DNA PACKAGING Molecular wt of nucleotide pair – 650 Da Wt of chromosome varies greatly No. of chromosome- –Bacteria – 01Human – 46 –Garden Pea – 14Chicken – 78 –Maize – 20Mouse – 40 –Frog – 26 Total contour Length – 2 meter / Human cell 1.7 mm / E. coli Cell DNA total length in Human – 2 X Km. Distance Sun – Earth – 1.5 X 10 8 Km.

DNA PACKAGING –The long stretch of DNA must be accommodated in very much smaller space of the nucleus –This made possible through the packing of DNA in compact structure –Packaging is by means of super coiling of DNA –Chromatin consists of fibers containing DNA and Protein –In 1975, P. M. Outdet used the term Nucleosome for the particles seen by earlier worker in Electron Microscope – First level of DNA compaction is formation of Bead String Model –Beads are of nucleosomes forming a string of beads by polymerization

DNA PACKAGING NUCLEOSOME COMPOSITION DNA- 200 BP (67nm) - 1,30,000 da HISTONES- 4 (2 Each) - 1,08,000 da NON HISTONES ,000 da Total - 2,62,000 da

Histones Small basic protein MW ranging between – da Rich in basic amino acid (Lysine, Arginine, etc) 25 Nucleotides can be accommodated per histone –TYPES OF HISTONES – – TypeMWAmino acids H 2 A H 2 B H H H

NUCLEOSOME Height – 6 nm Diameter – 11 nm Length of DNA around it – 67 nm (approx. 200 bp) 1.65 turns of DNA around nucleosome A=T rich region mostly attached to histones Histones are forming heterodimer –H 2 A and H 2 B; H 3 and H 4 Each heterodimer binds 30 bp DNA at 10 bp interval Phosphate backbone contacts with histones

NUCLEOSOME ASSEMBLY Nucleosome formation Nucleosome Test Nucleosome Structural features

DNA is embedded in chromatin

Regulation of gene expression at the level of chromatin Sequence-independent linker histones: control DNA compaction and accessibility to trans-acting factors post-translational modifications of histone tails: control compaction of DNA and serve as docking sites for trans-acting factors Range: Can act at the level of a single gene, often acts over groups of genes and over larger domains (20-200kb), and can affect gene expression over an entire chromosome