In eukaryotic cells, DNA is packaged through interactions with histone proteins to form nucleosomes, which are beads of DNA wound around histone octamers. Nucleosomes further condense into chromatin, and additional non-histone proteins compact chromatin into higher-order structures. In prokaryotes, DNA is loosely organized into loops associated with proteins in the nucleoid region, which lacks a membrane-bound nucleus. Eukaryotic chromatin also exists in two forms - loosely coiled euchromatin containing active genes, and tightly packed heterochromatin containing inactive regions.

1. DNA Packaging

2. 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

4. 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 octamer
to form a structure called nucleosome.

5. DNA in a eukaryotic
chromosomes

8.  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.

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

11. DNA in a prokaryotic
chromosomes

12.  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.

13. Fig. 9

14. 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.