Structure and Composition of Viruses

Viruses are smaller and simpler in construction than unicellular microorganisms, and they contain only one type of nucleic acid—either DNA or RNA—never both. As viruses have no ribosomes, mitochondria, or other organelles, they are completely dependent on their cellular hosts for energy production and protein synthesis. They replicate only within cells of the host that they infect. Unlike any microorganism, many viruses can, in suitable cells, reproduce themselves from their genome, a single nucleic acid molecule, that is, their nucleic acid alone is infectious. Outside a susceptible cell, the virus particle like a bacterial spore is metabolically inert; on the other hand, when replicating in a cell, it exhibits all the characteristics of life. The new group of microorganisms are known as the filterable viruses. The filtration studies has shown that virus particles (virions) range from about the size of the smallest unicellular microorganisms (300 nm) down to objects little bigger than the largest protein molecules (20 nm). In the simpler viruses, the virion consists of a single molecule of nucleic acid surrounded by a protein coat, the capsid; the capsid and its enclosed nucleic acid together constitute the nucleocapsid.