Packaging of DNA Helix and functions of histone

 Packaging of DNA Helix.                

      The distance between two consecutive base pair: “340m (0.34x10° m), if the length of DNA helix in a typical mammalian cell is calculated (simply by  multiplying the total number of bp with distance ; between two consecutive bp, that is, 6.6 x 10°9 bp x 0.34 * 10power 9 m/bp),  it comes out to be approximately 2.2 meter.                          A length that is far greater than  the “mension of typical nucleus  (approximately 10% a How is such a long  polymer packaged in a cell? If the length of E. coli DNA is 1.36 mm, can you

calculate the number of base pairs in E.coli? In prokaryotes, such as, E.

coli, though they do not have throughout the cell. DNA (being negatively

charged) wy is held with some proteins (that have positive . charges) in a

region termed as ‘nucleoid’, The DNA esi 3 in nucleoid is organised in

large loops held by Aes { proteins. Pe ve In eukaryotes, this organisation

is much more complex. There is a set of positively charged, basic proteins

called histones. A protein acquires charge depending upon the abundance of

amino acids ‘residues with charged side chains. Histones are rich i inthe

basic amino acid residues lysine and arginine. in their side chains.

Histones are organised to form a unit of eight molecules called histone

octamer. The negatively charged DNA is wrapped around the positively charged

histone octamer to form a structure called nucleosome .     A

typical nucleosome contains 200 bp of DNA helix. Nucleosomes constitute _

the repeating unit of a structure in nucleus called chromatin, thread- like

stained (coloured) bodies seen in nucleus. The nucleosomes in * chromatin

are seen as ‘beads-on-string’ structure when viewed under electron

microscope (EM)   - Theoretically, how many such beads  are present in a mammalian cell? tin is packaged to form The beads-on-string  structure in chroma’ ed and condensed at metaphase stage . chromatin fibers

that are further coil F of cell division to form chromosomes.                                The packaging of chromatin at higher _ level requires additional set of proteins that

collectively are referred to as » nucleosomes) do you imagine H1 histone

Core of histone molecules a defined nucleus, the DNA is not scattered. Nucleosome Histone octamer Both the amino acid residues carry

positive charges.