Question No. 20:

The DNA double helix structure was discovered by
(A) James Watson and Francis Crick
(B) Rosalind Franklin and Maurice Wilkins
(C) Linus Pauling
(D) Gregor Mendel Correct Answer: (A) James Watson and Francis Crick Explanation in Simple Sentences:

• James Watson and Francis Crick (A):
  Watson and Crick found the DNA double helix structure in 1953. They used X-ray data to build a model showing DNA’s twisted ladder shape. Their work explained how DNA stores genetic information.
  
  This is the correct answer.
  
• Rosalind Franklin and Maurice Wilkins (B): Franklin took X-ray pictures of DNA, showing it was a helix. Wilkins shared her data with Watson and Crick. They helped but didn’t propose the final model. This is incorrect.
  
• Linus Pauling (C): Pauling suggested a wrong triple-helix model for DNA. His idea didn’t match the real structure.
  This is incorrect.
  
• Gregor Mendel (D): Mendel studied how traits are passed in plants, founding genetics. He didn’t work on DNA’s structure.
  This is incorrect.

Reason: Watson and Crick correctly described DNA’s double helix, using data from others. Franklin and Wilkins provided key evidence, Pauling’s model was wrong, and Mendel worked on genetics, not DNA structure.