viruses

Meaning: A virus is an acellular particle: a DNA or RNA genome inside a protein capsid, sometimes with a lipid envelope from the host membrane. It has no ribosomes and little or no metabolism, so it is an obligate intracellular parasite that uses host machinery to make proteins and copy its genome. In the lytic cycle the virus takes over the cell, assembles new particles, and lyses it. In the lysogenic cycle the genome integrates as a prophage and is copied with host DNA until induction. Retroviruses such as HIV use reverse transcriptase to make DNA from an RNA genome.

On the MCAT: Passages typically ask you to compare lytic and lysogenic cycles, predict how a mutation in a receptor-binding protein changes host range, or read a plaque assay. Genome type also appears: a positive-sense RNA genome can act directly as mRNA, while a negative-sense genome must first be copied by a viral RNA polymerase.

Common confusion: Viruses are not prokaryotes; they are not cells at all. Because they lack their own ribosomes and cell wall, antibiotics that target bacterial structures typically do not work on them.