Structure of DNA:

It is a polymer. The monomeric unit of DNA is nucleotide. Many neucleotides are joined together to form a polynucleotides of DNA.

A nucleotide is made up of three molecules of the following:

(i) Deoxyribose Sugar


(ii) Phosphoric Acid(H3PO4)

(iii) Nitrogenous bases, such as Adenine, Guanine, thymine and Cytosine.

Based on four types of nitrogenous bases, there are four types of nucleotide in DNA. In a molecule of DNA, four types of thousands of nucleotides form a polymeric chain. There are two such long chains in a DNA molecules in which the base molecules are towards the axis. Base molecules of opposite chains are attached to one another by hydrogen bonds. The backbone of DNA is made up of sugar and phosphate molecules. X-ray diffraction studies have shown that DNA molecules exists as double helix where two arms are spirally twisted around each other.

Each larger helix has 10 pairs of neucleotides and distance between one pair and another is 3.4 A°. Hence, the two helixes have a distance of 34 A°. The two bases are always paired in a definite manner. Adenine always pairs with thymine with the help of two hydrogen bonds and guanine pairs with cytosine with the help of three hydrogen bonds. Adenine is complementary to thymine and cytosine to guanine.


Thus, the two chains are complementary to each other. The two strands of DNA run antiparallel.

Difference Between DNA and RNA:


1. It is generally double helix structure.


2. The sugar is deoxyribose.

3. It contains the bases adenine, thymine, cytosine, and guanine. Uracil is absent.

4. It is the genetic material except in certain viruses.

5. It is synthesized by itself.



1. It is generally single strand structure.

2. The sugar is ribose.

3. It contains the bases, adenine, cytosine, guanine and Uracil. Thymine is absent.


4. It is not the genetic material except in certain viruses.

5. It is synthesized on DNA template.