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Nucleic acids – Definition, Structure, Properties, Types

  • Last Updated : 22 Dec, 2021

Any molecule created by a living organism is referred to as a biomolecule. Large macromolecules including proteins, polysaccharides, lipids, and nucleic acids, as well as numerous smaller compounds, are included. Biogenic compounds is a more broad term for this type of chemical.

Nucleic Acids

Nucleic acids are macromolecules that are found in every living cell, either alone or in conjunction with other substances. End-to-end polymerisation of a vast number of units called nucleotides linked by phosphodiester linkages forms these lengthy strands. The word “nucleic acid” is used to describe specific big molecules found in cells.

Properties of Nucleic Acid:

  • Nucleotides are the building blocks of nucleic acid.
  • These make up all living things’ genetic material.
  • In a live cell, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are two forms of nucleic acids.
  • In 1969, Friedrich Miescher discovered both DNA and RNA.
  • A nucleotide is made up of three chemically different components. A heterocyclic base, or nitrogenous base, is one, a monosaccharide pentose sugar is another, and phosphoric acid, or phosphate group, is the third.
  • The nitrogenous bases are made up of one or two heterocyclic rings that include nitrogen atoms. Adenine (A), guanine (G), uracil (U), cytosine (C), and thymine (5-methyl uracil) are the five bases (T).
  • Adenine and guanine are substituted purines with two heterocyclic rings, whereas uracil, cytosine, and thymine are substituted pyrimidines with three heterocyclic rings (1 heterocyclic ring).
  • DNA has the nitrogenous bases A, T, G, and C, whereas RNA has the nitrogenous bases A, U, G, and C.
  • Polynucleotides either include beta-ribose sugar (in RNA) or beta 2′ deoxyribose sugar (in DNA) (in DNA).
  • Nucleosides: Sugar + Base
  • Nucleotides are made up of three parts: base, sugar, and phosphate.
  • The backbone of DNA strands is made up of phosphodiester linkages, which are sugar and phosphate residues.
  • Due to the presence of phosphate groups, they are acidic and negatively charged.

Functions of Nucleic Acids: Nucleic acids are genetic material for all living cells, meaning they pass on hereditary characteristics from one generation to the next. Nucleic acid can also determine an organism’s phenotypic. Some nucleic acids, such as ribozymes, may have enzymatic activity. Nucleic acids play a role in protein production, either directly or indirectly.

Structure of Nucleic Acid

The nucleotide is a tiny unitary structure made up of phosphodiester links that connect nucleic acids. Each nucleotide comprises 

  • A Nitrogen base
  • A Pentose sugar
  • Phosphoric acid

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

Nitrogen base + Pentose sugar = Nucleoside

  • Nitrogen Bases: The nitrogen bases are the nitrogenous components of the nucleotide. Purine and pyrimidine are two forms of heterocyclic bases that are used to make the five nitrogenous bases.
    • Purines have a two-ring structure. Adenine (A) and guanine (G) are the two purine bases found in DNA and RNA (G).
    • Pyrimidines are bases with only one ring. Cytosine (C) and thymine (T) are the pyrimidine bases found in DNA molecules, while cytosine (C) and uracil are the pyrimidine bases found in RNA molecules (U).

  • Pentose Sugar: Pentose sugar is a sugar molecule or monosaccharide having five carbon atoms. In nucleic acid, the pentose sugar is an aldose sugar. RNA is a nucleic acid that contains ribose sugar, whereas DNA is a nucleic acid that contains a Beta-2’–deoxyribose sugar. Chemically, these two sugars are not the same. Ribose sugar has the chemical formula C5H10O5, whereas Beta-2’–deoxyribose sugar has the molecular formula C5H10O4. With OH groups of 5′ and 3′ carbon, these sugars make bonds with phosphate groups, whereas with nitrogenous bases, they form bonds with OH groups of 1′ carbon.

  • Phosphoric Acid: H3PO4 is the formula. Three reactive -OH groups can be found in phosphoric acid. Two of these are involved in the formation of a sugar-phosphate backbone using phosphodiester bonds.

Bonds between Different Units of Nucleotides

The following types of linkage or bond can be found in a nucleotide unit’s components:

  1. N–glycosidic linkage: To generate a nucleoside, a nitrogenous base is attached to the pentose sugar via a N– glycosidic linkage. Purine nucleosides feature a 1’–9′ glycosidic bond (sugar carbon 1′, A/G nitrogen 9′). The 1’–1′ linkage (sugar carbon 1′ and 1′ nitrogen of T/C) is found in pyrimidine nucleosides.
  2. Phosphoester linkage: A matching nucleotide is generated when a phosphate group is attached to the 5′–OH of a pentose sugar of a nucleoside via phosphodiester linkage. A dinucleotide is made up of two nucleotides joined together by a 3′–5′ phosphodiester bond.

Types of Nucleic Acids

Nucleic acids are of two types:

  1. Deoxyribonucleic acid (DNA)
  2. Ribonucleic Acid (RNA)

DNA 

All living species have DNA as their primary genetic material. It’s a nucleic acid molecule with two strands.

Occurrence: DNA is mostly present in the chromosomes of plant and animal cells’ nuclei. It’s found in mitochondria and chloroplasts as well. It’s found in circular and supercoiled chromosomes in prokaryotes’ cytoplasm. However, it is found in eukaryotes with proteins such as histones and protamine.

Structure: Watson and Crick’s double-stranded double-helical model is the most widely accepted structural model of DNA (1953). The structure of DNA, according to the model, is as follows:

  • A right-handed helical spiral is formed by each chain of DNA, and two chains coil around each other to form a double helix.
  • The phosphodiester bond is the link between the sugar and phosphate molecules, and the bases project inside.
  • The chains run in antiparallel directions, with one strand coming from the 5’→3′ direction and the other coming from the 3’→5′ direction.
  • The nitrogenous bases on one strand form hydrogen bonds with the bases on the other strand. Adenine forms 2H -bonds with thymine (A-T), and guanine forms 3H -bonds with cytosine (G-C). The helical structure is stabilised by this coupling.
  • The chains are complementary because, for every adenine in one chain, there will be thymine in the other; for every guanine in one chain, there will be cytosine in the other, and so on.
  • DNA has a 2nm consistent thickness.
  • The pitch of the helix is 3.4nm for each round of the double helix.
  • Each turn comprises around 10 base pairs. The distance between two neighbouring base pairs is about 0.34 nanometers.
  • The helix’s backbone is made up of sugar and phosphate, with bases aligned along the axis.

RNA

RNA is a single-stranded nucleic acid found in a few viruses, such as retroviruses and viroids, as genetic material.

Occurrence: The majority of RNA is located in the cytoplasm of cells. The nucleolus and nucleoplasm both contain it. Except for a few viruses that have double-stranded RNA, it is generally found as a single-stranded polynucleotide.

Structure: 

  1. The single RNA strand is folded back on itself, generating hairpin-like structures fully or in parts.
  2. In some plant viruses, the genetic material is double-stranded but non-helical RNA.
  3. Each strand of RNA is made up of a large number of ribonucleotides that are bonded together by phosphodiester linkages.
  4. Adenine and uracil (A-U) form a pair, and guanine and cytosine form a pair (G-C).
  5. Messenger RNA, ribosomal RNA, and transfer RNA are the three kinds of RNA.

Difference between DNA and RNA

DNA

RNA

It contains deoxyribose sugar.It contains ribose sugar.
It can be present in the nucleus, mitochondria, and chloroplast chromosomes.It is related to chromosomes and can be found in the cytoplasm, nucleolus, and nucleoplasm.
Double-stranded structure.Single-stranded structures generally except a few viruses.
Adenine, guanine, cytosine, and thymine are the nitrogenous bases found.Adenine, guanine, cytosine, and uracil are the nitrogenous bases found.
A long molecule with high molecular weight.A relatively short molecule with low molecular weight.
Purines and pyrimidines occur in equal proportionPurines and pyrimidines do not occur in equal proportion.
DNA is the hereditary material.Only a few viruses and viroids have RNA as their genetic material.

Sample Questions

Question 1: What are Nucleic Acids?

Answer:

Nucleic acids are macromolecules that can be found alone or in combination with other chemicals in every live cell. These long strands are formed by the end-to-end polymerization of a large number of nucleotide units coupled by phosphodiester bonds. The term “nucleic acid” refers to a class of large molecules found in cells.

Question 2: What is nucleoside?

Answer:

A nucleoside is formed when a nitrogenous base is connected to a pentose sugar via an N-glycosidic bond.

Question 3: What is a nucleic acid’s primary function?

Answer:

Nucleic acid has the ability to store and transfer genetic information from one generation of cells to the next.

Question 4: What is DNA?

Answer:

The molecule found inside cells that contains the genetic information necessary for an organism’s development and function. This information is handed down through the generations thanks to DNA molecules. Adenine (A) coupled with thymine (T) and guanine (G) paired with cytosine (C) form a double-stranded helix that is kept together by weak hydrogen interactions between purine-pyrimidine nucleotide base pairs (C). Also known as deoxyribonucleic acid (DNA).

Question 5: What is Phosphoester linkage?

Answer:

When a phosphate group is connected to the 5′–OH of a nucleoside’s pentose sugar via phosphoester linkage, a matching nucleotide is formed. A dinucleotide is composed of two nucleotides linked by a 3′–5′ phosphodiester linkage.

Question 6: What is N–glycosidic linkage?

Answer:

A nitrogenous base is connected to the pentose sugar via a N– glycosidic bond to produce a nucleoside. A 1’–9′ glycosidic bond exists between purine nucleosides (sugar carbon 1′, A/G nitrogen 9′). Pyrimidine nucleosides have the 1’–1′ connection (sugar carbon 1′ and 1′ nitrogen of T/C).


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