Nucleic Acids: Structure, Classification and Biological Importance


Nucleic acids are the large biomolecules that store, encode and transmit genetic data from one generation to another in the form of DNA and RNA. Each functional unit of nucleic acid is called as nucleotide. Each nucleotide is made up of three parts: a phosphoric acid residue, a sugar and a nitrogen containing heterocyclic base. There are two types of nucleic acids one is deoxyribonucleic acid (DNA) and second is ribonucleic acid (RNA). DNA consist a deoxyribose sugar and RNA consist the ribose sugar. In this article we will discuss about nucleic acid structure, classification of nucleic acids and biological importance of nucleic acids.

Structure of nucleic acids

Nucleic acid is made up of nucleotide which is monomer. Each nucleotide consists of,

  • Pentose sugar
  • Nitrogen base
  • Phosphoric acid

N-glycosidic linkage connects a nitrogenous base to a pentose sugar to for a nucleoside.

Nucleoside = Nitrogen base + Pentose sugar


Nucleotide = Nitrogen base + Pentose sugar + Phosphoric acid

Pentose sugar

Pentose sugar is a monosaccharide sugar containing five carbon atoms. DNA nucleic acid contains Beta-2’-deoxyribose sugar (C5H10O4). RNA nucleic acid contains ribose sugar (C5H10O5). Hydroxy group (-OH) of 5’ and 3’ carbon make bond with phosphate group. Hydroxy group (-OH) of 1’ carbon make bond with nitrogenous base.

Nitrogen base

Nitrogen bases are nitrogen containing compounds. There are two types of nitrogen bases purine and pyrimidine. Purine is a two-ring structure containing Adenine (A) and Guanine (G). These two purine bases are found in both DNA and RNA. Pyrimidines are only one ring structures containing Cytosine (C) and Thymine (T) which are found in DNA molecule. In case of RNA Uracil (U) is found in place of Thymine (T).

Phosphoric acid

The chemical formula for phosphoric acid is H3PO4. There are three reactive hydroxyl groups (-OH) in it. With the help of phosphodiester bonds sugar and phosphoric acid produce initial structure of nucleotide.

Bonds between nucleotides

There are two important linkages or bonds found in nucleotides.

N-glycosidic linkage

To generate a nucleoside a nitrogenous base is connected to the pentose sugar with the help of N-glycosidic linkage.

Phosphodiester linkage

The 5’ hydroxyl group of pentose sugar of nucleoside is attached to a phosphate group with the help of phosphodiester linkage to form a nucleotide.

Types of nucleic acids

There are two types of nucleic acids,

  • Deoxyribonucleic acid (DNA)
  • Ribonucleic acid (RNA)

Deoxyribonucleic acid (DNA)

DNA contains the genetic information used in development and functioning of all living organisms. DNA is mostly present in the chromosomes of plant and animal cells.

Structure of DNA

Watson and Crick’s have given double-stranded double-helical model of DNA in 1953. According to this model the structure of DNA is as follows,

  • Backbone of DNA is made up of right-handed helical spiral of polynucleotides.
  • These chains are antiparallel to each other. Direction of one strand is 5’ to 3’ and direction of another strand is 3’ to 5’.
  • Sugar and phosphate molecule are link together by phosphodiester bond.
  • The double helical structure is stabilised by hydrogen bond couplings. The nitrogen bases on one strand forms the hydrogen bond with bases on another strand. Adenine forms 2H- bonds with thymine and guanine forms 3H- bonds with cytosine.
  • The two strands are complementary to each other. For every adenine in one strand, there will be one thymine in other strand and for every guanine in one strand there will be one cytosine in another strand.
  • The DNA has thickness of 2nm. In the helical structure of DNA each turn consists of 10 base pairs, separated apart at a distance of 0.34nm. Hence distance of each turn is 3.4nm.

Ribonucleic acid (RNA)

The RNA is a single stranded nucleic acid involved in protein synthesis, translation and transcription of genetic code. It is found in few viruses as a genetic material.

Structure of RNA

The single RNA strand is folded back on itself, forming heparin like structure. In some plant viruses RNA is double stranded but non-helical. Single strand of RNA is made up of ribonucleotides which are held together by phosphodiester linkage. Adenine pairs with uracil and cytosine pairs with guanine.

Types of RNA

Based on functions they perform RNA is classified in three different types,

  • Messenger RNA (mRNA): In the process of transcription, RNA transcript is formed which is known as messenger RNA. DNA uses this mRNA to communicate with other cells.
  • Transfer RNA (tRNA): The transfer RNA participates in translation of mRNA transcript during protein synthesis.
  • Ribosomal RNA (rRNA): It is one of the most important RNA. It is a component of ribosomes which helps to produce proteins in the body.
Difference between DNA and RNA
Difference between DNA and RNA

Biological importance of nucleic acids

  • Carriers of genetic information: Nucleic acids serves as carriers of genetic information which helps in determining functional characteristics of organisms.
  • Protein synthesis: It is the fundamental process for structure and function of cells. DNA provides instruction for the process of protein synthesis.
  • Inheritance of genetic information: Nucleic acids play an important role to pass genetic information from one generation to next.
  • Cell replication and division: Nucleic acids assures the accurate transmission of genetic material essential for cell replication and division.
  • Molecular biology and genetic engineering: with the help of genetic engineering, we can extract specific gene from an organism and to store it in bacteria for long time for research purpose.


Nucleic acids are the fundamental building blocks of life, holding genetic information which useful for cellular processes. Understanding of nucleic acids unfolds the way for exciting discoveries in medicine and technology.

Frequently asked questions

What is the difference between nucleotide and nucleoside?

Nucleotide is made up of pentose sugar, nitrogen base and phosphate group. Nucleoside do not contain phosphate group.

What are two types of nucleic acids?

There are two types of nucleic acids one is DNA and second is RNA.

What is the full form of DNA?

DNA full form is a Deoxyribonucleic acid.

What is the full form of RNA?

RNA full form is a Ribonucleic acid.

What is structure of DNA?

DNA have a double stranded double helical structure which was discovered by Watson and Crick’s.

What are the types of RNA?

There are three types of RNA,

  • Messenger RNA (mRNA)
  • Ribosomal RNA (rRNA)
  • Transfer RNA (tRNA)

What is Chargaff’s rule?

In DNA the ratio of adenine (A) and thymine (T) to the guanine(G) and cytosine(C) is constant.

[A] = [T] and [C] = [G]

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