(a)  Icosahedral symmetry

• •  Icosahedral is regular polyhedran with 20 equivalent triangular faces 12 vertices.

•  In the icosahedral structure, the individual polypeptide molecules form a geometrical structure that surrounds the nucleic acid

For example :Adenovirus has icosahedral structure (Fig. 2)

Fig. 12. Structure of Adenovirus

(b)  Helical symmetry

• •  In viruses with helical symmetry, the polypeptide units are arranged as a helix and form a rod like structure surrounding the nucleic acid genome

For example : TMV (Tobacco Mosaic Virus) has helical symmetry (Fig. 3)

Fig. 13. Structure of Tobacco Mosaic Virus