Biotechnology and Bioinformatics (1+1)

1.2.3. Properties of DNA

1. DNA denaturation or DNA melting

The temperature at which the DNA strands are half denatured is called the melting temperature, or Tm. When a DNA solution is heated, the bonds that hold the 2 strands together become weaker and finally break. This is known as DNA denaturation. The amount of strand separation or melting is measured by the absorbance of the DNA solution at 260 nm. Nucleic acids absorb light at this wave length. When cooled, the two strands will reunite. The GC content of a DNA has a significant effect on its Tm. The higher a DNA's GC content, the higher its Tm. C º G pairing form 3 hydrogen bonds, whereas A = T pairs have only 2. In addition to heating, DMSO and formamide also disrupt the hydrogen bonding between DNA strands and promote denaturation. Lowering the salt concentrations of the DNA solution also aids denaturation.

2. Annealing or Renaturation

Once the two strands of DNA separate, they can reunite under the proper conditions. This is called annealing or renaturation. Factors that contribute to renaturation are: 

1. Temperature - Best temperature for renaturation of a DNA is about 25^oC below its Tm.

2. DNA concentration - The higher the concentration, the faster the annealing.

3. Renaturation time - If longer time allowed for annealing, the more will occur.