In virology, what is the outer protein coat that surrounds the nucleic acid of a virus called?

Introduction / Context:
Viruses are simple infectious agents composed of genetic material surrounded by protective structures. To understand how viruses infect cells and how they can be targeted by treatments or vaccines, it is important to know the basic parts of a virus particle. One key structure is the outer protein coat that directly surrounds the viral nucleic acid. This question asks you to recall the correct term for that protein shell and to distinguish it from related terms in virology.

Given Data / Assumptions:

• The question focuses on the name of the outer protein coat around viral DNA or RNA.
• The options include capsid, virion, envelope, and none of the above.
• We assume standard virology terminology from textbooks.
• We recognise that some viruses also have lipid envelopes, while others do not.

Concept / Approach:
The capsid is the protein shell that encloses and protects the viral genome. It is made up of repeating protein subunits called capsomeres and often has a characteristic shape, such as helical or icosahedral. A virion is the complete, fully assembled virus particle outside a host cell, including the capsid and, if present, an envelope. The envelope is a lipid membrane surrounding the capsid in some viruses, derived from the host cell membrane and containing viral glycoproteins. The correct approach is to identify the capsid as the outer protein coat around the nucleic acid itself, regardless of whether an additional envelope is present.

Step-by-Step Solution:
Step 1: Recall that the core of a virus particle contains nucleic acid, which may be DNA or RNA. Step 2: Remember that this nucleic acid is directly surrounded by a protective protein shell called the capsid. Step 3: Recognise that the capsid is built from smaller protein subunits, capsomeres, which assemble into a specific geometric arrangement. Step 4: Understand that a virion refers to the entire virus particle, including the capsid and, in some viruses, an outer lipid envelope, so it is not just the protein coat. Step 5: Note that the envelope, when present, is composed mainly of lipids and embedded proteins and surrounds the capsid, not the nucleic acid directly. Step 6: Conclude that the correct term for the outer protein coat around the viral genome is capsid.

Verification / Alternative check:
Virology diagrams and electron micrographs often label virus structure using three key terms: genome, capsid, and envelope. The genome is shown at the centre, the capsid surrounds it as a protein shell, and the envelope, if present, appears as an outer membrane. Teaching descriptions repeatedly define capsid as the protein coat that protects the nucleic acid and enables the virus to attach to and enter host cells. In contrast, the term virion is used whenever authors want to describe the complete infectious particle. This consistent use confirms that capsid is the correct term for the outer protein coat.

Why Other Options Are Wrong:
Virion refers to the complete virus particle, including nucleic acid, capsid, and sometimes an envelope. It is not the name of the protein coat alone, so this option is incorrect for the structure specifically asked about.
Envelope describes the lipid membrane that surrounds the capsid in enveloped viruses. It is not present in all viruses and it is not purely a protein coat around the nucleic acid, making this option wrong in this context.
None of the above is incorrect because capsid is a well established term that precisely fits the definition of an outer protein coat around the viral genome, so we cannot claim that none of the options are right.

Common Pitfalls:
Students sometimes confuse capsid and envelope because both are outer layers of the virus, or confuse capsid with virion because they see both terms together in descriptions. Another pitfall is to think of virion as a structural part rather than the whole particle. To avoid these errors, remember that the capsid is the protein coat around the genome, the envelope is a lipid layer that may surround the capsid, and the virion is the entire assembled virus particle.

Final Answer:
In virology, the outer protein coat that surrounds the nucleic acid of a virus is called the capsid.