3 What are Viruses?A virus is a non-cellular particle made up of genetic material and protein that can invade living cells.
4 copyright cmassengale Size of Virusescopyright cmassengale
5 Are Viruses Living or Non-living? Viruses are both and neitherThey have some properties of life but not othersFor example, viruses can be killed, even crystallized like table saltHowever, they can’t maintain a constant internal state (homeostasis).
6 Characteristics: acellular somewhere between living & nonliving smaller than almost all living organismsstructure : protein coat called a capsid &nucleic acid (DNA or RNA)no growth or metabolism, no cytoplasm, no cell membranethey do reproduce, but only by using a living host cell
11 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
12 ReproductionLysogenic 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