Though DNA is the genetic material in most organisms,  procaryotes and eucaryotes, there are a few exceptions such as tobacco mosaic virus (TMV) , where RNA is the genetic material. Miescher in 1869 isolated  DNA from the fish sperm and called in nuclein. Later, he isolated a protein, protamine associated with nuclein. Since then continued interest in the area led to several new discoveries about its structure and ruction.
          Procaryotes (Escherichia coli and blue-green alga) have been subjective to intensive studies which show that the  bacterial chromosome is exclusively made of  DNA. It is made of two polynucleotide chains, twisted  together in a made of  DNA. It is made of two polynucleotide chains, twisted together in a double helix. Wilkins in England made some X-ray diffraction studies and concluded that DNA molecules from different sources have the same structure. However, the real breakthrough came in 1953 when Watson and Crick proposed a three –dimensional  double helical model of DNA with the following properties.

The watson  and Crick Model

2.    The polynucleotide chains consist of deoxyribose sugar, hence the name deoxyribotides which are joined by internucleotide linkages, called 3’ , 5’- phosphodiester bonds.

3.    The sugar phosphate backbone forms the outer edge of the helix and the bases project form it perpendicularly inwards.

4.    The polynucleotide chains are held together in a helical shape thorough noncovalent hydrogen bonds between the bases of two opposite chains. The two chains are complementary and run in opposite, that is if , one chains runs from 3’ and 5’ end, the other will run in the direction 5’ to 3’, establishing a polarity.

5.    A critical feature of the double helix in the base-pairing relationship; adenine pairs with thymine by two hydrogen bonds and guanine pairs with cytosine by three hydrogen bonds.

6.    Each turn of the helix (consists of 10pairs of nucleotides with an internucleotide space of 3.4 Å making each helix 34 Å long. The width of the double helix is 20 Å .

7.    The base sequence of one chains matches with the sequence of one chain is known, the sequence on the other can be easily predicted.

8.    The twisted in the helical configuration are formed in such a manner that a major and a minor groove are created.


Chargaff’s Rule

The sequence of bases in the DNA molecule is of  prime importance to the biological role it has to play. While studying the sequence analysis of DNA, Ervin Chargaff made an important contribution which demonstrated the relative amounts of bases in the molecule. He found that the ratios of the four bases from the DNA obtained from different sources in variable but that the base from the DNA obtained from different sources in variable but that the base composition of a particular species remains constant.  Further in quantitative terms, Chargaff concluded that the number of purines always equals the number of pyrimidines in given sample of DNA, that is, The number of adenine bases in equal to that of thymine bases and the number of cytosines equal the number of guanines. This has been named as Chargaff rule: