Question

In sea urchin DNA, which is double stranded, $17\%$ of the bases were shown to be cytosine. The percentages of the other three bases expected to be present in this DNA are:

• A. G - 8.5 %, A - 50 %, T - 24.5 %
• B. G - 34 %, A - 24.5 %, T - 24.5 %
• C. G - 17 %, A - 16.5 %, T - 32.5 %
• D. G - 17 %, A - 33 %, T - 33 %

Answer 1

The correct option is D G - 17 %, A - 33 %, T - 33 %

According to Chargaff's rule, [A] + [G] = [C] + [T].
However, Adenine binds to Thymine and Cytosine binds to Guanine. Therefore, [A] = [T], [G] = [C].
Total content of DNA = 100 %.
[A] + [T] + [G] + [C] = 100%
But, [C] = 17 % = [G]
[A] + [T] + 17% + 17 % = 100 %
[A] + [T] = 100 % - 34 % = 66 %
But, [A] = [T]
2[A] = 66 %
[A] = 33 % = [T]
Therefore, A - 33 %, T - 33 %, G - 17%, C - 17%