Viruses are infective micro-organisms that show several differences from typical bacterial cells.
1. Size. On the whole viruses are much smaller than bacteria. Most animal viruses and all plant viruses are invisible under the light microscope. Some of the smaller viruses are only 200A in diameter.
2. No independent metabolism. viruses cannot multiply outside a living cell. No virus has been cultivated in a cell-free medium. Viruses do not have an independent metabolism. they are metabolically inactive outside the host cell because they do not possess enzyme systems and protein synthesis machinery. thus viruses are obligatory intracellular parasites. Because of this fact viruses have been considered to be non-living by some.
3. Simple structure. Viruses have a very simple structure. They consist of a nucleic acid core surrounded by a protein coat. In this respect they differ from typical cells which are made up of proteins, carbohydrates, lipids and nucleic acids. Myxoviruses have a membranous envelope consisting of protein, lipid and carbohydrate outside the usual protein coat, but this envelope is derived from the host cell. Thus essentially viruses are lager nucleoprotein particles.
4. Absence of cellular structure. Viruses do not have any cytoplasm, and thus cytoplasmic organelles like mitochondria, golgi complexes, lysosomes, ribosomes, etc are absent. They do not have any limiting cell membrane. They utilize the ribosomes of the host cell for protein synthesis during reproduction.
5. Nucleic acids. Viruses usually have only one nucleic acid, either DNA or RNA. Typical cells have both DNA and RNA. Recently it has been found that certain cancer causing animal viruses e.g. Rous Sarcoma Virus (RSV) have both RNA and DNA and are therefore called RNA-DNA viruses.
6. Crystallization. Many of the smaller viruses can be crystallized and thus behave like chemicals.
7. No growth and division. Viruses do not have the power of growth and division. A ;fully formed virus does not increase in size by addition of new molecules. The virus itself cannot divide. Only in genetic material (RNA or DNA) is capable of reproduction and that too only in a host cell.
Viruses possess two fundamental characteristics of living systems. firstly they contain nucleic acid as their genetic material. The nucleic acid contains all the instructions for the structure and function of the virus secondly they can reproduce themselves even if only by using the host cell's synthesis machinery.