Final answer:
The shape of a virus is determined by its capsid. The capsid consists of protein subunits called capsomeres that form various shapes such as helical or icosahedral. Viral envelopes are taken from the host's membrane structures and may contain glycoproteins for attachment to host cells.
Step-by-step explanation:
The shape of a virus is primarily determined by its capsid, which is the protein coat that encapsulates the viral nucleic acid. While an envelope, if present, can influence the overall shape by adding an additional layer around the capsid, the fundamental geometric structure is attributed to the capsid itself. Capsids are made up of smaller protein subunits called capsomeres, which self-assemble into the characteristic shapes of the virus, such as helical or icosahedral forms.
Viral envelopes are derived from the host's membrane structures, which the virus acquires as it buds off from a host cell during the process of replication. This envelope may contain specialized viral glycoproteins that assist in the attachment and entry into host cells, which is crucial for the viral infection process.