Virus structure and life cycle

A virus is a tiny infectious particle consisting of a nucleic acid genome — DNA or RNA, single- or double-stranded — enclosed in a protein shell called a capsid. Some viruses also carry a lipid envelope stolen from host cell membranes, studded with viral glycoproteins. Because viruses lack ribosomes and metabolic machinery, they are obligate intracellular parasites: they can replicate only by hijacking a host cell.

The life cycle proceeds in ordered stages. Attachment comes first: viral surface proteins bind specific receptors on the host cell, which explains why each virus infects only certain species and tissues. Entry (penetration) follows, by membrane fusion, receptor-mediated endocytosis, or direct genome injection in bacteriophages. Uncoating then frees the genome from the capsid inside the cell.

Next comes replication and expression: the host's machinery — supplemented by viral enzymes when needed — copies the viral genome and translates viral mRNA into structural and catalytic proteins. The route to mRNA depends on genome type; retroviruses, for instance, must first reverse-transcribe RNA into DNA. In assembly, new genomes and capsid proteins self-organize into progeny virions. Finally, release occurs either by lysis, which destroys the host cell, or by budding through the membrane, which yields enveloped virions and can leave the cell alive. Bacteriophages add the lysogenic alternative, integrating their DNA into the host chromosome and lying dormant before entering the lytic cycle.

The MCAT tests virus structure and the life cycle within its biology foundations: expect questions on capsid and envelope structure, receptor-based specificity, the replication strategies of different genome types, and the lytic-versus-lysogenic distinction.