watson and crick model of DNA(molecular biology)

dicovery of double helical structure of DNA by watson and crick and its features

1. WATSON AND CRICK MODEL OF DNA D.INDRAJA

2. DNA MODEL The three-dimensional structure of DNA, first proposed by James D. Watson and Francis H. C. Crick in 1953, consists of two long helical strands that are coiled around a common axis to form a double helix. Each DNA molecule is comprised of two biopolymer strands coiling around each other. Each strand has a 5′end (with a phosphate group) and a 3′end (with a hydroxyl group). The strands are antiparallel , meaning that one strand runs in a 5′to 3′direction, while the other strand runs in a 3′to 5′direction. The diameter of the double helix is 2nm and the double helical structure repeats at an interval of 3.4nm(34A) which corresponds to ten base pairs which is none other than helix measure

3. The distance between two succesive base pairs is 0.34nm (3.4A) The two strands are held together by hydrogen bonds and are complementary to each other. The two DNA strands are called polynucleotides , as they are made of simpler monomer units called nucleotides. Basically, the DNA is composed of deoxyribonucleotides . The deoxyribonucleotides are linked together by 3′- 5′phosphodiester bonds. The nitrogenous bases that compose the deoxyribonucleotides include adenine, cytosine, thymine, and guanine. The structure of DNA -DNA is a double helix structure because it looks like a twisted ladder.

4. The sides of the ladder are made of alternating sugar ( deoxyribose ) and phosphate molecules while the steps of the ladder are made up of a pair of nitrogen bases. As a result of the double helical nature of DNA, the molecule has two asymmetric grooves. One groove is smaller than the other . The larger groove is called the major groove, occurs when the backbones are far apart; while the smaller one is called the minor groove, and occurs when they are close together . A always pairs with T with 2 hydrogen bonds and G pairs with C with 3 hydrogen bonds hence two strands are complimentary to each other Because of specificity in base pairing the amount of purines is equal to the amount of pyramidines this is called as chargaffs rule of base equivalence.

5. Double helical structure of DNA

6. COMPONENTS OF DNA Nucleoside is a compound formed by the combination of a pentose sugar and nitrogen base Nucleotide is compound formed by the combination of nucleoside and phosphate group Nucleotides are the building blocks of nucleic acids Nucleotide have three charecteristic components Nitrogen base Deoxyribose sugar Phosphate

7. 6 amino purine 2 amino 6 oxy purine 2,4 di oxy pyramidine 2,4 di oxy 5 methyl pyramidine 2 oxy 4 amino pyramidine

8. DEOXYRIBOSE SUGAR Deoxyribose , also known as D- Deoxyribose and 2-deoxyribose, is a pentose sugar (monosaccharide containing five carbon atoms) that is a key component of the nucleic acid deoxyribonucleic acid (DNA). It is derived from the pentose sugar ribose. Deoxyribose has the chemical formula C 5 H 10 O 4 . Deoxyribose is the sugar component of DNA Alternating with phosphate bases, deoxyribose forms the backbone of the DNA, binding to the nitrogenous bases adenine, thymine, guanine, and cytosine.

9. The sugar-phosphate backbone forms the structural framework of nucleic acids, including DNA. This backbone is composed of alternating sugar and phosphate groups and defines directionality of the molecule. PHOSPHATE GROUP

10. DNA are composed of nucleotides that are linked to one another in a chain by chemical bonds, called ester bonds These bonds are called phosphodiester bonds In double-stranded DNA, the molecular double-helix shape is formed by two linear sugar-phosphate backbones that run opposite each other and twist together in a helical shape. The sugar-phosphate backbone is negatively charged and hydrophilic, which allows the DNA backbone to form bonds with water.

11. Secret behind the discovery At King's College in London, Rosalind Franklin and Maurice Wilkins were studying DNA. Wilkins and Franklin used X-ray diffraction as their main tool -- beaming X-rays through the molecule yielded a shadow picture of the molecule's structure, by how the X-rays bounced off its component parts. Franklin, a shy and inward young woman, suffered from patronizing attitudes and sexism that forced her to do much of her work alone. And her senior partner, Wilkins, showed some of Franklin's findings to Watson in January 1953 without her knowledge .

12.  Shortly after, Watson and Crick made a crucial advance when they proposed that the DNA molecule was made up of two chains of  nucleotides  paired in such a way to form a double helix, like a spiral staircase. This structure, announced in their famous paper in the April 1953 issue of Nature, explained how the DNA molecule could replicate itself during cell division, enabling organisms to reproduce themselves with amazing accuracy except for occasional mutations. For their work, Watson, Crick, and Wilkins received the Nobel Prize in 1962. Despite her contribution to the discovery of DNA's helical structure, Rosalind Franklin was not named a prize winner: She had died of cancer four years earlier, at the age of 37.