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Question
In the Watson and Crick model of DNA, the ‘steps’ of the ladder are composed of ______.
Options
sugars
a purine and a pyrimidine
two purines
a sugar and a phosphate molecule
MCQ
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Solution
In the Watson and Crick model of DNA, the ‘steps’ of the ladder are composed of a purine and a pyrimidine.
Explanation:
In the Watson and Crick model, DNA resembles a twisted ladder where the ‘steps’ or rungs are formed by pairs of nitrogenous bases. To maintain a constant distance between the two strands, a double-ringed purine (Adenine or Guanine) always bonds with a single-ringed pyrimidine (Thymine or Cytosine). These base pairs are held together by hydrogen bonds, while the outer ‘rails’ of the ladder consist of a repeating sugar-phosphate backbone.
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