Viruses are only visible with an electron microscope and do not have a proper cell structure; no cytoplasm, no organelles and no chromosomes.
Viruses can only reproduce inside a host cell - only non-living thing that can reproduce. Outside the cell they exist as inert virus particles called virons. They are highly specific (bacteriophagesm specific to bacteria)
When they invade the host cell they are able to take over the cells metabolic machinery and make new virus particles. Either the virus brings the enzymes it needs or its nucleic acid codes for them.
VIruses consist of:
- A nucleic acid
- A protein coat or caspid
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