TSR Wiki > Study Help > Subjects and Revision > Revision Notes > Biology > Virii
- Viruses are not truly alive.
- They are much smaller and less complex than cells.
- Most viruses multiply exclusively inside living host cells.
- Viruses cause disease by the sum of the effects of damage to the host cells they invade and the effects of toxins which are produced in the process.
- They are DNA or RNA in a protein coat (capsid).
- They have no nucleus, cytoplasm, or membranes.
- They do not carry out cellular functions.
- Viruses contain very few enzymes and so, as intracellular parasites, they uses the host's enzymes for their own metabolism.
- Viruses are unable to move and so rely on passive dispersal, or a vector, to move them between host cells.
- Viruses reproduce by binding to specific cells, penetrating the cell membrane and then using the protein making machinery (transcription and translation) of the cell to build fresh viruses.
- DNA viruses, e.g. smallpox, do not mutate and can be transcribed directly.
- RNA viruses, e.g. AIDS, mutate regularly - have to be reverse-transcribed back to DNA.
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