Question

Describe DNA denaturation, renaturation (annealing) and how base composition (A & T v. C & G) affects DNA stability

Answer 1

DNA denaturation    it is the process of breaking down the DNA molecule, generally for the purposes of comparison or sequencing.

denaturation is done by different denaturating agents

Denaturation by Temperature

when the DNA solution is heated to approximately 90°C or above there will be enough kinetic energy to denature the DNA completely causing it to separate into single strands. This denaturation is very abrupt and is accelerated by chemical reagents like urea and formamide.

Denaturation by Chemical Agents

Denaturation of DNA double helix can also be brought about by certain chemical agents such as urea and formamide. These chemical reagents enhance the aqueous solubility of the purine and pyrimidine groups. The Tm value is lowered by the addition of urea. In 8M urea, Tm is decreased by nearly 20°C. DNA can be completely denatured by 95% formamide at room temperature only.

Effect of pH on Denaturation

Denaturation also occurs at acidic and alkaline solutions in which ionic changes of the purine and pyrimidine bases can occur. In acidic solutions at pH values 2-3 the amino groups bind with protons and the DNA double helix is disrupted. Similarly, in alkaline solutions at pH 12, the enolic hydroxyl groups ionize, thus preventing the keto-amino hydrogen bonding.

Renaturation   is the process in which the seperated complementary DNA strands can form a double helix

the rate of renaturation can give valuable information about the complexity of the DNA if there are repetitive sequences in the DNA, it shows less complexity in comparison to its total length, but the complexity is equal to its total length if all sequences are unique.

The degree of renaturation after a given time depends on C0, the concentration of double stranded DNA prior to denaturation, and t, the duration of the renaturation in seconds. The concentration is measured in nucleotides per unit volume.

in DNA the two polynucleotide chains are not identical but complementary to each other due to base pairing so these two strands are held together by hydrogen bonds formed by complementary base pairs the A-T pair has two hydrogen bonds while G-C has three hydrogen bonds so the G-C is stronger by about 50% than A-T bond .if the DNA structure has more G-C bonds means the molecule becomes more stable because the breaking of three hydrogen bonds requires more energy.