A typical virus with helical symmetry is the tobacco mosaic virus (TMV) illustrated in . it is an RNA virus in which the 2130 identical capsomeres are arranged in a helix. the overall dimensions of the TMV virion are 18X300 nm. the lengths of helical viruses are determined by the length of the nucleic acid, but the width of the helical virion is determined by the size and packaging of the capsomeres