Question

If the base adenine constitutes 31% of an isolated DNA fragment, then write what will be the expected percentage of the base cytosine in it. Explain how did you arrive at the answer given.

Answer 1

Given: Cytosine = 19%

According to Chargaff's rule in a DNA molecule, A + T + G + C = 100%

Percentage of A = Percentage of T

Percentage of G = Percentage of C

Therefore, If A = 31%, then T = 31%

G + C = 38%

Therefore, the total percentage of adenine and thymine together is 31% + 31% = 62%.

The total percentage of all four bases (adenine, thymine, cytosine, and guanine) in DNA is 100%, with adenine and thymine accounting for 62%. Cysine and guanine make up the remaining 100% − 62% = 38%.

To calculate the percentage of cytosine alone, divide the remaining percentage (38%), which is equivalent to guanine according to Chargaff's guidelines, by 2.
`(38%)/2` = 19%.

Thus, the predicted percentage of cytosine in the DNA fragment is 19%.