MDCAT notes

A virus is a piece of genetic material wrapped in protein. The genetic material is the genome, either DNA or RNA, and the protein coat around it is the capsid. The capsid is built from repeated protein subunits called capsomeres, which are proteins and nothing else. Some viruses add a layer outside that coat, the envelope, and the whole assembled particle in its complete, mature, infectious form is the virion, the phrase the papers also use as "complete virus". Four words, four levels of description, and almost every structure question in this chapter turns on picking the right one.

Viral Structure and Classification is the largest topic in this chapter at 106 distinct questions, 2006 to 2026, all six boards. The phrasing of the correct answer is itself a clue to the syllabus: "capsid" and "virion" are both recurring correct options, three and two times respectively, so the examiner wants the level of the term named, not described.

"Capsomere" was chosen as a wrong answer 3 times and "Capsid" 4 times, both inside this topic, and the two collapse for most students because the words look like the same word. What separates them is scale: the capsid is the whole coat, a capsomere is one protein subunit of that coat, and the virion is the entire particle including coat, genome and, where present, envelope. Asked for the complete mature infectious particle against capsomere, genome and capsid, three of the four options are parts of it, which is the standard construction.