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DNA Double Helix
Concepts tested here
- Salient features of the double helix
All Questions
2008 AIPMT 1 question
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In the DNA molecule:According to the Watson and Crick model, DNA is made of two polynucleotide chains coiled around a common axis.
The two chains have anti-parallel polarity: if one chain has the polarity $5' \rightarrow 3'$, the other has $3' \rightarrow 5'$.
The bases of the two strands are paired through hydrogen bonds: adenine pairs with thymine (two hydrogen bonds) and guanine pairs with cytosine (three hydrogen bonds).
As a result a purine always pairs with a pyrimidine, so the amount of adenine equals thymine and guanine equals cytosine (Chargaff's rule): $A + G = T + C$.
So the proportion of adenine to thymine is always 1 : 1 in every organism, and the total purines always equal the total pyrimidines; those two statements are wrong.
The strands do not run parallel in the same direction.
The correct statement is that there are two strands which run antiparallel, one in the $5' \rightarrow 3'$ direction and the other in the $3' \rightarrow 5'$ direction.