Viruses

Viruses

• Nonliving particles • Very small (1/2 to 1/100 of a bacterial cell)• Do not perform respiration, grow, or

develop• Are able to replicate (only with the help of

living cells)• Host cell—a cell where a virus replicates• Bacteriophage (phage)—virus that infects a

bacterium

T4 bacteriophage infecting an E. coli cell

0.5 m

Comparing the size of a virus, a bacterium, and an animal cell

0.25 m

Virus

Animalcell

Bacterium

Animal cell nucleus

Viral Structure• 2 main parts:

• inner core of nucleic acid (DNA or RNA)– instructions for making copies of the virus

• outer coat of protein (capsid)–determines shape of virus (which cells &

how cells are infected)»polyhedral»helical»envelope with projections»classic phage shape

Viral structure

18 250 mm 70–90 nm (diameter) 80–200 nm (diameter) 80 225 nm

20 nm 50 nm 50 nm 50 nm(a) Tobacco mosaic virus (b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4

RNA

RNACapsomereof capsid

DNACapsomere

Glycoprotein Glycoprotein

Membranousenvelope

Capsid DNA

Head

Tail fiber

Tail sheath

Infection by tobacco mosaic virus (TMV)

Attachment to Host Cell• Order of events:

• virus recognizes host cell• virus attaches to receptor site on membrane of host cell–Receptor site on host matches with viral proteins (like a puzzle)

• virus enters host cell• virus replicates inside host cell

Attachment is Specific• viruses have specifically shaped attachment

proteins• each virus infects only certain types of cells

– most are species specific• Smallpox, polio, measles—affects only humans

– although some are not• West Nile virus—mosquitoes, birds, humans,

horses– some are cell-type specific

• polio—affects intestine & nerve cells

Simplified viral reproductive cycleVIRUS

Capsid proteins

mRNA

Viral DNA

HOST CELL

Viral DNA

Entry into cell anduncoating of DNA

Replication Transcription

DNA

Capsid

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

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

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.

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

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.

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.

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4 3

5

Phage assembly

Head Tails Tail fibers

Lytic cycle of phage T4, a virulent phage

Lytic vs Lysogenic

• Lytic cycle (virulent phage)– Release of virus bursts and kills host cell

(lysis)• Lysogenic cycle (temperate phage)

– Viral DNA integrates into host genome (provirus)

– Can be transmitted to daughter cells – Can initiate lytic cycle in response to

environmental signal (stress)

The lytic and lysogenic cycles of phage , a temperate phage

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

The bacterium reproducesnormally, copying the prophageand transmitting it to daughter cells.

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

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

Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle.

Certain factorsdetermine whether

The phage attaches to ahost cell and injects its DNA.

Phage DNAcircularizes

The cell lyses, releasing phages.Lytic cycleis induced

Lysogenic cycleis entered

Lysogenic cycleLytic cycle

or Prophage/Provirus

Bacterialchromosome

Phage

PhageDNA

 The structure of HIV, the retrovirus that

causes AIDS

Reversetranscriptase

Viral envelope

Capsid

Glycoprotein

RNA(two identicalstrands)

Vesicles transport theglycoproteins from the ER tothe cell’s plasma membrane.

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

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The double-stranded DNA is incorporatedas a provirus into the cell’s DNA.

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

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Reverse transcriptasecatalyzes the synthesis ofa second DNA strandcomplementary to the first.

3

Reverse transcriptasecatalyzes the synthesis of aDNA strand complementaryto the viral RNA.

2

New viruses budoff from the host cell.9

Capsids areassembled aroundviral genomes and reverse transcriptase molecules.

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mRNA

RNA genomefor the nextviral generation

Viral RNA

RNA-DNAhybrid

DNA

ChromosomalDNA

NUCLEUSProvirus

HOST CELL

Reverse transcriptase

New HIV leaving a cell

HIV entering a cell

0.25 µm

HIV Membrane of white blood cell

The virus fuses with thecell’s plasma membrane.The capsid proteins areremoved, releasing the viral proteins and RNA.

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The reproductive cycle of HIV, a retrovirus

Lytic Cycle

Retrovirus CycleLysogenic Cycle

Complete the Following Venn Diagram. Describe in detail similarities and differences, give examples.