VIRUSES

Structure

Essentially, viruses consist only of a core of genetic material (DNA or RNA), surrounded by a protein coat (capsid). Sometimes the coat is enclosed in an 'envelope' originating as a piece of the plasma membrane from the host cell. Viruses reproduce only within living cells of hosts such as animals, plants and bacteria. A virus particle is not a cell; there is no cytoplasm or nucleus and no metabolic activity. Size varies widely, but 0.01-0.1 µm is the usual range: they can be seen only using the electron microscope . No further structural detail is required.

Multiplication

The following diagram shows, at an appropriate level for GCSE, multiplication of the influenza virus. It should be compared with the illustrated, textbook accounts of the structure and infective cycle which usually deal with the rather specialised case of bacteriophage, a type of virus that uses bacteria as host.