In helical viruses, the protomeres do not get grouped in capsomeres, it is bound for the formation of a ribbon-like structure folding into a helix as the protomers are thicker at one end compared to the other. The helical capsid’s diameter is found by the traits of its protomeres. The length is estimated by the length of the nucleic acid it encloses. For instance, a helical symmetry possessing virus is the one causing tobacco mosaic – in them, the nucleocapsid is wrapped as though a tight coil. Another example of this symmetry would be the rabies virus.
