Structure of DNA explained

Nucleotides are the building blocks of DNA, DNA is a polymer of these units. We had to understand the chemical structure of this polynucleotide chain(DNA).

The components of a nucleotide

A nucleotide showing base pairs.
A nucleotide

A nucleotide is made up of :

  1. a sugar (deoxyribose)
  2. a phosphate group
  3. one of four nitrogenous bases: adenine (A), thymine (T), guanine (G) or cytosine (C).

There are two types of nitrogenous bases –

  1. Purines(Adenine and Guanine)
  2. Pyrimidines (Cytosine, Uracil and Thymine).

Nitrogenous bases with one rings are called Pyrimidines (C, T & U) while those with two rings are called Purines (A & G).

Uracil is present in RNA at the place of Thymine.

Linking of Nucleotides

DNA nucleotides assemble in chains linked by covalent bonds, which form between the deoxyribose sugar of one nucleotide and the phosphate group of the next. This arrangement makes an alternating chain of deoxyribose sugar and phosphate groups in the DNA polymer, a structure known as the sugar-phosphate backbone.

Double-stranded polynucleotide chain showing hydrogen bonding between nitrogenous bases.
Double-stranded polynucleotide chain

Hydrogen bonding between nitrogenous bases on opposite strands make two stands of DNA double helix stable and held them together.

Picture showing Hydrogen bonding between base pairs.
H-bonding between base pairs

Adenine forms two hydrogen bonds with Thymine from opposite strand and vice-versa. Guanine forms three H-bonds with Cytosine and vice-versa.

It concludes, a purine always comes opposite to a pyrimidine. This generates approximately uniform distance between the two strands of the helix. These A-T  &  G-C associations are known as complementary base pairs.

        If you are interested in knowing the packaging of DNA in eukaryotes, go to this post(DNA Packaging).

Rakesh Barwar
Rakesh Barwar
Founder and Editor-in-Chief of The Last DNA. My passion for technology and biotechnology give rise to this awesome site. I write articles that can be easily understood by a majority of people, therefore making biotech simple and interesting!


  1. great submit, very informative. I’m wondering why the other specialists of this sector don’t notice this. You should continue your writing. I’m sure, you have a huge readers’ base already!


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