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الجمعة، 1 مارس 2019

Macromolecules and genetic information PART2

DNA : 

  • There are 3 levels for the DNA structure by incresing complexity known as primary,secondary and tertiary structures 

  2. SECONDARY: refer to stable 3 dimensional configuration of DNA 
  3. TERTIARY: complex packing arrangement of double stranded DNA in chromosome  

  1. Primary structure of DNA:

  • DNA is linear polymer, double stranded in its native state. 
  • consist of number of nucleotides linked together between '3 and '5 sites of deoxyribose by phosphodiester bond . 
  • the 2 directions are parallel but in opposite direction(antiparallel) '3 to '5, '5 to '3. 
  • a single bacterial DNA molecule is about 1mm long. 
  • The length of human chromosome is 8.2 cm. 
  • a human cell contains about 2 meters of DNA.
  • DNA in the body could stretch to the sun and back almost 100 times. so it is tightly backed.

      2. Secondary structure : 

(watson and crick 1953)
  •  The 2 strands are kept in position and the structure is stabilized by H bonds. 
  • Formation of 2H bonds between A and T and 3 bonds between G and C is maximally possible. 
  • the molar content of A = molar content of T,and the molar content of G = molar content of C.
  • The ratio (A+T)/(G+C)IS NOT equal and varies from DNA to DNA obtained from different sources .
  • the 2 poly nucleotide strands of  a DNA molecule are not identical but are complementary.
  • on the outside of B form DNA, the spaces between the intertwined strands form 2 helical grooves of different widths described as the major groove and minor groove. 

       3.tertiary structure:   


  • The length of DNA may be 1000 time longer than the cell itself.
How it packed into the cell?!!
  • The solution is supercoiling. 
  • Supercoiling is a state in which double stranded DNA are further twisted. 
  • Supercoiling puts the DNA molecule under torsion 
  • The Process generates a tightly coiled structure that is under positive or negative supercoils 
  • Negative supercoiling occur when the DNA is twisted about its axis in the opposite direction from that of the right handed double helix (anticlockwise).

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