Question

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

Hint:

Chargaff’s rule ensures that purines (A, G) and pyrimidines (T, C) are present in equal proportions in double-stranded DNA.

Answer 1

- DNA adheres to Chargaff’s rule: Adenine (A) equals Thymine (T), and Cytosine (C) equals Guanine (G). - Given that Adenine (A) is 31%, Thymine (T) is also 31%. - The combined percentage of Adenine and Thymine is 31% + 31% = 62%. - The remaining percentage of bases is calculated as: \[ 100\% - (A + T) = 100\% - 62\% = 38\%. \] - As Cytosine (C) equals Guanine (G), this remaining 38% is split equally: \[ C = G = \frac{38\%}{2} = 19\%. \] - Therefore, the percentage of cytosine in the DNA fragment is 19%.