Q.What is the shape of the T4 bacteriophage, and what is its genetic material?

Because viruses do not fit the schemes used for cellular organisms, the International Committee on Taxonomy of Viruses (ICTV) sets the standard rules for naming and classifying them, based on criteria such as the type of nucleic acid a virus carries (DNA or RNA), whether it has an outer envelope, its host range, and - most usefully - the symmetry of its protein coat, the geometric pattern formed by its repeating protein subunits (capsomeres). Helical symmetry arranges capsomeres in a spiral around the nucleic acid, producing a rod-shaped virus, as in TMV; icosahedral (polyhedral) symmetry arranges capsomeres into a near-spherical, many-faced particle, as in many animal viruses; and complex symmetry, seen in bacteriophages, combines both patterns within different parts of one virus particle.

Chemically, every virus (virion) consists of a nucleic acid core - DNA or RNA, never both - enclosed by a protein capsid built from capsomeres, with some viruses additionally carrying a lipid envelope. The Tobacco Mosaic Virus (TMV) is a rigid, rod-shaped particle whose genetic material is a single strand of RNA running along its central axis, surrounded by a helical arrangement of protein capsomeres forming the rod. T-even bacteriophages (T2, T4, T6, which infect Escherichia coli) show a far more elaborate, complex structure: a polyhedral (icosahedral) head enclosing double-stranded DNA, joined to a cylindrical tail made of a tail sheath, tail tube, base plate, and tail fibres, with the tail fibres recognising and attaching to specific sites on the host bacterium's surface.