The Genetics of Viruses & Prokaryotes
Chapter 13 — How viruses reproduce, how phage λ decides lysis vs lysogeny, how bacteria exchange genes, and how operons regulate expression.
How Do Viruses Reproduce & Transmit Genes?
Viruses are acellular obligate intracellular parasites — they hijack host machinery to replicate.
Why Prokaryotes & Viruses Are Model Organisms
Easier to study and sequence.
Generations in minutes to hours.
Mutations immediately visible.
What Are Viruses?
Viruses are acellular — not cells. They consist of nucleic acid (DNA or RNA) surrounded by a protein capsid. Outside the host cell, they exist as virions.
Viruses Do NOT:
No control of materials in/out.
No metabolic functions of their own.
They are obligate intracellular parasites — they require host cell DNA replication and protein synthesis machinery. They are not affected by antibiotics that target bacterial cell walls or ribosomes.
Discovery
First virus discovered in the 1890s — an agent causing tobacco mosaic disease. It could pass through a filter retaining bacteria, diffuse through agar gel, and was crystallized in the 1930s.
Virus Classification Criteria
| Criterion | Options |
|---|---|
| Genome type | DNA or RNA |
| Nucleic acid | Single- or double-stranded |
| Shape | Simple (helical, icosahedral) or complex |
| Envelope | With or without membrane |
| Host type | Bacteria, animals, plants |
| Infection manner | Lytic, lysogenic, etc. |