BRAINCAKEViruses
Viruses are tiny infectious particles, usually between about 20 nm and 300 nm across, much smaller than bacteria. They can only be seen with an electron microscope.
Viruses are acellular: they are not made of cells. A virus particle (a virion) is simply a core of nucleic acid, either DNA or RNA but never both, surrounded by a protein coat. Viruses have no cytoplasm, no ribosomes and no enzymes for respiration, so they carry out no metabolism of their own.
Because of this, viruses are obligate intracellular parasites. They can only reproduce inside a living host cell, where they use the host's ribosomes, enzymes, nucleotides, amino acids and ATP to make new virus particles. Outside a cell a virus is inert and some, such as tobacco mosaic virus, can even be crystallised.
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More in Prokaryotic Cells
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All 31 lessons in Prokaryotic Cells · All Edexcel AS-level Biology topics