What a virus is
A virus is an acellular particle: it has no cytoplasm, no ribosomes and no metabolism of its own. A virion (the complete particle) contains a genome of either DNA or RNA, surrounded by a protein coat called the capsid. Some viruses also wear an envelope, a lipid layer stolen from the host cell membrane, with viral proteins embedded in it. Because viruses cannot make proteins or energy without a host, they are obligate intracellular parasites. Viruses that infect bacteria are bacteriophages.
Host range
A virus attaches only if a protein on its surface fits a receptor on the host cell. This match largely sets which species and which tissues it can infect, a property called tropism.
Steps of replication
Most viruses follow the same five steps:
- Attachment to a receptor.
- Penetration or entry, so the genome gets inside.
- Biosynthesis: the host machinery copies the genome and makes viral proteins.
- Maturation: parts assemble into new virions.
- Release, by bursting the cell or by budding out through the membrane (taking an envelope with them).
Two life-cycle patterns
In the lytic cycle the virus replicates at once and the cell lyses, releasing progeny. In the lysogenic cycle a phage genome inserts into the host chromosome as a prophage and is copied every time the cell divides, staying silent until a trigger switches it to the lytic path.
Retroviruses have RNA genomes and carry reverse transcriptase, an enzyme that makes DNA from RNA. That DNA can integrate into the host genome.
Not quite viruses
Prions are infectious misfolded proteins with no nucleic acid at all.
Worked example
One phage infects a bacterium. The burst size, the number of new phages released per lysed cell, is 50. Assume every progeny phage then infects a fresh cell, and every infection is lytic.
- After the first cycle: 50 phages.
- After the second cycle: 50 times 50, which is 2,500 phages.
- After the third cycle: 50 times 2,500, which is 125,000 phages.
So the count after n cycles is 50 to the power n. This shows why a tiny initial infection can wipe out a culture in a short time. In real cultures not every phage finds a host, so the true numbers are lower, and a lysogenic infection would produce no burst at all.