The document describes the nucleosome solenoid model of DNA packaging in eukaryotic cells. It proposes that DNA wraps around histone proteins to form nucleosomes, which then further condense to form a solenoid fiber in a "beads on a string" structure. This allows the long DNA molecule to tightly pack into the small nucleus. DNA is first packaged into nucleosomes, then into solenoid fibers and further into higher order structures like chromatin and chromosomes.

1. NUCLEOSOME SOLENOID
MODEL
SUBMITTED BY –
Barsha Rani Barik,20C/15
Group- A, B.Sc. (Ag)

2. Introduction
 It was proposed by R.D.Kornberg and O.Thomas in 1974 and
was confirmed and coined by P.Oudet in 1975.
 Best accepted model for DNA packaging.
 Also called Nucleosome model/Oudet concept of chromatin
structure/beads on a string model
 Appears as “BEADS ON A STRING” of chromatin fibre under
electron microscope where DNA coils around Histone proteins.
• This model says that DNA is tightly bound to histone proteins
which serve to form a repeating array of DNA-Protein particles
called nucleosomes and further condensing makes it fit into the
small chromosome structure .

3. Why DNA packaging is required?
• DNA is about 2 m long & it has to
packed in a nucleus which is few
micrometer in diameter.
• Hence, highly coiled structure is
required
• There are various order of packaging
1. Nucleosome
2. Solenoid fibre
3. Scaffold loop
4. Chromatin
5. Chromosome

4. NUCLEOSOME
• Simplest packing str. Of DNA
• 146 bp DNA wrapped around the histone octamer in
1.75 turns.
• Octamer= 2 copies of 4 core histones(H2A,H2B,H3,H4)
• DNA length varies b/w species
• Core DNA= DNA associated with histone octamer
• Linker DNA= DNA b/w histone octamer-length varies
from 8 to 114 bp.
• H1 Histone is not part of the nucleosome core but
attached to nucleosome to strengthen it.

7.  So,there are 2 major types of proteins associated with DNA in the
chromatin. i.e, HISTONES and NON-HISTONES.
HISTONES-
 Most abundant proteins associated with the chromosomes
 Very rich in basic proteins i.e, lysines and arginines(+ve charge) and
lacks tryptophan.
 At normal pH of the cell they have net +ve charge that facillitates their
binding to the -vely charged DNA.
 Play structural role in the packaging of DNA
NON-HISTONES-
 These are the proteins req. at the packaging of chromatin at higher
level .
 Associated with DNA other than histones
 Acidic in nature & -vely charged and likely to bind with positively
charged histones .

8. First order of packaging :
NUCLEOSOME
Short segment of DNA
helix
Second order of packaging:
SOLENOID FIBRE
SCAFFOLD LOOP
CHROMATID
CHROMOSOME

9. CONCLUSION
 Chromatin is the complex combination of DNA and proteins that organizes
chromosomes which appear as many thread like coiled and elongated
structures suspended in the nucleoplasm. So the chromatin contains genetic
material instructions to direct cell function.
 The first level of packing in Chromatin involves the binding of DNA to
histones into fundamental packing unit particles called nucleosomes.
 The second level of packing involves packing of nucleosomes into 30 nm
thick chromatin fibre .
 The highest level of packing of chromatin in the chromosome is found at the
metaphase stage of cell division.
 There are 2 distinct types of chromatin- Euchromatin and Heterochromatin
which differ on their staining properties.
 In the chromatin, DNA and basic proteins called histones are present in
about equal amounts.
 DNA is the permanent component of chromosomes and is the sole genetic
material of eukaryotes.

10. Thank you