Viruses

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

copyright cmassengale Size of Viruses copyright cmassengale

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

Reproduction Lytic 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 viruses Steps: 1) adsorption – attachment of virus to host cell 2) 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 or may 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