NEET · Biology · STD 12 - 5. molecular basis of inheritance

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

A \(G\ 17\%, A\ 33\%, T\ 33\%\)
B \(G\ 8.5\%, A\ 50\%, T\ 24.5\%\)
C \(G\ 34\%, A\ 24.5\%, T\ 24.5\%\)
D \(G\ 17\%, A\ 16.5\%, T\ 32.5\%.\)

Verified Solution

Answer & Solution

Correct Answer

(A) \(G\ 17\%, A\ 33\%, T\ 33\%\)

Step-by-step Solution

Detailed explanation

(a) : According to Chargaff’s rule, the amount of adenine is always equal to that of thymine and the amount of guanine is always equal to that of cytosine, \( i.e., A = T\) and \(G = C\). Also, the purines and pyrimidines are always in equal amounts, \(i.e., A + G = T + C\). Now, given ds \(DNA\) has \(17\%\) cytosine and hence guanine will be also \(17\%\). So, \(A + T\) must be \(66\%\), therefore, percentage of adenine or thymine would be \(66/2 = 33\%\).

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

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Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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