A virion is a nucleic acid genome packaged inside a protein shell called the capsid. The capsid is not a passive container: it protects the genome from nucleases and from physical damage outside the cell, and in many viruses it also participates in recognizing the target cell and in releasing the genome once inside. Capsids are built from repeating protein subunits, which is why a small number of genes can produce a large, symmetrical shell.
Many viruses add a second layer: a lipid envelope borrowed from a host cell membrane, studded with viral glycoproteins. These glycoproteins are the attachment and fusion machinery — the parts that bind a receptor and later merge the viral membrane with a cellular membrane. Because the envelope is a lipid bilayer, enveloped viruses are sensitive to detergents, drying, and heat in ways that non-enveloped viruses are not.
Some virions also carry enzymes inside the particle. The most important is a polymerase, the enzyme that copies nucleic acid. Whether a virus needs to package one depends entirely on its genome type, which is the subject of the next page.
A useful way to hold this together: the capsid and envelope are the delivery system; the genome is the cargo; any packaged enzymes are tools the virus cannot make after entry because it has no ribosomes of its own.