Capsid

A capsid is the protein shell of a virus that encloses its genetic material (DNA or RNA). It is built from protein subunits called capsomeres and serves to protect the viral genome and aid in its delivery into host cells.

• All viruses have a capsid surrounding their nucleic acid. Some viruses are "enveloped" (they have a lipid membrane outside the capsid), while others are "naked" (no envelope, just the capsid).
• The capsid's shape can vary: often helical or icosahedral (spherical) in structure. The arrangement of capsomeres defines these shapes. For example, tobacco mosaic virus has a helical capsid, while poliovirus has an icosahedral capsid.
• Capsid vs. capsule: Don't confuse them. Capsid refers to a virus's protein coat. A capsule, by contrast, usually refers to a polysaccharide outer layer on some bacteria (or sometimes to a virus's envelope).

• If a question describes the "protein coat" of a virus or mentions capsomeres, it's referring to the capsid. For example, "a viral structure composed of repeating protein subunits" is the capsid.
• A common exam point is distinguishing enveloped vs. non-enveloped viruses. Remember: the capsid is present in both, but enveloped viruses have an extra lipid membrane around the capsid.
• Be prepared for terminology pitfalls. For instance, a question might use the term "nucleocapsid," which means the capsid + the enclosed genome. Recognize that's still essentially the viral genome inside its capsid.