Viruses exist in two different states, the extracellular infectious particle or virion and the intracellular state consisting of viral nucleic acid. The capsid may be a polyhedron or a helix, or a combination of both (in some phages). Viruses are infective micro¬organisms that show several differences from typical microbial cells.

1. Size. The size range of viruses is from about 20 to 300 nm. On the whole, viruses are much smaller than bacteria. Most animal viruses and all plant viruses and phages are invisible under the light microscope.
2. Simple structure. Viruses have very simple structures. The simplest viruses are nucleoprotein particles consisting of genetic material (DNA or RNA) surrounded by a protein capsid. In this respect they differ from typical cells which arc made up) of proteins, carbohydrates, lipids and nuc1eicacids.

The more complex viruses contain lipids and carbohydrates in addition to proteins and nucleic acids, e. g. the enveloped viruses

3. 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.

4. 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 metabo¬lically inactive outside the host cell because they do not posses enzyme systems and protein synthesis machinery.

Viral nucleic acid replicates by utilizing the protein synthesis machinery of the host. It codes for the synthesis of a limited number of viral proteins, including the subunits or capsomeres of the capsid, the tail protein and some enzymes concerned Viruses have only one nucleic acid, either DNA or RNA. Typical cells have both DNA and RNA. Genomes of certain with the synthesis or the release of virions.
5. Nucleic acids. RNA viruses can be transcribed into complementary DNA strands in the infected host cells, e. g. Rous Sarcoma Virus (RSV). Such RNA viruses are therefore also 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 its genetic material (RNA or DNA) is capable of reproduction and that too only in a host cell.

It will thus be seen that viruses do not show all the characteris¬tics of typical living organisms. They, however, possess two funda¬mental characteristics of living systems. Firstly, they contain nucleic acid as their genetic material.

The nucleic acid contains instructions for the structure and function of the virus. Secondly, they can reproduce themselves, even if only by using the host cells synthesis machinery.
Viral genomes
The nucleic acid comprising the genome may be single-stranded or double-stranded, & in a linear, circular or segmented configuration. Single-stranded virus genomes may be:
• positive (+)sense, i.e. of the same polarity (nucleotide sequence) as mRNA
• negative (-)sense
• Ambisense - a mixture of the two.
N/B. Virus genomes range in size from approximately 3,200 nucleotides (nt) to approximately 1.2 million base pairs
Unlike the genomes of all cells, which are composed of DNA, virus genomes may contain their genetic information encoded in either DNA or RNA. Since viruses are obligate intracellular parasites only able to replicate inside the appropriate host cells, the genome must contain information encoded in a form which can be recognized & decoded by the particular type of cell parasitized. Thus, the genetic code employed by the virus must match or at least be recognized by the host organism. Similarly, the control signals which direct the expression of virus genes must be appropriate to the host.
Many of the DNA viruses of eukaryotes closely resemble their host cells in terms of the biology of their genomes: Some DNA virus genomes are complexed with cellular histones to form a chromatin-like structure inside the virus particle