The structure of TMV can be described in terms of the number of subunits per turn of the helix i.e. 16.3. The pitch or rise per turn of the helix i.e. 2.28nm and the axial rise per subunit i.e. 0.14nm. It is possible that the nucleoprotein filament of HIV has a similar structure. Electron microscopy suggests that many viruses are roughly spherical. A detailed examination shows that they are actually icosahedral.
Icosahedral viruses are very common plant and animal viruses.
The HIV capsid layer is thought to have an icosahedral structure. An icosahedron has 20 equilateral triangles arranged around the face of a sphere. There are several reasons why viruses adopt icosahedral symmetry. One is that triangulating a dome into 20 is the best way of producing a shell of equivalently bonded identical structures. It is the minimum free energy structure.