Head (capsid)
Usually an icosahedron packed with DNA. T4's head is a textbook icon about 110 nm long.
A tailed phage binds a receptor, contracts, and injects nucleic acid. Minutes later in a rich broth, hundreds of new particles may assemble. In the ocean the same logic runs more slowly.
Lysogeny is a truce: the phage genome rides along as the bacterium divides. UV or starvation can flip the switch to lysis. Description, not a recipe.
Parts
Tail
A tube that can contract in myoviruses. Not every phage has a tail — filamentous phages are threads.
Tail fibres
The matching keys. Change the fibre and the host range can change — a research fact, not instructions.
Genome
Typically DNA, from a few kilobases to half a megabase in jumbo phages. RNA phages exist too (MS2, 1961).
Baseplate
A landing gear that signals 'we have docked' before DNA moves.
No 70S factory
Protein synthesis is entirely the bacterium's. Phages are instructions plus a syringe.
Life cycle
1Adsorb
Fibres find a receptor — LPS, a porin, a pilus. No match, no infection.
2Inject
Nucleic acid enters the cytoplasm. The empty capsid stays outside — Hershey–Chase used that fact.
3Take over or integrate
Lytic programmes copy phage DNA. Temperate programmes may insert as prophage.
4Burst or wait
Lysis releases a burst of tens to hundreds of particles. Lysogeny waits for a later cue.
When headlines say a phage 'evolved overnight', they mean huge numbers and tight host matching, not a cartoon villain.
Keep exploring
More in Tree of microbes
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