Arthur Kornberg in 1955 discovered the enzyme DNA polymerase from E. Coli for the synthesis of nucleotides as shown in Figure 5.5. DNA polymerase is 109 Kdal single polypeptide chain. It catalyses the step-by-step addition of polynucleotide units to a DNA chain.   (DNA)n residues + dNTP  (DNA)n+1 + PPi DNA polymerase requires the following components to synthesis DNA…

Denaturation of DNA helix also occurs at acid and alkaline pH values, at which, changes of the substituents on the purine and pyrimidine bases can occur. In acid solutions near pH 2 to 3, at which amino groups bind protons, the DNA helix is disrupted. Similarly, in alkaline solutions near pH 12, the enolic hydroxyl…

The DNA double helix, although stabilised by hydrogen bonding, can be denatured by heat. This occurs at characteristic temperature called denaturation temperature or melting temperature Tm. Tm is defined as the temperature at which the helical structure of DNA is lost. Since G—C base pairs are more stable than A-T base pairs, the Tm is greater for DNAs with…

DENATURATION The two strands of DNA helix are held together by hydrogen bonds (change in pH or increase in temperature) results in the separation of polynucleotide strands. This phenomenon of loss of helical structure of DNA is known as denaturation. The phosphodiester bonds are not broken by denaturation. Loss of helical structure can be measured…

Z-DNA is longer and thinner than B-DNA. It is a left-handed helix. A single turn of Z-DNA has twelve base pairs, and the pitch of the helix is 4.56 nm. The diameter of the double helix is 1.84 nm. The major groove of Z-DNA is not a groove but it simply forms a convex surface.…

H-DNA is found in polypyrimidine or polypurine segments that contain a minor repeat. An example is a long stretch of alternating T and C residues. The main feature of H-DNA is the pairing and interwinding of three strands of DNA to form a triple helix. H-DNA in triple helix contains pyrimidines in two strands and…

D-DNA is rarely present and has only eight base pairs per helical turn. This form is present in all DNA molecules irrespective of their base sequence. The axial rise per base pair is 3.03 Å with a tilt of 16.7° from the axis of the helix.

C-DNA is produced at 66% relative humidity and in the presence of Li+ ions. This form of DNA is known as the right-handed helix with an axial rise of 3.32 Å per base pair. Per turn of the helix contains 9.33 base pairs and the pitch of the helix is, therefore, 3.32 × 9.33 or 30.97…

The B-type of DNA double helix described by Watson and Crick is the most predominant type under physiological conditions. Adjacent nucleotides in each chain are rotated by 34.60 relative to each other. Double helix completes one turn approximately every 10.4 base pairs. A single turn of a double helix spans a distance of 3.4 nm.…

The number of base pairs per turn is about 11, and the pitch of the helix of A-DNA is 2.46 nm. In both A-DNA and B-DNA, the sugar moiety and base are on the opposite sides of the glycosidic bond. In the presence of high concentrations of cations, the nucleotides will rotate into syn-conformations. In…