Discussion
T-even bacteriophages have :
Answer: A
T-even bacteriophages have binal symmetry.
A T-even phage such as T4 has two parts with different symmetry: an elongated icosahedral (polyhedral) head that holds the DNA, and a tail whose contractile sheath is built helically, ending in a base plate with tail fibres. A virion combining cubic and helical symmetry in this way is said to have binal symmetry, the term used for these phages in the standard classification of virus architecture.
Helical symmetry alone describes viruses such as tobacco mosaic virus, and polyhedral (icosahedral) alone describes viruses such as adenovirus. Complex symmetry is the term kept for viruses like the poxviruses whose structure fits neither pattern; some books also apply it loosely to phages, but the specific term for the T-even head-and-tail plan is binal.
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