The biologists of twentieth century realized that the genetic material must have certain characteristics. These charabteristics can explain the properties of life:

1: First, the genetic material must be able to code for the sequence of amino acids in proteins. It must control protein synthesis.

  1. Second, it must be able to replicate itself before cell division.
  2. Third, the genetic material must be in the nucleus of eukaryotic cells.
  3. Fourth, it must be able to change over time for evolutionary changes.

DNA (deoxyribonucleic acid) is the only molecule which fulfills all of these requirements.


Two kinds of nucleic acids participate in protein synthesis. Both have similar building block called the nucleotide.

  • Deoxyribonucleic acid or DNA, is the genetic material
  • Ribonucleic acid or RNA is produced in the nucleus and moves to the cytoplasm. It participates in protein synthesis in cytoplasm.

DNA and RNA are large molecules made up of subunits called nucleotides. A nucleotide consists of a nitrogen containing organic base, a pentose sugar and phosphate group.



monophosphate ((LAMP) OH

It was discovered in 1950s that covalent bonds are present in a nucleic acid. Th( scientists started work on discovery of the three-dimensional structure of DNA Watson and Crick proposed the model of DNA:

  1. The DNA is helical in shape. It is made up of two strands.
  2. The helix has a uniform width of 2 rim.
  3. Its nitrogenous bases are 0.34 nm apart. Ten layers of base pairs are preser

on each turn of the helix.                                                       •



  1. The phosphate groups were present outside the helix. But the nitrogenou bases are present in the interior of the double helix.
  2. The double helix is a ladder like. It has rigid rungs. Its ladder twists in a spire fashion. The side ropes are the equivalent sugar—phosphate backbones. Thrungs are pairs of nitrogenous bases.
    1. Franklin’s X-ray data indicate that the helix makes one full turn after every 3.4 nm of its length.
    2. The pairing of the nitrogenous bases is complement. Adenine pair with thymine (T) and guanine (C) with cytosine. Adenine and Guanine are larger bases. They have two ringed structures. They are called purine base. On the other hand, the cytosine and thymine are pyrimidine bases. They have single ring. In this way one purine and one pyrimidine combination keep the diameter of the DNA uniform.
    3. Both strands of the DNA are anti parallel. One strand is in 5 – 3 direction. The other strand is in 3 – 5 strands.



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