Question

Two strands of DNA are held together by _________.
(a) Peptide bonds
(b) Phosphodiester bonds
(c) S-S bonds
(d) Hydrogen bonds

Answer 1

Hint: The DNA strands are held together by a type of bond that is usually seen between electronegative atoms like N, O, F, and another atom having low electronegativity which has properties similar to both group 1 and group 17 elements. This type of bonding is also seen in water and ammonia.

Complete answer:
Deoxyribonucleic (DNA) is a molecule composed of two polynucleotide chains that are coiled around each other to form a double-helical structure carrying genetic instructions for the development, functioning, growth, and reproduction of all the known organisms and viruses.
The two strands of a DNA helix are known as polynucleotides as they are composed of simpler monomers called nucleotides. Each nucleotide is composed of one of the four nitrogen-containing bases (adenine [A], thymine [T], guanine [G], or cytosine [C]), a sugar called as deoxyribose, and a phosphate group.
The nucleotides of a strand are joined to one another in a chain form by covalent bonds which are known as the phospho-diester linkage between the sugar of one nucleotide and the phosphate group of the next, resulting in an alternating sugar-phosphate backbone. The nitrogenous bases of the two separate polynucleotide strands are bound to each other, according to base-pairing rules according to which A pairs with T and C pairs with G. This pairing occurs by hydrogen bonds to make the double-stranded helical structure of the DNA.
So, the answer is, ‘Hydrogen bonds’

Note: The number of hydrogen bonds varies according to the base pairing. The number of hydrogen bonds in A-T bases is two while in G-C pairing they are bonded using three hydrogen bonds. There is also a 5^th nitrogenous base Uracil [U], which is seen in RNA.