Viruses & Genetic Variation
How viruses replicate, mutate, and shuffle genetic information between hosts.
Why viruses are not cells
A virus has no ribosomes, no metabolism, and no ability to reproduce on its own. It carries only a genome - DNA or RNA, single- or double-stranded - inside a protein capsid, plus in some cases a membrane envelope stolen from a previous host.
Because they depend entirely on host machinery, viruses are obligate intracellular parasites. Host range is determined by which surface proteins the virus can bind.
Replication strategies
- Lytic: attach, inject genome, hijack host machinery, assemble virions, lyse the cell.
- Lysogenic: viral DNA integrates as a prophage, replicates passively with the host genome, and can later switch to the lytic cycle under stress.
- Retroviral: RNA genome is reverse transcribed into DNA, then integrated into the host chromosome as a provirus.
Viruses as engines of variation
RNA viruses mutate rapidly because RNA-dependent polymerases lack proofreading, which is why influenza vaccines are reformulated each year (antigenic drift). When two viral strains infect the same cell, whole genome segments can swap - antigenic shift - producing sudden pandemic strains.
Viruses also move genes between hosts. Transduction carries bacterial DNA from one cell to another, contributing to horizontal gene transfer and the spread of antibiotic resistance.
Key terms
4
- Capsid
- The protein coat surrounding a virus's genetic material.
- Lytic cycle
- Replication path in which new virions burst the host cell.
- Lysogenic cycle
- Viral DNA integrates into the host genome as a prophage and is copied with it.
- Reverse transcriptase
- Enzyme that synthesizes DNA from an RNA template; used by retroviruses like HIV.
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