DEOXYRIBOSE NUCLEIC ACID: AN INTRODUCTION
DNA is formed in the cells of all animals, plants, prokaryotes and in most viruses.
In eukaryotes, it is present in combination with proteins forming nucleoproteins. In
prokaryotes (e.g., E. coli), the genetic material consists of a single giant molecule of DNA
about 1000 microns in length, without any associated proteins. DNA is present mainly
in the chromosomes. It has also been reported in cytoplasmic organelles like mitochondria
and chloroplasts. The DNA of all plants and animals and many viruses (polyoma virus,
smallpox virus, bacteriophages T2, T4 and T6) is double stranded. In the bacteriophage
f X 174, however, it is single stranded. In some viruses, the genetic material is RNA as
in the tobacco mosaic virus (TMV). In tobacco virus, influenza virus, poliomyelitis virus
and the bacterial viruses F2 and R17, the RNA is single stranded. In the reovirus and
the wound tumour virus (in bacteria) and in higher plants and animals both DNA
and RNA are present. Viruses usually contain either DNA or RNA.