• Infect prokaryotes
• Head, or capsid, contains genome
• Cylindrical tail sheath and hexagonal end plate facilitate infection.
• DNA passes through sheath during injection
• Tail fibers bind to target’s membrane.

• Frequently have an envelope, either made of virus-produced glycoprotein or host-derived membrane lipids
• Envelope has “spikes” that bind to target cell; envelope then fuses with target cell, carrying capsid and genome inside.

• Can be either RNA or DNA, and either single- or double-stranded
• Reverse transcriptases translate viral RNA into DNA for insertion into host genome.
• Genome codes for a replication protein (polymerase) and all proteins required for virion production However a lot of bacteriophages code for sigma factor that guide host polymerase to viral genes

• Lytic phase: virions are actively produced by the host’s synthetic machinery; when enough viruses are present, the cell ruptures, releasing virions
• Lysogenic phase: virus genome is inserted into the host’s genome and lie dormant; in bacteriophage, the inserted sequence is called a prophage.

• based on homology of DNA polymerase and/or type of nucleic acid
• Viral genomes are extremely malleable; also, their ultimate origin is unknown.

• Regressive hypothesis
• Cellular origin hypothesis
• Coevolution hypothesis

Viruses may have once been small cells that parasitised larger cells. (rickettsia and chlamydia)

Over time, genes not required by their parasitism were lost

Dependence on parasitism caused loss of genes that enabled them to survive outside a cell

• Viruses may have evolved from bits of DNA or RNA that "escaped" from the genes of a larger organism.
• The escaped DNA could have come from plasmids or transposons (molecules of DNA that replicate and move around to different positions within the genes of the cell)

• Viruses may have evolved from complex molecules of protein and nucleic acid at the same time as cells first appeared on earth
• Would have been dependent on cellular life for billions of years.
• Molecules of RNA that are not classified as viruses because they lack a protein coat. They do not code for proteins but interact with the host cell and use the host machinery for their replication.

• Genetic transduction through phage has probably been important in the propagation of novel genes (lysogenic phage inserting randomly into host genome, and packaging part of chromosomal DNA)
• In aquatic ecosystems, viral lysis has been shown to be important in nutrient cycling through the microbial loop
• Viruses divert the flow of carbon and nutrients from secondary consumers by destroying host cells and releasing the contents of these cells into the pool of dissolved organic matter (DOM) in the ocean. DOM is then used as a food source by bacteria, which transfers some of this material back into the food web.

• The l-phage has been invaluable in genomic science.
• Genes of interest are ligated with the “arms” of the phage genome (cos sites/arms), recognized by l-phage for genome-packing
• Only the ends of the l-phage are necessary for infection and replication. The middle section encodes lysogenic functionality and is replaced with the DNA of insert.
• Recombinant phages are then used to transfect E. coli, which amplify the genomic DNA for sequencing.

• viruses can only be cultured inside living cells.
• Viruses are cultured by obtaining an enriched viral preparation using filtration of actively growing host cells.
• A lawn of host cells are prepared on a petri dish; enriched virus is added.
• Plaques form around VFU (virus forming units). The bacteria around the plaque are subcultured and should actively produce virus.