Class 12 Chemistry Biomolecules Nucleic acids

Nucleic acids

  • Nucleic acids are the polymers in which nucleotides are monomers. These are biomolecules present in nuclei of all living cells in the form of nucleoproteins .They are also called as polynucleotides .

They help in the role of transmission of hereditary characters and synthesis of proteins.

Each nucleotide consists of 3 parts:

  • A pentose sugar
  • A nitrogenous base
  • A phosphate group
  • The nitrogenous base and a pentose sugar are called as nucleoside.

Nitrogenous bases are of two types: Purines and Pyrimidines

  • Purines: adenine and guanine
  • Pyrimidines: cytosine , thiamine and uracil


Please note that Purines and Pyrimidines are linked together by hydrogen bonds

  • Adenine always bond with thiamine by double bond or vice versa.
  • Cytosine always pairs with guanine by triple bond or vice versa.

Types of nucleic acids

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


  • It occurs in nucleus of cell. It has double stranded helical structure

DNA contains:

  • Deoxyribose sugar
  • Nitrogenous bases :
  • Purines (adenine and guanine ), Pyrimidines (thiamine and cytosine )
  • A phosphate group
  • It can undergo replication
  • It helps in transfer of genetic information from parents to offspring



  • It occurs in cytoplasm of cell

It consist of:

  • Ribose sugar
  • Nitrogenous base
  • Purines: adenine and guanine
  • Pyrimidines: cytosine and uracil
  • A phosphate group
  • It has a single strand helical structure
  • It doesn’t undergo replication
  • It controls synthesis of proteins

The structure of deoxyribose and ribose sugar is given:

Structure of nucleic acids

  1. Primary structure
  • The nucleic acids are formed by the condensation of thousands of molecules of nucleotides.
  • On hydrolysis the nucleotides produces phosphoric acid and nucleoside .it means nucleosides on hydrolyses form Purine and Pyrimidines base and sugar moiety.
  • A nucleic acid-- NH3ànucleotides –aq NH3à nucleosides + phosphoric acid –dilute HCL--àPurines + Pyrimidines + sugar.
  • Nucleotides are building blocks of nucleic acids.
  • These nucleotides are linked together with one another in a particular sequence, phosphate groups forming bridges between C-5 of the sugar residue of the one nucleoside and C-3 of the sugar residue of the other nucleoside.
  • The manner in which the sugar, phosphate and bases are linked with one another in nucleic acids is known as primary structure of nucleic acids.


  1. Secondary structure:
  • Watson and Crick explained the double helix structure of DNA. The nucleotides in each strand are connected by phosphate ester bond and bases of one strand by hydrogen bonds.
  • Adenine pairs with thiamine through two double hydrogen bonds whereas cytosine pairs with guanine by triple hydrogen bonds.
  • The two strands of DNA are complementary to each other that is if one side there is Purine then on other side at same position Pyrimidine is present. For example if base sequence on strand is ACTCGCCA, then on the other strand the sequence will be complementary that is: TGAGCGGT
  • The primary and secondary structure is shown below:


Watson and Crick model of DNA

Functions of nucleic acids

  •  Replication: The genetic information of cell is contained in the sequence of bases A, T, C and G in DNA molecule .In the division of cell, DNA molecules replicate and makes exact copies of themselves so that each daughter cell will have DNA identical to that of the parent cell.
  • Protein synthesis: The specific information coded on DNA has to be translated and expressed in the form of synthesis of specific proteins which performs various functions in the cell. This synthesis is done in two steps:
  • Transcription and translation.
  • Gene and genetic code: Each segment of DNA molecule that codes for specific protein or a polypeptide is known as The relationship between nucleotides triplets and the amino acids are called the genetic code .This is gene and genetic code.
  • Mutation: It is a chemical change in DNA molecule, which leads to the synthesis of proteins with a changed amino acid sequence.
    • These changes are caused by radiation, viruses or chemical agents.
    • The majority of changes in DNA are replicated by special enzymes in the cell, but if there is failure to repair by the enzymes then it can cause mutation.









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