Genetics of Viruses

What Are Viruses? Viruses are tiny (20 - 400 nm) particles composed of a nucleic acid core (either DNA or RNA) surrounded by a protein coat (capsid) and sometimes a membrane envelope (derived from its host cell) Viruses are incapable of reproduction independent of host cells whose cellular machinery is taken over by the virus and used as a factory for producing new viruses

Viral Structure Viruses are made of a maximum of three basic parts: Genetic material: DNA viruses - Single or double stranded DNA RNA viruses - Single or double stranded RNA Capsid - A protein coat made up of one or a few capsomere proteins Envelope - Not all viruses have an envelope which is derived from the host cell’s membrane The envelope may contain proteins which assist in attachment and entry into new host cells

Phage Reproduction: The Lytic Cycle Destruction of the bacteria’s DNA Infection Replication of the viral genome Production of viral parts Packaging Lysis

Phage Reproduction: The Lysogenic Cycle Circularization of phage DNA Infection Many generations of bacteria Temperate phage Exit of phage Integration of phage DNA into the bacterial genome On to the lytic cycle

Are Viruses Alive? Viruses require cells to reproduce and do not have cells of their own, thus they do not fit the Cell Theory which states that being made out of cells is a characteristic of living things Are viruses “missing links” between living and non-living things? No, viruses require cells to reproduce, so cells had to come first then viruses thus viruses cannot serve as a “missing link” between living and non-living things