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Molecular Basis of Inheritance — BIOLOGY STD 12 Science — Question

Bihar BoardEnglish MediumSTD 12 ScienceBIOLOGYMolecular Basis of Inheritance1 Mark
Question
For question, two statements are given-one labelled Assertion and the other labelled Reason. Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below.
  1. Both assertion and reason are true, and reason is the correct explanation of assertion.
  2. Both assertion and reason are true, but reason is not the correct explanation of assertion.
  3. Assertion is true, but reason is false.
  4. Both assertion and reason are false.
Assertion: B-DNA has a constant diameter of 20 A.
Reason: A pyrimidine always pairs with a purine.
✓

Answer

  1. Both assertion and reason are true, and reason is the correct explanation of assertion.

Explanation:

B-DNA is a helical structure with diameter of 20 A. If pairing occurs between two purines, it would need too much space and if pairing occurs between two pyrimidines, it would occupy too little space. The constancy of DNA diameter is maintained only when a pyrimidine always pairs with a purine.

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