA or RNA).

• Viruses are similar in size to a large protein macromolecule, generally smaller than 200 nm in diameter.

Discovery of Viruses

• Search for cause of tobacco mosaic disease led to viruses

• The elusive virus was crystallized in 1935 by Stanley

Fig. 19-2

RESULTS

1 2 3Extracted sapfrom tobaccoplant withtobaccomosaic disease

Passed sapthrough aporcelain filter knownto trapbacteria

Rubbed filteredsap on healthytobacco plants

4 Healthy plantsbecame infected

Viral Capsids• Capsids are built from protein

subunits called capsomeres• May be rod-shaped (helical viruses),

polyhedral (icosahedral viruses) or more complex

• Some viruses have membranous envelopes that help them infect hosts (flu virus

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

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

• Viruses are obligate intracellular parasites, which means they can reproduce only within a host cell

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

Viral Reproduction

• Once a viral genome has entered a cell, the cell begins to manufacture viral proteins using the host cell’s materials (enzymes, ribosomes, tRNAs, amino acids, ATP, etc.)

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

• 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 culminates in the death of

the host cell by producing 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

Animation: Phage T4 Lytic CycleAnimation: Phage T4 Lytic Cycle

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

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 and is called a prophage.

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

• Viruses that can be lysogenic or lytic are called temperate phages.

Animation: Phage Lambda Lysogenic and Lytic CyclesAnimation: Phage Lambda Lysogenic and Lytic Cycles

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.

Animal Viruses

• Classified as DNA or RNA viruses, single or double-stranded

• Many have envelopes with glycoproteins that are specific for receptors.

• The glycoproteins are made by the ER and added to the host cell’s membrane which envelopes the emerging viruses.

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

Table 19-1a

Table 19-1b

RNA Viruses

• 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 is ex.)

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

• Unlike a prophage, a provirus remains a permanent resident of the host 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

Evolution of Viruses

• Since viruses can reproduce only within cells, they probably evolved as bits of cellular nucleic acid

• Candidates for the source of viral genomes are plasmids and transposons, small mobile DNA segments

• Mimivirus, a double-stranded DNA virus, is the largest virus yet discovered

mimivirus

Which came first,the cell or the mimivirus?

The virus was first isolated in 1992 from amoebae growing in a water tower.

Viral diseases in animals

• Symptoms caused by - toxins - body’s defense mechanisms• Vaccines – weakened or derivatives of

viral particles capable of causing an immune response

• Antibiotics not effective• Some antiviral medications interfere

with viral nucleic acid synthesis

Where do new viruses come from?

• Mutations of existing viruses

• The dissemination of an existing virus to a more widespread population

• Or spread between species

• Epidemic – general outbreak of a disease

• Pandemic – global epidemic

Fig. 19-9

(a) The 1918 flu pandemic

(b) Influenza A H5N1 virus

(c) Vaccinating ducks

0.5 µm

Plant viruses

• More than 2,000 types of viral diseases of plants are known and cause spots on leaves and fruits, stunted growth, and damaged flowers or roots

• Most plant viruses have an RNA genome

• Plant viral disease can spread by vertical transmission from parent plant or by horizontal transmission from an external source.

Fig. 19-10

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 misfolded proteins that cause brain diseases in mammals

• Prions propagate by converting normal proteins into the prion version

• Scrapie in sheep, mad cow disease, and Creutzfeldt-Jakob disease in humans are all caused by prions

Viroids in Plants

Fig. 19-11

Prion

Normalprotein

Originalprion

Newprion

Aggregatesof prions

Misfolding of proteins to form prions

Scrapie in sheep