What gives definite shape to a virion?

Explanation

The capsid is the protein shell that surrounds and protects the viral genome, and it plays a crucial role in giving the virion its definite shape. It is composed of repeating protein subunits called capsomeres, which self-assemble in a precise and symmetrical manner. This assembly follows geometric rules, ensuring that each virus particle has a consistent and stable structure. Depending on the arrangement of capsomeres, the capsid can form three main types of shapes: helical, icosahedral, or complex. In helical viruses, the capsid proteins arrange around the nucleic acid in a spiral, producing rod-like or filamentous virions, as seen in the tobacco mosaic virus and rabies virus. In icosahedral viruses, capsomeres form a spherical structure with 20 triangular faces, providing a highly stable and efficient design, as in adenovirus and poliovirus. Some viruses, such as bacteriophages and poxviruses, exhibit complex structures that combine helical and icosahedral features. Thus, by self-assembling into specific symmetries, the capsid provides the virion with its distinct shape, stability, and ability to interact with host cells.