Question

The two long polynucleotide chains of DNA are held together by
A. Hydrogen bonds
B. Covalent bonds
C. Metallic bonds
D. Ionic bonds
E. Polar covalent bonds

Answer 1

Hint: DNA also known as deoxyribonucleic acid is a long polymer of deoxyribonucleotides. It is considered as the genetic material in all eukaryotes. Its length is usually defined as a number of nucleotides present in it which varies from species to species.

Complete Answer:
The DNA is the polymer of nucleotides i.e. comprises a nitrogenous base, a pentose sugar (deoxyribose sugar in case of DNA while ribose sugar in RNA) and a phosphate group. The nitrogenous bases present can be either purine or pyrimidine. Purine includes adenine and guanine while pyrimidine includes cytosine, uracil and thymine. Cytosine is found in both RNA and DNA while the thymine is exclusively present in DNA while uracil is exclusively present in RNA. These nucleotides are forming two chains that are held together by the hydrogen bonds.

The correct option is A i.e. hydrogen bonds.

Additional information: The pentose sugar of DNA is linked to a nitrogenous base by the help of N-glycosidic linkages that are responsible for the formation of nucleoside. When the ${5'}$OH group of this nucleotide is linked with a phosphate group, a corresponding nucleotide is formed.
Two nucleotides of the DNA are linked through ${3'} - {5'}$ phosphodiester bond to form a dinucleotide that are further joined with another nucleotide and form a polynucleotide chain. The chain has a free phosphate group at ${5'}$ end of pentose sugar which is called as ${5'}$ end of polynucleotide chain. Similarly, at another end the ${3'}$OH group is pentose sugar is free which is referred to as ${3'}$ end of the polynucleotide chain. Thus, form the backbone of the polynucleotide chain, by the help of sugar and phosphate and the bases project inside.

Note: The polynucleotide chain is antiparallel to each other and the bases of these antiparallel strands are held together by hydrogen bond in which adenine form 2 hydrogen bond with thymine and guanine are linked by 3 hydrogen bonds with cytosine.