What is the primary function of a protein capsid?

The primary function of a protein capsid is to protect the viral genetic material. The capsid is a proteinaceous shell that encases and safeguards the viral genome, which can be composed of DNA or RNA. This protective structure is crucial for the stability of the virus, as it helps shield the genetic material from environmental factors that could lead to degradation or inactivation, such as enzymes within the host organism and external physical conditions. Additionally, the capsid plays a key role in the virus's ability to infect a host, as it facilitates attachment and entry into host cells, ensuring that the viral genetic material is delivered effectively to initiate the replication process.

In contrast, options suggesting that the capsid provides energy or functions as an antibiotic do not reflect the biological role of the capsid in viral structure and function. Furthermore, while the capsid does assist in the process of replication by delivering the viral genome to host cells, it does not directly replicate the virus; that function is carried out by the cellular machinery of the host once the virus has entered the cell. Thus, the protection of the viral genetic material is the fundamental role served by the protein capsid.