Unit – II A

Nucleic Acids

  • In 1869, Friedrich Miescher separated cellular substance from the nucei of pus cell and called it ‘Nuclein’. Due to acidic nature, the substance is further called as nucleic acid.
  • There are two types of nucleic acids a) Deoxyribonucleic acid (DNA) found primarily in the nucleus of cells and b) Ribonucleic acid (RNA) found mainly in the cytoplasm of living cells.

Chemical Components of Nucleic Acids:


  • Nucleotides are the structural units of nucleic acids. Each nucleotide has three components

Sugars :

  • The five-carbon sugar (pentose) in nucleic acids is ribose or a ribose derivative. It has pentagonal ring structure.
  • In RNA the sugar is ribose, in DNA it is 2deoxyribose.
  • The only difference between these two sugars is found at the 2-carbon of the ribose ring. Ribose has a hydroxyl group (-OH) bound to this carbon, while deoxyribose has a hydrogen atom (“deoxy” means no oxygen).

Phosphate Group:

  • The second component of a nucleotide is derived from phosphoric acid (H3PO4).
  • Phosphoric acid contains three hydroxyl groups attached to phosphorous.

Phosphoric acid             Phosphate group

  • From these three OH groups, two are responsible for strand formation.

Nitrogen or Organic Bases:

  • The organic bases found in nucleic acids are derivatives of pyrimidine or purine.
  • Pyrimidine is a six-membered heterocyclic ring. A heterocyclic ring is a ring compound containing atoms that are not all identical. Purine is a fused ring compound containing a six-membered ring connected to a five-membered ring.

  • The three pyrimidine derivatives found in nucleic acids are cytosine (C), thymine (T), and uracil (U).

  • The two purine derivatives found in nucleic acids are adenine (A) and guanine (G).

  • Adenine, guanine, and cytosine are found in both DNA and RNA. Thymine is found only in DNA, while uracil is found only in RNA.
  • Thymine and uracil are often used to differentiate DNA from RNA.
  • The nucleotides are named according to their nitogenous base. For e.g. a nucleotide containing thymine is called thymine nucleotide.
  • Thus Nucleotide = Pentose Sugar + Nitrogen Base + Phosphate Group
  • or Nucleotide = Nucleoside + Phosphate Group


  • When ribose or 2-deoxyribose is combined with a purine or pyrimidine base, then the combination is called nucleoside.
  • A nucleoside is basically a nucleotide that is missing the phosphate portion.
  • Thus Nucleoside = Sugar + Nitrogen Base

New Naming System for Nucleosides:

New Naming System for Nucleotides:


  • A polynucleotide chain is formed by connecting several nucleotides in succession.
  • RNA is a polynucleotide that, upon hydrolysis, yields D-ribose, phosphoric acid, and the four bases adenine, guanine, cytosine, and uracil.
  • DNA is a polynucleotide that yieldsD-2-deoxyribose, phosphoric acid, and the four bases adenine, guanine, cytosine, and thymine.


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