Chemical Nature of DNA/ Biochemical organization of DNA

Structure of DNA :

Biochemical composition of DNA

  • Discover – Miescher- Pus-cell -Name Nuclein 
  • Altman -Nucleic Acid 
  • Each nucleotide unit consists of three types of compounds which are as follows :

(1) A pentose sugar -the 5 carbon deoxyribose sugar

(2)  2. Phosphoric Acid (H₃PO₄)

(3)  Nitrogenous base-a heterocyclic ring containing nitrogen

1. Pentose sugar molecule

  • Pentose sugar was identified by Lewine (1909). 
  • It is 5 Carbon Sugar 
  • These are Two type –
  1. Deoxyribose Sugar  – In DNA always deoxyribose sugar is found which is lacking in one oxygen atom at 2’ carbon 
  2. Ribose sugar – It is Found in RNA but in this oxygen is present at 2’ carbon
  • The deoxyribose sugar has a pentagonal ring with 5 carbon atoms (1″, 2′, 3′, 4′ and 5 carbons). 
  • Among which 1 carbon is attached to the nitrogenous base and 3′ and 5′ carbons are attached to phosphoric acid. 
  • At carbon 1′ always the OH group is present which is the point of attachment of the base. 
  • This is linked to the 1-nitrogen (1-Nitrogenation) atom of the pyrimidine bases and to the 9-nitrogen atom (9-Nitrogenation) of purine bases.

(2)  2. Phosphoric Acid (H₃PO₄)

  • It found in the form of H₃PO in DNA
  • It joins sugar at 3’ and 5’ carbon by the condensation reaction.
  • DNA has an acidic nature due to the presence of phosphoric acid because it have the negatively charged oxygen molecule.

3. Nitrogenous bases

  • It has nitrogen in their ring so these are basic nature.
  • These are of two types : 

(a) Purines and (b) Pyrimidines.

(a) Purines : 

  • Purines are double ringed nitrogen compounds,
  • in which two benzene rings (5 and 6 membered) are fused. 
  • One ring is called Pyrimidine Ring (2N +4C ) and another is called Imidazole Ring (2N +3C )
  • In DNA two types of purines : Adenine (A) and Guanine (G) are found. 

(b) Pyrimidines : 

  • Pyrimidines are 6 membered single benzene ring compounds. 
  • They are Thymine (T) and Cytosine (C). But in RNA instead of thymine, Uracil (U) is present. 


  • A molecule of deoxyribose sugar attached with a molecule of nitrogenous base without phosphate group is known as nucleoside. 
  • In a molecule of DNA four types of nucleosides are found, which are :

(a) Adenosine : Adenine + Deoxyribose

(b) Guanosine : Guanine + Deoxyribose

(c) Cytidine : Cytosine + Deoxyribose

(d) Thymidine : Thymine + Deoxyribose


  • A nucleotide consists of one molecule of deoxyribose sugar, one molecule of phosphoric acid and one of the four nitrogenous bases. 
  • In DNA molecule following

four types of nucleotides are present :

(a) De-oxyadenylic acid : Adenine+Deoxyribose +Phosphoric acid (DAMP)

(b) De-oxyguanylic acid : Guanine+Deoxyribose +Phosphoric acid (dGMP)

(c) De-oxycytidylic acid : Cytɔsine+Deoxyribose +Phosphoric acid (DCMP)

(d) De-oxythymidylic acid : Thymine+Deoxyribose +Phosphoric acid (dTMP)

Chargaff’s Rule

  • According to this rule the amount of Purine is equals to that of Pyrimidine in DNA 
  •  According to this law A=T and GC are always equal to one  except for o x 174 Coliphage here A:T and G:C ratio is not one. 
  • Chargaff’s rules are also called as “Base rule” or ‘base ratios’.

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