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Nucleic Acid

Till recently it was stated that viruses contain either DNA or RNA, never both together. On this basis two classes of viruses were recognized, DNA and RNA viruses. Thus with respect to the nucleic acids present three types of viruses have been recognized, DNA viruses, RNA viruses and DNA-RNA viruses.

1. DNA viruses contain DNA as the genetic material. On replication this DNA produces new DNA. DNA transmits information for protein synthesis through RNA. (DNA→RNA→Protein).

2. RNA viruses contain RNA as the genetic material. The RNA replicates directly to produce new RNA. Information for protein synthesis passes from RNA to protein without involvement of DNA (RNA→RNA→Protein).

3. DNA-RNA viruses. In a group of RNA tumour viruses, called leukoviruses or rousviruses, the genetic material is alternately DNA and RNA. In addition to the normal mode of transfer found in DNA viruses (DNA→RNA→Protein) the rousviruses also transfer information from RNA to DNA: (RNA→DNA→RNA→Protein).

With respect to the number of strands, four types of nucleic acids have been found in viruses, double stranded DNA, single stranded DNA, double stranded RNA and single-stranded RNA.

1. Double stranded DNA has been found in pox viruses. The DNA is present in the form of a single large molecule. The DNA molecule may be cyclic (ring-like), as in SV-40 and papilloma viruses, or linear as in the coliphage lambda. In cyclic DNA the two ends are linked by covalent bonds. It has been suggested that all natural DNA linked by covalent bonds. It has been suggested that all natural DNA is cyclic, and the linear form is the result of breakage during extraction or analysis.

2. Single stranded DNA is found in the bacteriophages phiX/174 and M-13, and is cyclic.

3. Double stranded RNA has been found within the viral capsid in the reoviruses and animals, and in the wound tumor virus and rice dwarf virus of plants. It should however, be noted that single stranded RNA becomes temporarily double stranded during synthesis.

4. Single stranded RNA is found in most of the RNA viruses e.g. tobacco mosaic virus (TMV), influenza virus, poliomyelitis virus, the bacteriophage MS-2, the bacterial viruses F2 and R17 (coliphage), and the avian leukemia virus.

Plant viruses contain only RNA. Most bacteriophages contain only DNA. The RNA of several viruses is folded upon itself to form helices in some regions.

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