Viruses Are obligate intracellular parasites Some are small as ribosome Some bigger than some bacteria

anatomy of a virus All contain a sort of genetic material Could be DNA (single or double stranded) Could be RNA (single or double stranded) All contain a protein coat enclosing the genetic material=capsule Some have a viral envelope that is derived from a host cell membrane

BACTERIOPHAGES (Phage) are the best understood viruses An example of a phage is the T4 phage Its life cycle always results in the death of host cell This cycle is called the lytic cycle If this is the only life cycle the phage is a virulent phage

Viral life cycles

Lysogenic cycle -temperate phages -DNA is incorporated into host genome -called prophage -may enter the lytic cycle -environmental triggers

Viral envelopes Have glycoproteins that help bind to cell membrane Capsid enters cell Genetic material copied New capsid proteins made Glycoproteins produced in ER Viruses bud out of cell

Retroviruses Single stranded RNA that acts as template for DNA