Viruses

• Small fixed size • Genetic material (nucleic acid: DNA/RNA) • Capsid (protein) • No cytoplasm • Few/no enzymes • Requires host cell to reproduce

1) Phage attaches to specific receptors on bacterial cell 2) Penetration: viral DNA injected into host cell (capsid left outside) 3) Viral DNA takes over host cell machinery 4) Viral DNA replicates; host cell’s DNA broken (DNA hydrolysis) 5) Phage proteins synthesized & assembled into new phages 6) Cell undergoes lysis → new phages released

1) Phage injects DNA into cell 2) Viral DNA integrates into host cell’s DNA, forming prophage 3) Phage’s viral DNA replicates with host’s DNA 4) Cell divides, passing phage’s DNA on to daughter cells 5) Under stress (induction), prophase excised, entering lytic cycle

• Viruses existed before cells cuz they’re simpler • May have contributed genetic material to first cells • Explains unique viral genes • Weakness: viruses rely on cells to reproduce

• Viruses may have evolved from escaped DNA/RNA from cells • Supported by genetic exchange in bacteri • Explains viral diversity • Weakness: many viral genes not found in cells

• Viruses may have evolved from small parasitic cells • Unneeded genes lost over time • Supported by giant viruses with genes similar to parasitic bacteria • Weakness: smallest parasitic cells do not resemble viruses

• DNA viruses (adenoviruses, most bacteriophages) • RNA viruses (HIV, coronaviruses) • Positive-sense = acts as mRNA → translated immediately • Negative-sense = complementary to mRNA → must be converted→ positive-sense before translation

• Protein coat enclosing viral genome • Made of capsomeres

• Encloses nucleocapsids • Contain host phospholipids & viral spike proteins • Spike proteins help bind and enter host cells • Envelope protects the virus from damage

• Rapid replication and high mutation rate • Little/no proofreading mechanism • Immune systems selects against recognized viruses • Mutated viruses not recognized selected for

Influenza: - Enveloped virus with negative-sense, ssRNA - HA binds to sialic acid receptors on host cells Antigenic drift: - Small mutations in genes HA/NA - Gradual antigen changes reduce immune recognition Antigenic shift: - Sudden big antigen change in influenza A - 2 strains infect same host cell → RNA reassortment - New HA/NA combinations create new subtype

• Causes AIDS • Retrovirus with RNA genome • Reverse transcriptase converts viral RNA → DNA • Viral DNA integrates into host DNA (provirus) • High mutation rate (no proofreading) • → many variants → difficult to treat • Makes HIV hard to treat