DNA structure


  • A polymer of deoxyribonucleotides.
  • Length is defined as the number of nucleotides or nucleotide pairs present. This is the characteristic of an organism.
  • Bacteriophage Ø x174 has 5386 nucleotides.
  • Bacteriophage λ has 48502 base pairs.
  • Escherichia coli has 4.6 x 106 base pairs.
  • The haploid genome of human DNA is 3.3 x 109 base pairs.

    Structure of polynucleotide chain

  • A nucleotide has 3 components:
  1. A nitrogenous base,
  2. A pentose sugar &
  3. A phosphate group.

Nitrogenous bases are of two types:

  1. Purine: adenine and guanine
  2. Pyrimidine: cytosine, thymine, and uracil.
  • Thymine is present in DNA and uracil is present in RNA. Thymine is 5-methyl uracil.
  • Pentose sugar is deoxyribose sugar in DNA and ribose sugar in RNA. Ribose sugar has an additional OH group at 2′ position.

A nucleoside is a nitrogenous base linked to pentose sugar through an N-glycosidic linkage.

  • A glycosidic bond is formed between C at 1st position of sugar and N at 1st position of pyrimidine / N at the 9th position of purine.

A nucleotide is formed by the linking of a phosphate group to the 5OH group of ribose sugar of the nucleoside by a phosphoester linkage.

Nitrogenous base
Adenosine/ DeoxyadenosineAdenylic acid/ Deoxyadenylic acid
Guanosine/ DeoxyguanosineGuanylic acid/ Deoxyguanylic acid
Cytidine/ DeoxycytidineCytidylic acid/ Deoxycytidylic acid
DeoxythymidineDeoxythymidylic acid
UridineUridylic acid


Polynucleotide chain: In a polynucleotide chain, adjacent nucleotides are joined by 3-5phosphodiester linkage formed between 3C of one sugar and 5C of adjacent sugar.

  • It has a free PO4 group at 5C of the ribose sugar, it is the 5end of the polynucleotide chain.
  • It has a free OH group at the 3C of the ribose sugar, it is the 3end of the polynucleotide chain.


First discovered by Fredrick Meischer in 1869. He named it as “nuclein”.

  • The structure of DNA was given by James Watson and Francis Crick in 1953, for which he received the Nobel prize, basing on the discoveries of:
  • Maurice Wilkins and Rosalind Franklin by X-ray diffraction method gave the double helical nature of the DNA.
  • Astbury gave the 3D structure of DNA.
  • Hallmark of their proposition was the base pairing between the two strands of polynucleotide chains based on observations of Chargaff.
  • Erwin Chargaff proposed the base equivalence rule: according to which in a DNA molecule the amount of purine equals to the pyrimidine.
  • A=T & G=C
  • and the A+T/G+C ratio is constant for a species.

Structure of DNA

  • It is made of two polynucleotide chains. Sugar and phosphate form the backbone of the DNA molecule and the bases project inside.
  • DNA is double stranded, right-handed coiled
  • The two chains are antiparallel., they are opposite in direction. One chain is in 5→3 direction, the other is in 3→5′ direction. This is in relation to the position of sugar in the two strands.
  • The two strands are held together by hydrogen bonds formed between nitrogenous bases forming base pairs. Where A always pairs with T double bond and G always pairs with C triple bond.
  • The two strands are complementary to each other i.e. if you know the base sequence of one strand you can find out the base sequence of the other strand. This is due to specific base pairing.

  • The diameter of a DNA molecule is 20A°.
  • Space occupied by a turn is 34A°.
  • There are 10 nucleotide pairs in a turn, so space occupied by a nucleotide is 3.4A°. (1nm = 10A°, 1nm =10-9m).
  • Hydrogen bonds and the plane of one base pair stacks over the other in double helix provides stability to the helical structure.
  • Helical nature of DNA is due to the rotation shown by glycosidic bonds between base and sugar.
  • Bases are at right angles to long axis of the helix & are at an angle of 36° with respect to its adjacent nucleotide.
  • There are 5 forms of DNA: A, B, C, D & Z. Of which ‘B’ is the native form.
  • DNA uncoils on heating at above 80-90° (denaturation) and rejoins on cooling (renaturation).
  • DNA content of a cell is known as its ‘C’ value.

ALSO WATCH:DNA StructureDNA Replication.


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