A sample of DNA has 35 mole percent of GC sea urchin and has

A sample of DNA has 35 mole percent of G+C (sea urchin), and has a T_m of 86 Degree C. A sample of a different DNA has 50 mole percent of G+C (mold) and a T_m of 91 Degree C. Estimate the A content of a sample of DNA with a T_m of 83 Degree C. Two samples of DNA have the same T_m and density. Does this prove they are from the same species? Explain.

Solution

Tm or melting temperature of DNA is defined as the temperature at which 50% of the double stranded DNA molecules are denatured to single strands. The melting temperature is said to increase with higher GC content. The formula for melting temperature of DNA can be given as below:

Tm= 64.9 + 0.41 % GC - (500/ length of DNA)

Therefore, you would probably need the length of the DNA to determine the adenine content.

Melting temperature is characteristic of the GC content of the DNA. The higher the GC content, the higher is the melting temperature. This is because the GC base pairs are connected by three hydrogen bonds, which require more heat or enery to disscociate as compared to AT base pairs which have two hydrogen bonds. The melting point of a DNA moleculae can therefore aid in the estimation of base composition. It is also known that the each species have a characteristic melting temperature. Melting temperature is often calculated to determine the genetic distance between to organisms. In addition to this, bouyant density of DNA also provides a clue to the base composition (GC content). Therefore, if two samples of DNA have the same Tm and density, they can be said to belong to the same species.