DNA Supercoiling | BiotechStudies

DNA SUPERCOILING

DNA Supercoiling

• Nucleosomes are:- (i) condensation of chromatin into a cell nucleus with the help of histone proteins may it packed into nucleus of 2micrometer in diameter.

(ii) Method used:- X-ray diffraction; electron microscopy, cross Linking reagents and nuclease action, polyacrylamide gel electrophoresis (PAGE) and Agrose gel electrophoresis.

• There is octamer of histone of composition H2A, H2B, H3, H4 [2 copies of each] linked by H1 linker and is resistant from nuclease action.

Here, H2A and H4 is Linker and H2B and H3 is octamer.

• Nucleosomes :- it is a shallow structure V shaped only 146 vo DNA wrapped around a core of 8 histone. This wrapping make 1-3/4 turns. Double helix make several kinds and turns.
• Histone:- histones are variable according to species and with tissue. H1 is the Linker protein which hold the nucleosomes together. Chromatosome (166bp). It ranges from 212bp in chick erythrocyte and 165bp for Yeast.
• Lysine/Arginine ratio:- H1 = 20.0 [Linker] H2A=1.25, H2B= 2.5, H3=0.72, H4=0.79 [All are core]

• Histones are Basic proteins have low molecular weight easily isolated by changing salt or acid concentration from chromatins and separated on basis of size and charge.
• The complex(DNA + histone= chromatin)
• Histone have unusual amino acids composition. Extremely rich in +ve amino acid (Lysine, Arginine).
• +Ve charge on histone allow them to bind -ve charge DNA through phosphate bond.
• Chromatin=(0.5m/Nacl) Breakage of electrostatic interaction of free DNA and Histones and then both is purified.
• Chromatin= (2M/Nacl) All histones will be removed and purified then add 0.5MNacl so at last, H1 is dissociated.
• All histones protein of different organism have sequence is conserved.
• H5 is the special type of histone formed in nucleated erythrocytes of fishes, Amphibian, Birds.
• Histones can be phosphorylated, Methylated, Acetylated; any modification can be done to affect their interaction with DNA.
• In sperm cells, histone protein can be placed by protamines.