DNA Structure and the Polynucleotide Molecule

What is DNA?

DNA – Deoxyribonucleic acid, is a long, double helix polymer carrying the unique genetic information required for the human growth and development. It is a double helix which has two ribbon-shaped structures called as strands and each strand is helically (spirally) wound. The structure of DNA is given in the following figure.

DNA structure

What is the Structure of DNA?

Deoxyribonucleic acid (DNA) is composed of nucleic acids found in the nuclei of all living cells. They are the vehicles of genetic inheritance.

Each strand of a DNA is the polymer of nucleotides and each nucleotide is made of a sugar, nitrogenous base, and a phosphate group.


A nitrogenous base is a nitrogen-containing organic compound containing one (called a pyrimidine) or two rings (called a purine). Pyrimidines can be either thymine or cytosine, and purines can be either adenine or guanine. The sugar is a 5 carbon sugar or pentose sugar called deoxyribose.

Deoxyribose DNA

The backbone of the strand is formed by the sugars and the phosphates. The nitrogenous bases project out of the backbone.The hydrogen bonds are formed from the opposite strands of the nitrogenous bases, thus forming a ladder-like structure, the nitrogenous bases being the rungs of the ladder.

The phosphate groups are linked to the 5′- and 3′- carbon deoxyribose. These are called 5′- and 3′- carbons, and not 5- and 3- carbons, to distinguish from the constituents of the rings of the nitrogenous bases. As seen in the figure, there is a phosphate at the 5′-end, and no phosphate at the 3′-end. Thus, each strand has two distinct ends. The two strands have opposite polarities, i.e., they have their 5′- and 3′-ends in the opposite directions.

As mentioned earlier, the opposite strands of the nitrogenous bases produce the hydrogen bonds and Adenine always bond with thymine, forming 2 hydrogen bonds, and guanine pairs with cytosine, forming 3 hydrogen bonds.

What is a Polynucleotide Molecule?

Polynucleotide molecules are a biopolymer consisting of an elongated, linear sequence of nucleotides that are linked together by the ester linkages between the hydroxyl group of the sugar component of the next nucleotide and the phosphoryl group of nucleotide.

Why is DNA called a polynucleotide?

DNA Structure

The DNA called a polynucleotide because the DNA molecule is composed of nucleotides -deoxyadenylate (A), deoxyguanylate (G), deoxycytidylate (C), and deoxythymidylate (thymidylate) (T) which are combined together to create long chains called a polynucleotide. As per the DNA structure, the DNA consists of two chains of the polynucleotide, each in the form of a spherical spiral.

Stay tuned with BYJU’S to learn more in detail about the structure of DNA.

Practise This Question

Which of the following is the example of structural protein?