polynucleotide chains that are coiled around a common axis to form right handed B form of DNA. The coils are held together by hydrogen bonds which occur between complementary pairs of nitrogenous bases. The sugar is called ′ -deoxyribose because there is no hydroxyl at position ′ . Adenine and thiamine base pairs has two hydrogen bonds while guanine and cytosine base pairs have three hydrogen bonds.
As published by Erwin Chargaff in , a purine pairs with pyrimidine and vice versa . Adenine (A) always pairs with Thymine (T) by double bond and Guanine (G) always pairs with Cytosine (C) by triple bond. Figure . : Erwin Chargaff Figure .
: Rosalind franklin - - . . PM In 1950s, Maurice Wilkins and Rosalind Franklin of Kings College, London studied the X-ray crystallography and revealed experimental data on the structure of DNA . .
Features of DNA • If one strand runs in the ′ - ′ direction, the other runs in ′ - ′ direction and thus are antiparallel (they run in opposite direction). The ′ end has the phosphate group and ’end has the OH group. • The angle at which the two sugars protrude from the base pairs is about ° , for the narrow angle and ° for the wide angle. The narrow angle between the sugars generates a minor groove and the large angle on the other edge generates major groove.
• Each base is . nm apart and a complete turn of the helix comprises . nm or base pairs per turn in the predominant B form of DNA. • DNA helical structure has a diameter of A ° and a pitch of about A ° .
X-ray crystal study of DNA takes a stack of about bp to go completely around the helix ( ° ). • Thermodynamic stability of the helix and specificity of base pairing includes (i) the hydrogen bonds between the complementary bases of the double helix (ii) stacking interaction between bases tend to stack about each other perpendicular to