• Very simple structures consisting essentially of a nucleic acid genome, protected by a shell of protein. • May or may not have a lipoprotein envelope. • Has no organelle. • Very small, sizes range 20 - 200 nm, beyond the resolving power of the light microscope. • Metabolically inert and can only replicate inside a host cell. • Genome consists of ONLY one type of nucleic acid; either RNA or DNA. • Viral genome codes for the few proteins necessary for replication: some proteins are non-structural e.g. polymerase, and some are structural, i.e. form part of the virion structure.

• Very simple structures consisting essentially of a nucleic acid genome, protected by a shell of protein. • May or may not have a lipoprotein envelope. • Has no organelle. • Very small, sizes range 20 - 200 nm, beyond the resolving power of the light microscope. • Metabolically inert and can only replicate inside a host cell. • Genome consists of ONLY one type of nucleic acid; either RNA or DNA. • Viral genome codes for the few proteins necessary for replication: some proteins are non-structural e.g. polymerase, and some are structural, i.e. form part of the virion structure.

Very simple structures consisting essentially of a nucleic acid genome, protected by a shell of protein.