viruses: a kind of “borrowed life”

Viruses: a kind of “borrowed life”

HIV infected T-cell

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Viral Structure

• not cells

• small infectious particles w/

– DNA or RNA enclosed in a protein coat (capsid)

– in some cases, a membranous envelope

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Viral Genomes

• genomes may be 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

RNA

Capsomere

Capsomereof capsid

DNA

Glycoprotein18 250 nm 70–90 nm (diameter)

Glycoproteins80–200 nm (diameter) 80 225 nm

Membranousenvelope RNA

Capsid

HeadDNA

TailsheathTailfiber

50 nm50 nm50 nm20 nm(a) Tobacco mosaic virus

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

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• help viruses infect hosts

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

• derived from the host cell’s membrane

– contain a combination of viral and host cell molecules

Membranous Envelopes

Fig. 19-7

Capsid

RNA

Envelope (withglycoproteins)

Capsid and viral genomeenter the cell

HOST CELL

Viral genome (RNA)

Template

mRNA

ERGlyco-proteins

Capsidproteins Copy of

genome (RNA)

New virus

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• AKA phages• are viruses that infect bacteria

• most complex capsids found among viruses

– w/ elongated capsid head that encloses their DNA

– A protein tail piece attaches to host and injects the phage DNA inside

Bacteriophages

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Bacteriophages

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Viral Reproduction• obligate intracellular parasites

– can reproduce only within a host cell

• Each virus has a host range

– a limited number of host species that it can infect

Transcriptionand manufactureof capsid proteins

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

Entry anduncoating

VIRUS1

2

3

DNACapsid

4

Replication

HOST CELL

Viral DNA

mRNA

Capsidproteins

Viral DNA

• Viruses use the host’s cellular machinery to reproduce itself• enzymes,• ribosomes,• tRNAs,• amino acids,• ATP, • and other

molecules

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Two Reproductive Mechanisms1. the lytic cycle

2. lysogenic cycle

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The Lytic Cycle

• A reproductive cycle that kills the host cell by lysing it or bursting it open with new viral progeny

• Bacteriophages and other viruses can reproduce in this way

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

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The Lysogenic Cycle

• At the start, this cycle replicates viral genome without destroying the host

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

– This integrated viral and host DNA is known as a prophage

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

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

The Lysogenic Cycle

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.

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Viral Classification

• two key variables used to classify viruses

– DNA or RNA?

– Single-stranded or double-stranded?

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RNA Viruses (Retroviruses)

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

– Ex. HIV

Glycoprotein

Reversetranscriptase HIV

RNA (twoidenticalstrands)

Capsid

Viral envelope

HOST CELLReversetranscriptase

Viral RNA

RNA-DNAhybrid

DNA

NUCLEUSProvirus

ChromosomalDNA

RNA genomefor thenext viralgeneration

mRNA

New virus

The viral DNA that is permanently integrated into the host genome is called a provirus (unlike prophage)

HIVMembrane ofwhite blood cell

HIV entering a cell0.25 µm

New HIV leaving a cell

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Viral Diseases

• Viruses damage or kill cells and tissues:

– reproduction

– release of hydrolytic enzymes from lysosomes

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

• Others have envelope proteins that are toxic

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• derivatives of pathogens that stimulate the immune system to mount defenses against the actual pathogen

• Vaccines can prevent certain viral illnesses

• Viral infections cannot be treated by antibiotics

• Antiviral drugs can help to treat, though not cure, viral infections

Vaccines

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Emerging Viruses• appear suddenly and come to the attention of

scientists

• Outbreaks of “new” viral diseases in humans are usually caused by existing viruses that expand their host territory

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• Flu epidemics come from new strains of influenza virus to which people have little immunity

• New viral diseases can emerge when viruses spread from animals to humans

• Viral strains that jump species can exchange genetic information with other viruses to which humans have no immunity

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• These strains can cause pandemics, global epidemics

• The “avian flu” is a virus that recently appeared in humans and originated in wild birds

(a) The 1918 flu pandemic

(b) Influenza A H5N1 virus

(c) Vaccinating ducks

0.5 µm