Question

In the Watson and Crick model of DNA, the steps of the ladder are composed of

• A. sugars
• B. a purine and a pyrimidine
• C. two purines
• D. a sugar and a phosphate molecule

Answer 1

The correct option is B a purine and a pyrimidine

In the Watson and Crick model of DNA, the "steps" of the ladder are composed of a purine and pyrimidine and the “backbone” is made up of a sugar and a phosphate molecule. The model is called a double helix because of two long strands of DNA twist around each other like a twisted ladder.

So, the correct option is 'a purine and a pyrimidine'.