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] = [T], [G] = [C].

[A] + [G] = [C] + [T]
Purines = Pyrimidines.

Total content of DNA = 100 %

[A] + [T] + [G] + [C] = 100%

[C] = 17 % (information given in the question) and hence [G] = 17%

Hence, [A] + [T] + 17% + 17 % = 100 %

[A] + [T] = 100 % - 34 % = 66 %

[A] + [T] = 66 %

Since, [A] = [T]

66/2 = 33%

Hence, [A] = [T] = 33 %

Therefore, A - 33 %, T - 33 %, G - 17%, C - 17%