In 1953 Watson and Crick proposed the model of DNA. It was a revolutionary event in the history of genetics. In this article I am providing a detail structure of DNA model proposed by Watson and Crick with suitable diagrams.
Introduction
DNA stands for Deoxyribose nucleic acid and are considered as life origin molecules. DNA is the genetic material of all plants, animals, prokaryotic and many viruses. It is generally found on chromosomes inside the nucleus. DNA is considered as polymer of nucleotides.
Chemical composition
DNA, the genetic material consists of two polynucleotide chains arranged in a double helix. Each of the DNA chain is composed of many units called nucleotides each of which consisting of three smaller molecules:
Nitrogen bases (nitrogen containing compounds)
Deoxyribose sugar ( a five carbon sugar, C 5 H10O 4) and
Phosphoric acid (H3PO4)
Nitrogen bases
Two types of nitrogen bases are found in DNA.
Purine: It is two ringed heterocyclic nitrogenous base. E.g. Adenine (A) and Guanine (G)
Pyrimidines: it is single ringed nitrogenous base DNA contain two pyrimidines. E.g. Thymine (T) , Cytosine ( C) .
The structure of nucleotide is given in the following figure.
In DNA above four nitrogenous bases join with deoxyribose sugar to form nucleosides . When these nucleosides join with phosphoric acid it results in the formation of nucleotides.
Watson and crick Model of DNA
The Molecular Structure of DNA was proposed by Watson and crick. Watson and Crick (1953), based on X-ray diffraction method, proposed a double helical model of DNA to explain molecular structure of DNA for which they got Nobel prize in 1962. The main points of model are given below:
Each molecule of DNA consists of two helical polynucleotide chains or strands. These strands run anti parallel. i.e. their 3'-5' phosphodiester bonds are in opposite direction and 3' end of one strand lies besides the 5' end of the other.
In each polynucleotide chain the phosphate molecules lie on the outer side of deoxyribose and the nitrogenous base inward, perpendicular to the helical axis. X-ray data on DNA was obtained by Rosalind Franklin. It provides three –dimensional organization of macromolecules. X-ray photographs of DNA showed that it has helical structure and the width of helix was found to be 2nm.
The nitrogenous bases of two strands are linked together with the help of hydrogen bonds between oxygen and nitrogen atoms of the adjacent bases. Since there is fixed distance between the two strands (11 A0 or 10.8 A0), only specific base pairs can fit into the space. So the base pairing is very specific.
Punrine ( Adenine and guanine) , two ringed nitrogenous compounds pair with pyrimidine ( Cytosine and thymine)
Adenine pairs with thymine and guanine pairs with cytosine. There are two hydrogen bonds between A & T and three hydrogen bonds between C & G. Therefore, the two strands of the DNA molecules are complementary to each other.
In the DNA molecule, the two complementary chains are twisted around each other and form a double helix around a common central axis.
One turn of the helix measures about 34 A0 and contains 10 nucleotides. . Therefore, the distance between the adjacent nucleotides is 3.4 A0 or 0.34 nm.
Diameter of the helix is about 20 A0 . The double helix shows a major or wide groove and a minor or narrow groove. The narrow groove is the distance between the paired molecules while the wide groove is the space between successive turns when the pairs is wound into a helix.
only one of the two strands of DNA possess correct hereditary information. It is known as sense strand. Its complementary strand is called anti-sense strand.
Article by Hakimuddin Kuwakhedawala Hakimuddin Kuwakhedawala is based in India with over 15 years experience as a teacher. Teaching and writing are his passion. Most of the articles of Hakimuddin are related to education, exam, environmental problems, etc.
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