1. Dr. Saji Mariam George
Associate Professor (Retired)
Assumption College Autonomous
Changanacherry
CHROMOSOME MODELS
2. CHROMOSOME MODELS
1. Dupraw model (Folded fibre model)
• Proposed by Dupraw (1965)
• The body of the chromatid is represented by
a single DNA – Protein Fibre.
• The 20 A° DNA double helix is spirally
packed in protein to form a fibril.
• This fibril is coiled to form a 80 – 100 A °
fibre called Type A fibre – DNA is packed in a
packing ratio of at least 6: 1 (Extended
length / Packed length) .
3. • Type A fibre is coiled to form a 200 – 350 A °
Type B fibre – Packing ratio 56 : 1 –
extensively folded both longitudinally and
transversely to form the chromatid.
• The folded fibre model applies to both the
interphase and metaphase chromosome.
5. Nucleosome Model
• Proposed by Roger Kornberg (1974)
• Electron microscopic studies have shown that
chromatin is formed of a series of repeating units –
Nucleosomes - bead like structures which are about
10 nm in diameter.
• Each Nucleosome contains a histone octamer
consisting of two of each of the four histones H2A,
H2B, H3 and H4 with about 200 base pairs of DNA
coiled on the outside of the nucleosome (Repeating
units of chromatin consisting of 200 bp of DNA coiled
around a histone octamer ).
7. • After extensive nuclease digestion, a 146
nucleotide pair long segment of DNA remains
present in each nucleosome. This nuclease –
resistant structure is called the nucleosome
core – It consists of a 146 – nucleotide pair
length of DNA and two molecules each of
histones H2A, H2 B, H3 and H4. The whole
structure has the shape of a flattened disc.
• The Histones protect the segment of DNA in
the nucleosome core from cleavage by
endonucleases.
9. • Each nucleosome is separated from the next
by a region of linker DNA (spacer DNA) which
is generally 54 bp (bp=base pair) long and
contains single H1 histone protein molecule.
• The formation of nucleosome converts a DNA
molecule into a chromatin thread about
one – third of its initial length.
10.
11. • The chains of nucleotides can
be folded into a 30nm fiber
forming a solenoid structure
having six nucleosomes per
turn.
• This structure is stabilized by
interactions between different
H1 molecules. The folding of
the nucleosome chain to a
solenoid structure results in
condensation of the chromatin.
The solenoid is further coiled
into a super solenoid structure
which results in extreme
condensation.
