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Nucleic Acids

Biomolecules like carbohydrates, lipids, proteins, vitamins, hormones are basically organic compounds which are mainly composed of carbon and hydrogen with some other atoms like oxygen, sulphur, nitrogen, phosphorus etc. Some Biomolecules like adenosine triphosphate (ATP) and adenosine diphosphate (ADP) act as energy coin and mainly involve in the energy transfer process during metabolic activities. Carbohydrates are composed of monomer units which are bonded with each other through glycosidic linkage.

On the basis of number of monomer units, they can be classified as monosaccharide, oligosaccharide and polysaccharides. Another Biomolecule, proteins are also formed by condensation polymerisation of amino acid molecules. The amino acid molecules are bonded with each other through peptide linkage which is an amide bond and formed by the condensation reaction between amino group and the carboxyl group of similar or different types of amino acids.

Nucleic acids are little different from other Biomolecules in their structure and functions. They are mainly formed by three structural units; phosphate group, sugar molecule and nitrogenous bases. These units are bonded with each other in a certain manner to give the specific geometry to the molecule. Let’s discuss both types of nucleic acids; DNA and RNA with their structure and functions in the human body. Some common functional groups present in different bio molecules are as follows.

Functional group Chemical formula Chemical property Structure Biomolecules
Hydroxyl -OH Polar Hydroxyl Carbohydrate, proteins, Nucleic acid
Sulfhdryl -SH Polar Sulfhdryl
Vitamins, hormones
Phosphate -HPO4- Polar Phosphate
Nucleic acid
Carboxyl -COOH Acidic Carboxyl Proteins
Amino -NH2 Basic Amino Proteins


Nucleic Acids Definition

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  1. Every generation shows some resemblance to their ancestors which is because of nucleus of living cell.
  2. Nucleus has some bio molecules which can transmit the characteristics from one generation to next one, called as heredity.
  3. There are some particles which are responsible for heredity is called as chromosomes.
  4. Chromosomes are made up of proteins and some bio molecules known as nucleic acids.
  5. Nucleic acids are bio polymers made up of due to polymerization of monomer unit called as nucleotides.
There are mainly three components in each nucleotide, a sugar molecule, a heterocyclic nitrogenous base and a phospaht group. Mainly two types of nucleic acids are present in nucleus which involve in heredity process; DNA (deoxyribose nucleic acid) and RNA (ribose nucleic acid).

Both nucleic acids have their special features. DNA is a hereditary molecule which used to store and transmits genetic information, thus it is also termed as “code of life”. During the cell division in a living cell each DNA forms exact copy of that which carry information from parent cell to daughter cell.

RNA can be three types; mRNA, tRNA and rRNA. mRNA involves in protein synthesis in the cell nucleus with the use of specific part of DNA by using transcription process. tRNA involves in transfer of amino acids to the exact place in the cytoplasm where the proteins are synthesized.

Building Blocks of Nucleic Acids

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Nucleic acids are bio polymers composed of monomer units called as nucleotides, thus they are the building blocks of all nucleic acids. Each nucleotide has three components which are bonded together in a certain manner to form complete unit. These components are as follow.

1. Nitrogen-containing "base"

There are two type of nitrogenous base present in nucleotides, pyrimidine (one ring) or purine (two rings) which are quite differ from each other in there structures.

(a) Purine

There are two purine base commonly found in nucleic acid, Adenine (A) and guanine (G).
Both purine bases bonded with sugar through the N9 of base with C1 of the sugar.


(b) Pyrimidine

There are three pyrimidine base, Thymine (T) and cytosine(C) present in DNA and Uracil (U).


2. Five carbon sugar

Two pentose sugars are present in nucleic acids, ribose or deoxyribose sugar. DNA contains β-D-2-deoxyribose sugar while RNA contains β-D-ribose sugar. Both sugars are differing only in the presence of one oxygen atom at C2 position.

Five carbon sugar

  • In any nucleotide, the combination of these two components, a base and sugar is known as a nucleoside.
  • The bonding between nucleosides and phosphoric acid molecules results the formation of nucleotides.
  • In nucleosides, the 1-position of pyrimidine and 9-position of purine bonded with C1 of the sugar molecule through a β-linkage also known as N-glycosidic linkage.

On the basis of five different bases, there are five nucleosides are possible in DNA and RNA.

Abbreviation Base Nucleoside Nucleic Acid Structure of nucleoside
A Adenine Deoxyadenosine DNA Adenine
Adenosine RNA Adenosine
G Guanine Deoxyguanosine DNA Guanine
Guanosine RNA Guanosine
C Cytosine Deoxycytidine DNA Cytosine
Cytidine RNA Cytidine
T Thymine Deoxythymidine (thymidine) DNA Thymine
U Uracil Uridine RNA Uridine

3. A phosphate group

A phosphate group acts as a linking chain between two sugar molecules to make a complete strand of nucleic acid. The oxygen part of phosphate group linked with carbon atom of sugar to form nucleotide. A Nucleotide can also exist in activated forms with two or three phosphates, known as nucleotide diphosphates or triphosphates.

On the basis of sugar, they also termed as deoxynucleotide and ribonucleotide.


  • Three components of nucleotide are bonded by esterification of C5’ –OH of the sugar of nucleoside with phosphoric acid.
  • Thus nucleotides are nucleoside monophosphate and can be written as Base-Sugar-Phosphate.
  • On the basis of sugar, nucleotide can be abbreviated by using three capital letters with prefix d- in case of deoxy series.
For example, nucleotides and nucleosides of DNA are as follow.

Base Nucleoside 5'-Nucleotide Abbreviation
Adenine 2'-Deoxyadenosine 2’-Deoxyadenosine-5'-monophosphate dAMP
Cytosine 2'-Deoxycytidine 2'-Deoxycytidine-5'-monophosphate dCMP
Guanine 2'-Deoxyguanosine 2'-Deoxyguanosine-5'-monophosphate dGMP
Thymine 2'-Deoxythymidine 2'-Deoxythymidine-5'-monophosphate dTMP

During the formation of polynucleotide like DNA or RNA, monomer units (nucleotides) are bonded with phospho diester bond in which a bond formed between the 3' -OH group and the 5' phosphate group through condensation reaction.


Structure of Nucleic Acids

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The structure of nucleic acid is two types.
  1. Primary structure
  2. Secondary structure

1. Primary structure

Primary structure represents the backbone of polynucleotide which forms with bonding between three components of nucleotides. In the polymerization of nucleotides, the C5 carbon atom of sugar get condensed with hydroxyl group (-OH) of phosphate group which present at C3’ of the other nucleotide.

Hence the backbone of the nucleic acids made up of sugar and phosphate residue arranged in an alternate manner. Further each sugar bonded with nitrogenous base to form complete primary structure of nucleic acid.

Condensation Reaction

2. Secondary structure

  • Secondary structure of nucleic acid is based on Chargaff rule, given by E. Chargaff ; which states that in all cases the amount of adenine is equal to thymine (A=T) and cytosine is equal to guanine(C=G).
  • Hence the base composition of DNA varied from one species to another but the total amount of purines is equal to the total amount of pyrimidine.
  • Secondary structure shows the complete helix form of nucleic acid.
  • DNA exist as double helical form while RNA as single strand. DNA consist of two strands runs in opposite direction (the free phosphate residues at 3’ or 5’ positions of the two strands lie on the opposite sides of the α-helix) giving a double helix structure.
  • The nitrogenous base gets pair with each other through hydrogen bonding.
  • A purine base of one strand paired with a pyrimidine base of another strand.
  • Due to size and geometry of bases, guanine and cytosine get paired through three hydrogen bonds.
  • Similarly adenine and thymine are bonded through two hydrogen bonds.
  • The two helix of DNA are complementary to each other.

Primary Structure of Nucleic Acid

  • These two strands of DNA are not identical and the base sequence in one strand automatically fixes due to the base pairing.
  • The distance between base pairs is about 0.34 nm and the distance between two successive turns of the helix is 3.4 nm with the diameter 2.0 nm.
  • Because of the winding of strands of DNA around each other, some gaps created between each set of phosphate backbones which called as groove.
  • There are two types of grooves; major and minor.
  • A wide and deep groove is called as major groove while a shallow and narrow is termed as minor groove.

Secondary Structure of Nucleic Acid

Examples of Nucleic Acids

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Nucleic acids are mainly two types.

  1. Deoxyribose nucleic acid (DNA) and
  2. Ribose nucleic acid (RNA).

Both nucleic acids are polynucleotide and differ from each other in the presence of different nitrogenous base and sugar. Since each nucleotide has three components; a five member sugar molecule, a nitrogenous base and a phosphate group. In both nucleic acids (DNA and RNA), the phosphate group is same.

DNA consists of deoxyribose sugar, however a ribose sugar is present in RNA backbone. Purine bases are similar in both nucleic acids while pyrimidine base are different. DNA contains thymine (T) and cytosine(C) but RNA is made up of cytosine(C) and Uracil (U). Unlike DNA, RNA is a single strand nucleic acid involves in protein synthesis.

Two Strands of DNA

Function of Nucleic Acids

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There are two important functions of nucleic acids in any living cells; Replication and protein synthesis.

1. Replication

The process of formation of two identical copies of DNA strand by a single DNA strand is known as replication or mitosis or cell division. These identical DNA strands transfer to daughter cell and responsible for hereditary.

Replication process completed in certain steps.
  • Unwinding of DNA strands.
  • Each strand acts as template and involve in synthesis of two new strands.
  • New copy of DNA strand passes to daughter cell.
Hence replication is a semi conservative process as only one half of the parental DNA s conserved and one half gets synthesizes. This process is an enzyme catalyzed process involves many enzymes like,
  1. Topoisomerase: Initiates unwinding of the double helix of DNA.
  2. Helicase: Assists the unwinding by cleavage of hydrogen bonds between base pairs.
  3. Single-strand binding-protein: Stabilizes the separated strands by preventing them to recombine again.
  4. DNA Polymerase: Check for errors and make corrections during replication process.
  5. Ligase: United with small unattached DNA segments on a strand.

Examples of Nucleic Acids

2. Synthesis of proteins

RNA involves in protein synthesis and the message for the synthesis of proteins is coded in DNA. There are three types of RNA’s involves in protein synthesis.
  • Transfer RNA (tRNA): It is a small but stable and long lived molecule contains around 65-110 nucleotides. It used to carry activated amino acids to the site of protein synthesis.
  • Ribosomal RNA: it involves in the formation of the ribosome which is a main site for protein synthesis.
  • Messenger RNA (mRNA): It is a short-lived RNA which acts as a carrier of genetic information on the primary structure of proteins from DNA with special features which allow it to attach to ribosomes and function in protein synthesis.
More topics in Nucleic Acids
Types of Nucleic Acids Nucleic Acid Structure
Nucleic Acid Function
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