Question

DNA strands are antiparallel because of

A. Peptide bonds

B. Disulphide bonds

C. H-bonds

D. Phosphodiester bonds

Answer 1

Hint: The structural unit of DNA or monomer is called a nucleotide, which is composed of a deoxyribose sugar molecule, a nitrogenous base and a phosphate group.

Complete Answer:

- Antiparallel is defined as the case where DNA strands are found to orient in opposite directions, one of the strands has a polarity from 5ʼ to 3ʼ while another from 3ʼ to 5ʼ. The 5ʼ end has phosphate (PO₄⁻) on the 5th carbon of sugar and the 3ʼ end has a hydroxyl group (OH)  on the 3rd position of sugar. 

- The antiparallel nature of DNA strands is because of hydrogen bonds. The hydrogen bonds are responsible for binding the complementary bases (pyrimidine and purine). The nucleotides which form the DNA strand are connected hydrogen bonds( non-covalent bonds). 

- A single hydrogen bond is weaker than a covalent bond like phosphodiester bond but when there are many H-bonds, this makes the DNA polymers to be connected together. 

The binding occurs according to Chargaff’s rule, where A binds with T and G with C by two and three H-bonds respectively. 

So, the correct answer is option (C) H-bonds.

Note: There are three major forms of DNA that are double-stranded and connected by interactions between the complementary base pairs. These are: the A-form, B-form, and Z-form of DNA.