Viruses

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What are Viruses?

• A virus is a non-cellular particle made up of genetic material and protein that can invade living cells.

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Size of Viruses

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Are Viruses Living or Non-living?

• Viruses are both and neither• They have some properties

of life but not others• For example, viruses can be

killed, even crystallized like table salt

• However, they can’t maintain a constant internal state (homeostasis).

Characteristics:• acellular• somewhere between living & nonliving• smaller than almost all living organisms• structure : protein coat called a capsid &

nucleic acid (DNA or RNA)• no growth or metabolism, no cytoplasm, no cell

membrane• they do reproduce, but only by using a living host

cell

T4 Bacteriophage

Characteristics• Identified by shape: each depends on capsid arrangement and composition

1) many-sided (polyhedral) ex: adenovirus

2) rod ex: TMV (tobacco mosaic virus)

3) combination of 1 & 2 ex: bacteriophage

4) filovirus ex: ebola

Ebola

ReproductionLytic cycle – performed by virulent viruses Steps:

1) adsorption – attachment of virus to host cell 2) entry – viral nucleic acid enters host cell 3) replication – viral nucleic acid chops up host DNA & makes viral nucleic acid (NA) & viral proteins 4) assembly – viral capsids & NA combine 5) release – viruses lyse (break open) host cell

and are released to infect new cells

Reproduction

Lysogenic cycle – performed by temperate virusesSteps:1) adsorption – attachment of virus to host cell2) entry – viral nucleic acid enters host cell

*viral NA integrates into host DNA as a prophage (in prokaryote) or provirus (in eukaryote)

*may remain as prophage indefinitely ormay be triggered to break out and complete steps like the lytic cycle (replication,

assembly, release) triggers: stress, increased temperature

Viral Reproduction

• retrovirus – a virus containing RNA that makes DNA off of RNA using reverse transcriptase (an enzyme) that does transcription in reverse

• ex: HIV