Question

If a double stranded DNA has 40% guanine calculate the % of adenine in the DNA.

Answer 1

Answer:

According to Chargaff’s rule, the DNA molecule should have an equal ratio of pyrimidine (cytosine and thymine) and purine (adenine and guanine). It means that the number of adenine molecules is equal to thymine molecules and the number of guanine molecules is equal to cytosine molecules.

% A = % T and % G = % C

If dsDNA has 20% of cytosine, then according to the law, it would have 20% of guanine.

Thus, percentage of G + C content = 40%

The remaining 60% represents both A + T molecule. Since adenine and guanine are always present in equal numbers, the percentage of adenine molecule is 30%.

Answer 2

Answer:

As per Chargaff’s rule, DNA molecules are required to have an equal ratio of purine(adenine and guanine) and pyridine(cytosine and thymine). This is to say that the number of adenine molecules is equivalent to the cytosine molecule.

Percentage of adenosine = percentage of thymine,

Percentage of guanine = percentage of cytosine

Hence, according to the law, if the double stranded DNA has 20% of cytosine, it should have 20% of guanine. Therefore, the percentage of G + percentage of C = 40%

The other 60% indicates both A + T percentage molecule. As adenine and thymine is always found in equal numbers, the adenine content is 30%.