Parts of a Bacteriophage – T4 Labeled, 9 Parts

All 9 parts on the diagram

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Start here: 5 main parts 5

Head (capsid) The icosahedral protein shell, also called the capsid, that holds the DNA.
DNA T4's genome, packed tight inside the head. Double-stranded DNA about 169,000 base pairs long, encoding 289 proteins.
Tail sheath The contractile outer tube of the tail. It surrounds the tail tube until infection.
Baseplate The plate on the tail's end. A signal from the long tail fibres reaches it, and it changes shape to start infection.
Long tail fibres They recognize receptors on the surface of the bacterium. So they decide which bacteria the phage can infect.

The phage: 6 1

Short tail fibres Folded under the baseplate until the phage lands. They then unfold and bind the cell for good.

Infecting a cell: 3 3

Contracted tail sheath After the phage lands, the sheath contracts. That drives the tail tube into the cell.
Tail tube (core) The inner tube of the tail. Its tip punctures the outer membrane, and the DNA passes down it into the cell.
Outer membrane of the bacterium The outer membrane of the E. coli cell that the phage infects. The tail tube punctures it.

How this list was compiled: Read 2026-09-26. Wikipedia, Escherichia virus T4, https://en.wikipedia.org/wiki/Escherichia_virus_T4 : about 90 nm wide and 200 nm long; a double-stranded DNA genome of about 169 kbp encoding 289 proteins, held in an icosahedral head, also called a capsid; a contractile tail; long tail fibres that recognise host receptors and decide the host range; a recognition signal sent to the baseplate, which unravels the short tail fibres that bind irreversibly; the baseplate changing shape and the tail sheath contracting, so that the end of the tail tube punctures the outer membrane of the cell; the DNA travelling from the head through the tail tube into E. coli; the head assembled empty around a scaffolding protein that is later degraded, and DNA packed through a tiny pore by a motor at up to 2000 base pairs per second. Wikipedia, Bacteriophage, https://en.wikipedia.org/wiki/Bacteriophage , read the same day for the general account. Licence: own work

The drawing: Drawn for this site by script, 2026-09-26. Own work, nothing traced. Licence: own work

How it works

The parts of a bacteriophage, shown on T4, a virus that infects E. coli. The top figure is the phage on its own; the bottom figure shows it on a bacterium, tail contracted, passing its DNA into the cell.

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How the parts work together

The long tail fibres find receptors on the bacterium and signal the baseplate. The short fibres unfold and grip the cell, the sheath contracts, the tail tube punctures the outer membrane, and the DNA passes from the head down the tube.

How the head is filled

The head is built empty around a scaffolding protein, which is then broken down. A motor at a tiny pore packs the DNA in at up to 2,000 base pairs a second.

Sheath and tube are easy to mix up: the sheath is the outer tube that contracts, the tube is inside it and carries the DNA.

Questions and answers

How big is a T4 bacteriophage?

About 90 nanometres wide and 200 nanometres long, which makes it a relatively large virus.

What is the head of a bacteriophage called?

The capsid. On T4 it is icosahedral and holds a double-stranded DNA genome of about 169,000 base pairs.

What do the tail fibres of a bacteriophage do?

The long tail fibres recognise receptors on the surface of the bacterium, so they decide which bacteria the phage can infect. The short tail fibres then bind the cell for good.