A complete, fully assembled virus particle is called a
Answer: A
A virion is the complete infectious particle: the nucleic acid together with its protein coat and, in some families, an envelope. A protomer is a single coat protein subunit and a capsomere a cluster of them, so both are parts of the particle rather than the whole. A viroid is a separate class of agent altogether.
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Viruses are described as obligate intracellular parasites because they
Answer: B
The defining constraint is metabolic, not physical: a virus carries no machinery for generating energy or synthesising proteins, so it can only replicate inside a cell that supplies both. Size and visibility are consequences of its simplicity, and many viruses leave the host cell alive.
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The nucleic acid of a virus consists of
Answer: C
A virion carries one kind of genome, either DNA or RNA, and this is one of the clearest differences from a cell, which always contains both. Whichever it carries may be single or double stranded, which is what the classification into genome classes records.
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The protein coat that surrounds a viral genome is the
Answer: D
The capsid is the virus-coded protein shell enclosing the nucleic acid, and the two together are called the nucleocapsid. The envelope is a lipid layer outside that, present only in some families, and peplomers are the glycoprotein spikes projecting from it.
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An oligomeric cluster of coat protein subunits, visible as a structural unit of an icosahedral capsid, is a
Answer: A
Individual subunits are protomers, and they gather into capsomeres, which are what electron microscopy resolves on the capsid surface. Counting capsomeres was historically one way of characterising a virus. The nucleocapsid is the capsid with the genome inside it.
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Which symmetry describes a capsid whose identical subunits self-assemble into a helical array around the nucleic acid?
Answer: B
In helical symmetry the protomers wind around the genome in a spiral, so the length of the particle is set by the length of the nucleic acid rather than by the coat. Icosahedral capsids instead form a closed shell of fixed size, and complex describes particles that are neither.
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An icosahedral capsid has rotational symmetry described as
Answer: C
The icosahedron has axes of fivefold, threefold and twofold rotation, written 532, which is what allows a closed shell to be built from many copies of one protein. That economy matters because a small genome cannot encode a large, unique container.
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The lipid envelope of an enveloped virus is derived from
Answer: D
The envelope is taken from the plasma membrane or another host membrane as the particle leaves, and the virus contributes only the glycoproteins embedded in it. Because the lipid is host derived, an enveloped virus is readily inactivated by solvents and detergents.
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Compared with a naked virus, an enveloped virus is generally
Answer: A
The envelope is a lipid bilayer, so anything that disrupts lipid destroys the particle's ability to attach and fuse. Naked viruses have only protein on the outside and survive drying, acids and detergents far better, which is reflected in how each group is transmitted.
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Glycoprotein projections on the surface of an enveloped virion are called
Answer: B
Spikes, also called peplomers, are virus-coded glycoproteins embedded in the host-derived envelope, and they carry the attachment function. Capsomeres and protomers are parts of the protein capsid, and fimbriae are bacterial structures.
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