NUCLEOSOME MODEL OF CHROMOSOME

Basics of Undergraduate/university fellows

Nucleosome model of chromosome is proposed by ROGER KORNBERG (son of Arthur
Kornberg) in 1974.
 It was confirmed and crystalised by P. Oudet et al., (1975).
 Nucleosome is the lowest level of Chromosome organization in eukaryotic cells.
 Nucleosome model is a scientific model which explains the organization of DNA and
associated proteins in the chromosomes.
 Nucleosome model also explains the exact mechanism of the folding of DNA in
thenucleus.
 It is the most accepted model of chromatin organization.

1.  The chromosomes are the nuclear components of the special organization,
individuality, and function that are capable of self-reproduction and play a vital role
in heredity, mutation, variation and evolutionary development of the species.
 A human cell (diploid) contains approximately 6.4 billion base pairs in 23 pairs (2n =
46) of chromosomes.
 The total length of DNA of a single human cell is approximately 2.2meters long (when
all 46 DNA strands are joined end to end).
 The size of the nucleus in which the chromatin situated is about 10 μm in diameter.
Thus, itis evident that the 2.2 m long DNA should fold several times to fit in the
nucleus of 10μm diameter.
 Each chromosome contains a single linear segment of DNA, tightly coiled many times
around proteins that support its structure.
NUCLEOSOME MODEL OF CHROMOSOME
 Nucleosome model of chromosome is proposed by ROGER KORNBERG (son of Arthur
Kornberg) in 1974.
 It was confirmed and crystalised by P. Oudet et al., (1975).
 Nucleosome is the lowest level of Chromosome organization in eukaryotic cells.
 Nucleosome model is a scientific model which explains the organization of DNA and
associated proteins in the chromosomes.
 Nucleosome model also explains the exact mechanism of the folding of DNA in
thenucleus.
 It is the most accepted model of chromatin organization.
FEATURES OF THE NUCLEOSOME MODEL OF CHROMOSOMES
 Nucleosomes are the fundamental packing unit particles of the chromatin and
give chromatin a “beads-on-a-string” appearance in electron micrographs taken
after treatments that unfold higher-order packing.
 In eukaryotes, DNA is tightly bound to an equal mass of histones, which serve to
form a repeating array of DNA -protein particles, called nucleosomes. If it was
stretched out, the DNA double-helix in each human chromosome would span the cell
nucleus thousands of time.
 Histones play a crucial role in packing this very long DNA molecule in an orderly way
(i.e., nucleosome) into nucleus only a few micrometres in diameter. Thus,
nucleosomes are the fundamental packing unit particles of the chromatin and give
chromatin a “beadson- a-string” appearance in electron micrographs.
 Each nucleosome is a disc-shaped particle with a diameter of about 11 nm and 5.7
nm in height containing 2 copies of each 4 nucleosome histones–H2A, H2B, H3, and
H4.
 The histone octamer forms a protein core [(i.e., a core of histone tetramer (H3, H4)
and the apolar regions of 2(H2A and H2B)] around which the double-stranded DNA
helix is wound 1¾ time containing 146 base pairs.
 The DNA helix is wrapped as super helical left handed turn around this histone
octamer core.
 In chromatin, the DNA extends as a continuous thread from nucleosome to
nucleosome.
 Each histone core is encircled by 1.8 turns of DNA, represents about 146 base pairs.

2.  The H1 histone stays outside the histone octamer. The H1 histone plays very
important role in the formation of the 30-nm fiber. The linker DNA regions in 30-nm
structure are variably bent and twisted to attain the folding pattern.
 H1 histone molecule has an evolutionarily conserved globular core or central region
linked to extended amino-terminal and carboxyl-terminal “arms”, whose amino acid
sequence has evolved much more rapidly.
 Each H1 molecule binds through its globular portion to a unique site on a
nucleosome and has arms that are thought to extend to contact with other sites on
the histone cores of adjacent nucleosomes, so that the nucleosomes are pulled
together into a regular repeating array.
 The binding of H1 molecule to chromatin tends to create a local polarity that the
chromatin otherwise lacks.
 Each nucleosome bead is separated from the next by a region of linker DNA which is
generally 54 base pair long and contains single H1 histone protein molecule.
 Adjacent nucleosomes are connected by a short stretch of DNA called linkerDNA.
Linker DNA is about 10 to 80 bp in length.
 Each nucleosome bead is separated from the next by a region of linker DNA which is
generally 54 base pair long and contains single H1 histone protein molecule.
 Generally, DNA makes two complete turns around the histone octamers and these two turns
(200 bp long) are sealed off by H1 molecules. (Note : In some organisms nucleosome DNA may
vary from 162 base pairs (e.g., rabbit cortical neurons) to 242 base pairs (e.g., sea urchin
sperm); Reid and Leech, 1980).
 Thus, on an average, nucleosomes repeat at intervals of about 200 nucleotides or
base pairs.
 On average, nucleosomes repeat at intervals of about 200 nucleotides or base pairs.
 DNA in the chromatin attain a packing ratio of about 7:1 (seven fold packing) by the
formation of nucleosomes
 The 10-nm fibre: When nucleosomes are in close apposition, they form the 10-nm
filaments, in which packing of DNA is about five-to seven-fold, i.e., five to seven
times more compact than free DNA. Nucleosome units organized into more compact
structure of 30 nm in diameter called 30 nm fibers (proposed by Rachel Horowitz &
Christopher Woodcock in 1990).
 The 30-nm fibre: the chromatin is seen to be in the form of a fibre, with a diameter
of about 30 nm. Such 30-nm fibres can be observed in metaphase chromosomes and
in interphase nuclei and it probably represents the natural Confo rmation of
transcriptionally inactive chromatin. The 30-nm fibre consists of closely packed
nucleosomes.
 Radial loops of 30-nm fibre and metaphase chromosome : The probable nature of
one further level of folding seem to contain a series of looped domains - loops of
chromatin that extend at an angle from the main chromosome axis.
 In principle, looped domains in chromatin could be established and maintained by
DNA binding proteins that clamp two regions of the 30- nm fibre
 Structural non-histone proteins could be involved in organizing the 30-nm fibres into
loops.
 A central core of scaffold 300nm surrounded by a halo made of hoops of DNA. The
scaffold is made of non-histone proteins and retains the general shape of the
metaphase chromosome.
 Each chromosome has two scaffolds, one for each chromatid, and they are connected
together at the centromere region.
 The final level of packaging is characterized by the 700 nm followed by 1400nm
structure seen in the metaphase chromosome. The condensed piece of chromatin has

3. a characteristic scaffolding structure that can be detected in metaphase
chromosomes. This appears to be the result of extensive looping of the DNA in the
chromosome.