1.7 - Viruses
- 1The structure of viruses
- 2How viruses replicate
Viruses
Viruses are microscopic infectious agents that can only replicate inside the cells of living organisms.
Viruses are considered acellular (not made up of cells), and although we sometimes refer to them as microorganisms, they are not living and do not count as 'organisms'.
There are many types of viruses, each of which can have quite different shapes and sizes. In general though viruses are much smaller and simpler than cells.
The diagram below shows the basic structure of a virus, using HIV as an example.
Always present:
- Genetic material - Viral genomes may be DNA or RNA and single or double-stranded.
- Capsid - A layer of protein molecules that surrounds and protects the genetic material.
Sometimes present:
- Envelope - An outer layer made up of phospholipids (only present in some viruses).
- Glycoproteins - Also known as surface, attachment, or envelope proteins, these help viruses bind to host cells.
- Enzymes - Some viruses carry enzymes, like reverse transcriptase, which allows them to convert RNA into DNA.
Viral replication
As viruses are not alive, they do not undergo cell division like living organisms do. Instead, they rely entirely on the host cell's machinery to make copies of themselves.
The viral replication process involves:
- The virus uses its attachment proteins to bind to specific receptor proteins on host cells.
- The virus injects its viral DNA or RNA into the host cell.
- The virus uses the host cell's machinery and resources, such as enzymes and ribosomes, to replicate viral particles.
- New viral particles are released from the host cell.