Term was given by Zacharis, which is found in the cells of all living organisms except plant viruses,where RNA forms the genetic material and DNA is absent. In bacteriophages and viruses there is a single molecule of DNA, which remains coiled and is enclosed in the protein coat. In bacteria, mitochondria, plastids and other prokaryotes, DNA is circular and lies naked in the cytoplasm but in eukaryotes it is found in nucleus and known as carrier of genetic information and capable of self replication. Isolation and purification of specific DNA segment from a living organism achieved by Nirenberg H.Harries is associated with DNA-RNA hybridization technique.

The chemical analysis has shown that DNA is composed of three different types of compound.
(i) Sugar molecule : Represented by a pentose sugar the deoxyribose or 2-deoxyribose which derived from ribose due to the deletion of oxygen from the second carbon.
(ii) Phosphoric acid : H3PO4 that makes DNA acidic in nature.
(iii) Nitrogeneous base : These are nitrogen containing ring compound. Which classified into two groups:

(a) Purines : Two ring compound namely as Adenine and Guanine.
(b) Pyramidine : One ring compound included Cytosine and Thymine in RNA uracil is present instead of Thymine.

# Nucleosides : Nucleosides are formed by a purine or pyrimidine nitrogenous base and pentose sugar. DNA nucleosides are known as deoxyribosenucleosides.
# Nucleotides : In a nucleotide, purine or pyrimidine nitrogenous base is joined by deoxyribose pentose sugar (D), which is further linked with phosphate (P) group to form nucleotides.

In 1953 Watson and Crick suggested that in a DNA molecule there are two such polynucleotide chains arranged antiparallal or is opposite directions i.e. one polynucleotide chain runs in 5’-->3’ direction, the other in 3’--> 5’ direction. It means the 3’ end of one chain lies beside the 5’ end of other in right handed manner.
(i) Important features of Watson and Crick double helical model of DNA
There are important features of DNA double helix.
(a) The double helix comprises of two polynucleotide chains.
(b) The two strands (polynucleotide chains) of double helix are anti-parallel due to phosphodiester bond.
(c) Each polynucleotide chain has a sugar-phosphate ‘backbone’ with nitrogeneous bases directed inside the helix.
(d) The nitrogenous bases of two antiparallel polynucleotide strands are linked through hydrogen bonds. There are two hydrogen bonds between A and T, and three between G and C. The hydrogen bonds are the only attractive forces between the two polynucleotides of double helix. These serve to hold the structure together.
(e) The two polynucleotides in a double helix are complementary. The sequence of nitrogenous bases in one determines the sequence of the nitrogenous bases in the other. Complementary base pairing is of fundamental importance in molecular genetics.

Forms of DNA : Five different morphological forms of DNA double helix have been described. These are A, B, C, D and Z forms. Most of these forms (except B, and Z) occur in rigidly controlled experimental conditions. Watson and crick model represents commonest form, Biotic-form (B-form or B-DNA) of DNA. Some DNA forms are inter convertible also. The differences in these DNA forms are associated with:
(i) The numbers of base pairs, present in each turn of DNA helix.
(ii) The pitch or angle between each base pair.
(iii) The helical diameter of DNA molecule.
(iv) The handedness of double helix. Which is mentioned in table.

Characteristics of DNA
(i) Denaturation or melting : The phenomenon of separating of two strand of DNA molecule by breaking of hydrogen bond at the temp. 900C.
(ii) Renaturation or annealing : Separated strands reunite to form double helix molecule of DNA by cooling at the room temp. i.e. 250C.
These properties help to form hybrid from different DNA or with RNA.

RNA is found in the cytoplasm and nucleolus.Inside the cytoplasm it occurs freely as well as in the ribosomes. RNA can also be detected from mitochondria, chloroplasts and associated with the eukaryotic chromosomes. In some plant viruses RNA acts as hereditary material.

More commonly RNA is a single stranded structure consisting of an unbranched polynucleotide chain, but it is often folded back on itself forming helices. DNA is a double stranded structure and its two polynucleotide chains are bounded spirally around a main axis. It is made up by :
(i) Sugar : Ribose
(ii) Phosphate : In the form of H3PO4.
(iii) Nitrogenous base : Two types: (a) Purine, (b) Pyramidine

(a) Purine is further divided into Adenine and Guanine.
(b) Pyramidin divided into Cytosine and Uracil.

Types of RNA : RNA can be classified into two types.
(i) Genetic RNA : Which established by Conrat. In most of the plant viruses, some animal viruses and in many bacteriophages DNA is not found and RNA acts as hereditary material. This RNA may be single stranded or double stranded.
(ii) Nongenetic RNA : In the all other organisms where DNA is the hereditary material, different types of RNA are nongenetic. The nongenetic RNA is synthesized from DNA template. In general, three types of RNAs have been distinguished:
(a) Messenger RNA or nuclear RNA (mRNA)
(b) Ribosomal RNA (rRNA)
(c) Transfer RNA (tRNA)

# (a) Messenger RNA or Nuclear RNA : mRNA is a polymer of ribo-nucleotide as a complementary strand to DNA and carries genetic information in cytoplasm for the synthesis of proteins. For this reason only, it was named messenger RNA (mRNA) by Jacob and Monod is 5% of total RNA. It acts as a template for protein synthesis and has a short life span.
(b) Ribosomal RNA : rRNA constitutes redundant nature upto 80% of total RNA of the cell. It occurs in ribosomes, which are nucleoprotein molecules.
Types of rRNA : Inside the ribosomes of eukaryotic cells rRNA occurs in the form of the particles of three different dimensions. These are designated 28S, 18S, and 5S. The 28S and 5S molecules occur in large subunit (60S subunit) of ribosome, whereas 18S molecules is present in the small subunit (40S subunit) of ribosome. In prokaryotic cells there are only 23S and 16S rRNA are found. Which are synthesized in Nucleolus / SAT region.