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Structure of DNA

DNA – A double helix

  1. DNA stands for Deoxyribonucleic acid.It is a nucleic which is used for storing information for long term in all living beings and some viruses.



  1. Base composition in DNA varies from one species to other but in all the cases the amount of adenine is equal to thymine and the amount of cytosine is equal to guanine.


  1. The total amount of purines is always equal to pyrimidines.
    A + G = C + T 
    Ratio AT/CG varies between the species. 
    In man this ratio is 1.52
    In E Coil the ratio is 0.93


  1. The double helix is composed of two right handed helical polynucleotide chains coiled around the same central axis.


  1. The two strands are held together by hydrogen bonds.


  1. The two strands are antiparallel i.e.  5′ ——> 3′ phosphodiester linkages running in opposite directions.


  1. Two hydrogen bonds are formed between A and T and three are formed between C and G.


  1. In addition to hydrogen bonds other forces like hydrophobic interactions between stacked bases are also responsible for the stability and maintenance of double helix.


  1. The diameter of double helix is 2nm and the double helical structure repeats at an interval of 3.4nm which corresponds to ten base pairs.


  1. DNA helices can be right handed or left handed. But the b – conformation of DNA having the right handed helices is the most stable.


  1. On heating the two strands of DNA separate from each other and on cooling these again hybridize.


  1. The temperature at which the two strands separate completely is known as melting temperature (Tm). Melting temperature is specific for each specific sequence


  1. The B sample of DNA having higher melting point must have more C-G content because C-G pair has 3 hydrogen bonds.


  1. The sequence of bases along the DNA molecule encodes for the sequence of amino acids in every protein in all organisms.