Double helical DNA - Structure and Definition.

ABOUT THE DOUBlE HELICAL DNA. STRUCTURE OF DNA.
MINOR AND MAJOR GROOVE.

1. DNA DOUBLE HELICAL STRUCTURE PRESENTED BY, JAMUNA S M.Sc BIOCHEMISTRY

2. INTRODUCTION : Chromosomes are the carrier of genetic material. Chromosome contain DNA, RNA, proteins. DNA is the genetic material in most of the organism and higher organism. In 1953 , Jameson Watson and Francis Crick proposed double helix DNA model on the basis of X-ray diffraction studies with photographs of DNA taken by Wilkins and Franklin. It is also known as Watson and Crick model DNA structure. Watson and Crick shared a noble prize in 1962.

3. DNA is a double stranded structure in which the two strands are coiled around each other forming a double helix. The DNA duplex is “coil of life”. There are two grooves in DNA namely, 1. Major groove, 2. Minor groove. The backbone of the helix is formed of sugar and phosphate molecule. The nitrogenous bases are attached to sugar molecule. The two nucleotide strands are held together by unstable hydrogen bonds.

4. DNA DOUBLE HELIX DIMENSION OF DNA

5. The two strand run antiparallely in opposite direction i.e., they run in opposite direction 5’ to 3’ end and 3’ to 5’ end. The strand are interwined in clockwise direction. The width of the DNA molecule is 20 Å . The strand completes a turn every 34 Å along its length. There are ten nucleotide per turn. The internucleotide distance is 3.4 Å. Watson and Crick model of DNA is called B-form DNA. The chain in B-form DNA are in right handed orientation.

6. STABILITY OF DOUBLE HELIX STRUCTURE : Internal and external hydrogen bonds Negative charge of phosphate groups Base pair stacking. MAJOR AND MINOR GROOVE :

7.  Each nucleotide is made up of pentose sugar, a phosphate group and a nitrogenous base. Deoxyribose is the constituent sugar in DNA. The nitrogenous bases are of two kinds, Purines Pyrimidines  Adenine and guanine are the purines and thymine and cytosine are pyrimidines . The nitrogenous bases found in DNA are adenine, guanine, cytosine and thymine.

8. There are two hydrogen bonds between adenine and thymine (A=T) and there are three hydrogen bonds between guanine and cytosine (G  C) pairing.  The sub-unit containing only sugar and nitrogenous base is known as nucleoside. A nucleoside combines with phosphate to form a nucleotide.  Nucleotides are building blocks of DNA.

9. CHARGAFF’S LAW : Erwin Chargaff in 1949 showed that (i) The bases pair in specific manner. Adenine always pairs with thymine and guanine pairs with cytosine. (ii) Total amount of purine nucleotides is always equal to the total amount of pyrimidine nucleotides i.e.[A] + [G] = [T] + [C].  (iii) The proportion of adenine is equal to thymine and so also of guanine is equal to cytosine. But the [A] + [T] need not necessarily be equal to [G] + [C]. These empirical rules regarding the composition of bases in DNA is collectively known as Chargaff’s law or Base pair rules .