Viruses and Horizontal Gene Transfer

Viruses are not cells

A virus is a nucleic acid genome inside a protein capsid, sometimes wrapped in a stolen membrane envelope. It has no ribosomes, no metabolism, and cannot reproduce alone — it must use a host cell's machinery. Its genome may be DNA or RNA, single- or double-stranded, which is one of the few places in biology where DNA is not the universal genetic material.

• Lytic cycle — the virus takes over at once, mass-produces new particles and bursts the cell.
• Lysogenic cycle — the genome integrates into the host chromosome as a prophage and is copied every time the host divides, silently, until a trigger switches it to the lytic cycle.

Retroviruses such as HIV carry RNA plus reverse transcriptase, which makes DNA from RNA — the reverse of the usual flow — and that DNA integrates into the host genome. RNA viruses mutate fast because their polymerases do not proofread, which is why influenza needs a new vaccine each year.

Prokaryotes swap genes sideways

Bacteria also acquire genes from other cells, not just from a parent:

• Transformation — naked DNA is taken up from the surroundings.
• Transduction — a phage accidentally carries host DNA to the next cell it infects.
• Conjugation — a pilus joins two cells and a plasmid is copied across.

Transposons move within a genome and can switch genes off or change how much of a product is made. Together these mechanisms explain how antibiotic resistance can spread through a bacterial population — and even between species — far faster than mutation alone could manage.