DNA supercoiling
DNA is a big molecule

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Ruptured viral
particle
(bacteriophage)
X95 000
DNA about 1.7µm
© 2010 Paul Billiet ODWS
Some DNA molecules
Organism
Virus
ФX174
Molecular Base pairs
mass
x 106
/ 106 Daltons
32
0.000533
Length
1.7µm
E. coli
2800
4.2
1.2mm
Human
1 800 000
3000
2m
(46 molecules)
© 2010 Paul Billiet ODWS
THE NUCLEOSOME
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Uncoiled the DNA of a human would
stretch 2m
The average diameter of a nucleus is
10µm
The problem: To pack the DNA into
the nucleus and yet have access to the
genetic information.
© 2010 Paul Billiet ODWS
THE NUCLEOSOME

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Histones are made of
a lot of basic amino
acids
Combined with acidic
DNA they make a
stable nucleoprotein
called chromatin
Histone molecules
© 2010 Paul Billiet ODWS
Histone core =
an octomer
(8 histone molecules)
DNA wraps twice round the
core
DNA strand
© 2010 Paul Billiet ODWS
DNA held in place by another
histone (H1) = the nucleosome
H1
© 2010 Paul Billiet ODWS
Nucleosomes are joined by
linker DNA
Linker DNA
© 2010 Paul Billiet ODWS
DNA super coiling
DNA double helix
Nucleosome beads
on a string
30-nm chromatin fibre
© 2010 Paul Billiet ODWS
DNA super coiling
30-nm chromatin fibre
Chromosome in
extended form
Condensed section
of chromosome in
mitosis
© 2010 Paul Billiet ODWS
DNA super coiling
Condensed section
of chromosome in
mitosis
Condensed
chromosome
© 2010 Paul Billiet ODWS
Nucleosome functions

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Nucleosomes help to supercoil the
DNA molecule
Nucleosomes help to regulate gene
expression (transcription)
© 2010 Paul Billiet ODWS