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
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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.
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5. DNA in a eukaryotic
chromosomes
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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.
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10. NHC Proteins
• The packaging of chromatin at higher
level requires additional set of proteins
called Non histone chromosomal proteins
(NHC proteins)
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11. DNA in a prokaryotic
chromosomes
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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.
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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.
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