tail sheath

Meaning: In a T4-type bacteriophage, the tail sits below the head (capsid) and ends in a base plate with tail fibers. The tail sheath is the contractile protein layer wrapped around the hollow inner tube. Tail fibers bind specific receptors on the bacterial surface, and the base plate changes shape. The sheath then contracts, becoming shorter and wider, and pushes the tube through the cell wall and membrane. Viral DNA passes through the tube into the cytoplasm. The empty capsid and sheath stay outside as a "ghost." Some phages have tails that do not contract, so the sheath is a feature of contractile-tailed phages, not of every phage.

On the MCAT: Expect a labeled phage diagram. You match each part to its job: capsid holds the genome, tail fibers and base plate attach to the host, sheath and tube inject. The sheath belongs to the penetration step of the lytic cycle, which runs attachment, penetration, biosynthesis, assembly, then lysis. It also appears in Hershey-Chase reasoning. Protein labeled with ³⁵S stayed outside the cell, and DNA labeled with ³²P entered, which supported DNA as the genetic material.

Common confusion: Students call the sheath the capsid or say it carries the genome. The capsid holds the nucleic acid, and the sheath is a protein machine that moves the tube. The sheath also does not recognize the host. The tail fibers do the binding. The whole phage does not enter the bacterium.