3. Viruses: Structure and the Lytic Cycle

A virus is the ultimate parasite. It carries only the instructions for making more viruses — none of the machinery to do so. That machinery belongs to the host cell it invades.

Viral structure:

• Genetic material — either DNA or RNA (never both), which holds the instructions for making new viruses.
• Capsid — protein coat that protects the genetic material and helps the virus attach to cells.
• Envelope (some viruses only) — an extra lipid layer borrowed from the host cell; the flu virus and HIV have one.

The lytic cycle (how most viruses replicate):

1. Attachment — the virus binds to a specific receptor on the host cell surface. This is why most viruses infect only certain species or cell types.
2. Entry — viral genetic material is injected into or enters the host cell.
3. Replication — the host cell's own machinery reads the viral genes and makes hundreds of copies of the virus.
4. Assembly — new virus particles are built inside the cell.
5. Release (lysis) — the cell bursts open, releasing new viruses that go on to infect neighboring cells.

[missing figure: lytic_cycle]

Why antibiotics do NOT work on viruses: antibiotics target bacterial structures (cell walls, ribosomes, etc.). Viruses have none of those structures, so antibiotics have nothing to attack.

Antiviral drugs work differently — they block specific steps of the viral cycle (e.g., blocking attachment or preventing the virus from copying its genes).