• “Chromosomes are thread like structures;
appear at the time of cell division, in the
Chemical composition of
• A significant amount of RNA is also associated
with chromosome because these are the sites
of RNA synthesis.
Deoxyribonucleic acid (DNA):
• The main genetic constituent of the cells is
DNA. It coded information from cell to cell and
organism to organism. DNA is complexed with
proteins in a structure called Chromatin.
• It is about 40% of chromosomes.
• It is about 60% of the chromosome. Histone
proteins are present in chromosome. Histones
are positively charged due to abundance of
positive amino-acids, arginine and lysine, on
Condensed and non-condensed portions:
• These are highly condensed portions of
• These are portions other than
• Each chromatid is made up of many coils called
• Turned fibers present in supercoils are called as colis,
which are in actual case chromatin fibers. This coiling
helps DNA to be present in small space of nucleus.
• It is basic unit of chromosome or chromatin fiber. It is
DNA; duplex is coiled around a core of eight histone
• Nucleosomes are repeated after every 200
hundred nucleotides. Positively charged
histones are linked with negative charged
phosphate groups of DNA. The histone cores
thus act as magnetic forms that promote and
guides the coiling of DNA.
Histones or Histone proteins
• These are highly alkaline proteins found
in eukaryotic cell nuclei that package and
order the DNA into structural units called
• Histones are a group of basic proteins that
associate with DNA and help the DNA to
condense it into chromatin.
• Some histone proteins function as spools for the
thread-like DNA to wrap around.
• Chromatin, under the microscope in its extended
form, looks like beads on a string. These beads
are called nucleosomes.
• Each nucleosome is composed of DNA wrapped
around eight histone proteins, functions like a
spool and called a histone octamer.
• Each histone octamer is made of two copies each
of the histone proteins H3, H4, H2A, and H2B.
• The nucleosomes are then wrapped, into a 30 nm
spiral, called a solenoid, where additional H1
histones are associated with each nucleosome to
maintain the chromosomal structure.
• When salt concentration is higher, the
nucleosome bead necklace gradually assumes a
coiled form, a solenoid.
• In fact, histone proteins could act as gatekeepers
to the DNA, determining which portions of the
DNA were available for protein expression.
Classes of Histones:
• There are two main classes of Histones:
• Core Histones
• Linker Histones
In core histones following families are included
• Two of each of these core histone proteins assembles to
form one octameric nucleosome core particle, and
147 base pairs of DNA wrap around this core particle.
Linker histone included:
• The linker histone protein H1 binds the
nucleosome at the starting and ending sites of
the DNA, thus locking the DNA into place and
help in the formation of higher order structure.
• H5 histiones are individual proteins involve in the
packaging of specific region of DNA.
Packaging of Histones
• In the core of nucleosomes the two dimers
H2A and H2B and two tetramers H3 and H4
are involve and form the tertiary structure.
• Above mention histones are relatively similar
Histone interactions with DNA:
There are following types of interactions:
• Hydrogen bonds between the backbone of the
DNA and the amide group on the main chain of
• Non-polar interactions between the histone
proteins and deoxyribose sugars on DNA
• Salt bridges and hydrogen bonds between basic
amino acids which are actually the side chains
(especially lysine and arginine) and phosphate
oxides on DNA.
• Highly active genes have less histone while
non-active genes have highly linked with
histones during interphase. Histone proteins
has a highly positively charge on N-terminus
having lysine and arginine residues.
Types of modification in Histones:
• Histones can be changed to alter how much
packing the DNA is capable of. There are many
modifications that affect how well DNA is
• The three main types of modifications can be
seen in the following table:
• Normally histones are positively charged but
with the modification of methylation it
becomes hydrophobic which enable the
histone to more tightly pack.
• Acetylation and phosphorylation make the
histone more negative which weakens the
packing ability of histones due to the repelling
of negative-negative charges.
• It is complex nucleic acid and protein which
condenses to form chromosome during cell
• In eukaryotes it is found within cell nucleus
whereas in case of prokaryotes it is present in
• It can easily recognize through staining
therefore its name, literally means colored
• To package DNA into smaller volume so that
they fit easily into the cell.
• It strengthens DNA to allow mitosis and
• To control expression and DNA replication, it
serves as a mechanism.
• It is the tightly packed form of DNA, which
comes in different varieties.
• These varieties come between the two
continuous extremes of constitutive and
• Their function is in gene expression.
• It is not active and under specific
environmental and developmental signaling,
loses its condensed structure and become
• Centromere and telomere both are
heterochromatin as in the Barr body off,
second inactivated X chromosome in female.
• Gene regulation and protection of
• Dense packing of DNA makes less accessible to
protein factors that bind with DNA or its
• It results in formation of epigenetic
• All of its cells pack in the same region of DNA
so any gene in all cells would be poorly
expressed.i.e.1, 9, 16 and Y human
chromosome contain large region of
• In most organisms, it is present around
chromosome centromere and near telomeres.
• Constitutive chromatin affect the nearer genes
and usually repetitive and form centromere or
telomeres in addition to acting as an attractor
for gene expression and repression signals
• Formation of facultative heterochromatin is
regulated and associated with morphogenesis or
differentiation.i.e. X chromosome inactivation in
• facultative chromatin is the result of genes
silenced in a mechanism of histone methylation.
• One X chromosome is packs as silences in case of
facultative and other cell packed as euchromatin
• It is lightly packed form of DNA, under active
transcription with rich in gene concentration.
• 92% of human genome is euchromatic.
• Active transcription of DNA to mRNA products.
• Its unfolded structure allows the gene
regulatory proteins and RNA polymerase to
bind with DNA sequence so to initiate the
These are highly condensed portions of
These are loosely packed portions of
These are darkly stained regions of
They are lightly stained regions of
They remain permanently condensed.
It is condensed only during cell division when
compact packaging facilitates the movement of
Their DNA is never exposed.
At time other than division, it is present in open
configuration and its genes can be expressed.
It is found in Eukaryotes.
It is found in Prokaryotes and Eukaryotes both.
It is genetically inactive form of
It is genetically active form of chromatin.
It replicates late.
It is earlier replicative