These viruses usually occur in two structural forms:

Explanation

Bacteriophages, like other viruses, usually occur in two structural forms: cuboidal (icosahedral) and helical. In many bacteriophages, these two forms are combined into a characteristic head-and-tail structure. The head, which is icosahedral in shape, contains the viral DNA, while the tail is helical, functioning as a tube through which the nucleic acid is injected into the bacterial host. For example, the T-even phages (T2, T4, and T6) have a large icosahedral head and a long contractile helical tail, giving them the appearance of a tadpole. In contrast, some other phages like T3 and T7 are simpler and mostly icosahedral with only a very short, non-contractile tail. Thus, bacteriophages illustrate how the two fundamental viral structural types-cuboidal (icosahedral) and helical-can exist either separately or in combination, depending on the type of phage.