DNA Packaging

Packaging of DNA double helix

Human genome (in diploid cells) = 6 x 109 bp

6 x 109 bp X 0.34 nm/bp = 2.04 x 109 nm = 2 m/cell

Diameter of nucleus = 5-10 mm

Arrangement of DNA in a eukaryotic chromosomes

• In eukaryotes, the DNA is arranged around a set of positively charged basic proteins

called histones .

• Histones are organized to form a unit of eight molecules called, histone octamer.

• The negatively charged DNA is wrapped around the positively charged histone octamerto form a structure called nucleosome.

DNA in a eukaryotic chromosomes

A typical nucleosome contains 200 –bp of DNA helix.

• Nucleosomes constitute the repeating unit of structure in nucleus called chromatin.

• The nucleosomes in chromatin are seen as ‘beads on strings’ structure under

electron microscope.

NHC Proteins

• The packaging of chromatin at higher level requires additional set of proteins called Non histone chromosomal proteins(NHC proteins)

DNA in a prokaryotic chromosomes

Prokaryotic cells lack a nuclear membrane and defined nucleus, even though DNA

is not scattered throughout the cell.

• DNA, being negatively charged is held with some proteins having positive charges in a region termed as nucleoid.

• The DNA in nucleoid is organized in large loops held by proteins.

Fig. 9

Euchromatin and heterochromatin

Euchromatin

In a typical nucleus, some regions of chromatin are loosely packed and lightly stained known as euchromatin. This region contains active chromatin.

Heterochromatin

The chromatin that is more densely packed and stains dark are called heterochromatin. This region contains inactive chromatin.