Introduction:
Viruses are infectious agents that replicate inside the cells of living organisms. They consist of a core of genetic material, either DNA or RNA, surrounded by a protein coat called the capsid. Some viruses also have additional structures like an envelope or tail sheath. In this response, we will discuss which of these viral structures are not completely coded by the virus's own nucleic acid.

Capsid:
The capsid is the protein coat that surrounds the viral genetic material. It is composed of repeating protein subunits called capsomeres, which are encoded by the viral genome. Therefore, the capsid is completely coded by the virus's own nucleic acid.

gp120:
Gp120 is a glycoprotein found on the surface of the human immunodeficiency virus (HIV). It plays a crucial role in the attachment of the virus to host cells. Gp120 is not directly coded by the virus's own nucleic acid. Instead, it is synthesized as a precursor protein called gp160, which undergoes post-translational modifications in the host cell to form gp120. Therefore, gp120 is not completely coded by the virus's own nucleic acid.

Viral envelope:
The viral envelope is a lipid membrane that surrounds some viruses. It is derived from the host cell's plasma membrane or from internal cellular membranes during the process of viral budding. The envelope contains viral glycoproteins, including gp120 in the case of HIV. While the viral genome provides the instructions for the synthesis of these glycoproteins, the viral envelope itself is not completely coded by the virus's own nucleic acid.

Tail sheath:
The tail sheath is a structure found in bacteriophages, which are viruses that infect bacteria. It is responsible for delivering the viral genetic material into the host bacterium. The tail sheath is composed of proteins encoded by the viral genome. Therefore, the tail sheath is completely coded by the virus's own nucleic acid.

Conclusion:
In summary, the viral structures that are not completely coded by the virus's own nucleic acid are gp120 and the viral envelope. While the viral genome provides the instructions for the synthesis of these structures, they require additional processing or components derived from the host cell. The capsid and tail sheath, on the other hand, are completely coded by the virus's own nucleic acid.

A capsid