Polymorphism of DNA helix

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Subsequent experiments have shown that DNA is much more polymorphic than Expected. Thus, DNA can be of following types:                                                                                                                      

  • A-DNA                                                                                                                                              It has 11- base pairs which are considerably found at 75% relative humidity in the presence of Na+, K+ or Cs+ ions.
  • B-DNA                                                                                                                                                    It has 10- base pairs which are found at 92% relative humidity and low ionic Strength.
  • C-DNA                                                                                                                                             It has 9.33 base pairs which are found at 66% relative humidity in the presence of lithium ions.
  • D-DNA                                                                                                                                              It has 8- base pairs which are found at extreme variant.
  • Z-DNA                                                                                                                                             It has 12 base-pairs. It is left handed coiling with a zigzag phosphate backbone.


Wooddock (1973) has shown that chromatin consists of a repeating pattern of bodies known as the Nucleosomes. Nucleosome mainly consists of the DNA and Histone molecules.

Histones play important role in packaging of DNA in ordered way (Nucleosome) inside the nucleus. This model on chromatin gives appearance as beads-on-a-sting. Each nucleosome is a disc shaped particle with a diameter of about 11 nm and 5.7 nm in height containing 2 copies of each 4 nucleosome (Histones-H2A, H2B, H3 and H4). Histone octamer forms the protein core.

Double stranded DNA helix wound 1 ¾ time containing 146 base pairs. Generally, DNA makes two complete turns around the Histone octamer. Each nucleosome beads is separated by linker DNA which is generally 54 base Pairs long and contain H1 Histone protein molecule. P.Oudet gave the term Nucleosome for its visibility under E. Microscope.