What is a capsid?

• A. cell wall of bacteria
• B. lipid envelope surrounding some viruses
• C. genetic material inside a host cell nucleus
• D. protein coat that protects a virus

Answer: (D)

A capsid is the protein coat that protects a virus's genetic material and helps deliver it into a host cell. Built from protein subunits, the capsid encases the viral genome and determines the virus’s shape, such as icosahedral or helical. This protective shell keeps the genome safe from damage and plays a key role in attaching to and entering a host cell during infection. Some viruses also have an outer lipid envelope around the capsid, derived from the host cell membrane; that envelope is separate from the capsid but can assist entry. The other descriptions point to parts of cells or locations inside a cell, not the viral particle itself.