Question

How are the two strands of DNA positioned with respect to each other in the double helix?

• A. They run parallel to each other.
• B. They run antiparallel to each other.
• C. They are perpendicular to each other.
• D. They intersect each other at an angle.

Answer 1

The correct option is B They run antiparallel to each other.

DNA is known as deoxyribonucleic acid. It has a deoxyribose sugar (a pentose sugar) as its central molecule. Adenine, guanine, cytosine, thymine are the nitrogenous bases involved in the case of DNA. The structure of DNA was given by Watson and Crick who stated that there are 2 polynucleotide chains that form a double helix. Purines of one strand bond with pyrimidines of the other strand with the help of hydrogen bonds. This is done so that the uniformity in distance separating both the strands can be maintained. Purines (double ringed structures) always pair with pyrimidines (single ringed structures). A purine-purine or pyrimidine-pyrimidine pairing will distort the double helical structure. Adenine pairs with thymine with the help of 2 hydrogen bonds and guanine pairs with cytosine with the help of 3 hydrogen bonds. A phosphodiester bond is established between the phosphate group of one nucleotide and the -OH group of the sugar of the next nucleotide in the polynucleotide chains. So, a 3’-5’ phosphodiester bond is formed between the nucleotides in a polynucleotide chain. 3’ and 5’ notations help us to indicate the polarity of the polynucleotide chains. While one chain runs from 3’-5’, the other runs from 5’-3’. Thus they run parallel to each other, but in opposite directions. Hence, we can say that they run in antiparallel directions.
The distance between both the strands is maintained uniformly by the base pairing of purines and pyrimidines in both the strands and hence they cannot intersect each other at any point ever and run antiparallel to each other.