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.For these reasons, many biologists do not consider viruses to be living organisms. They show only some characteristics of life: they contain genetic material, they reproduce (inside hosts) and they evolve, but they do not respire, grow, excrete or respond on their own.Every type of organism can be infected by viruses. Viruses that infect bacteria are called bacteriophages (phages); others infect plants (e.g. tobacco mosaic virus) or animals (e.g. influenza virus, HIV). Most viruses are specific to one host or cell type, because their surface proteins must bind to particular receptors on the host cell.Viruses are classified mainly by the type of nucleic acid they contain, which is covered in the following lessons.
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.For these reasons, many biologists do not consider viruses to be living organisms. They show only some characteristics of life: they contain genetic material, they reproduce (inside hosts) and they evolve, but they do not respire, grow, excrete or respond on their own.Every type of organism can be infected by viruses. Viruses that infect bacteria are called bacteriophages (phages); others infect plants (e.g. tobacco mosaic virus) or animals (e.g. influenza virus, HIV). Most viruses are specific to one host or cell type, because their surface proteins must bind to particular receptors on the host cell.Viruses are classified mainly by the type of nucleic acid they contain, which is covered in the following lessons.