2. Contents
• Introduction
• Heterochromatin- Structure And Function
• Euchromatin – Structure And Function
• DNA Packing
• Chromatin Structures
• Difference Between Euchromatin And
Heterochromatin
3. Introduction
• Term coined by = Emil Heitz(1928)
• Chromatin consists of Hetrochromatin and
Euchromatin.
• In eukaryotes this DNA protein complex is
observed.
• These were take part in the protection of DNA
inside the nucleus.
7. Heterochromatin
• It is a tightly packed form of DNA in the nucleus.
• So compactly organised that they are inaccessible
to the protein involved in gene expression.
• It is of two types-
Facultative Heterochromatin and Constitutive
Heterochromatin.
• It has high DNA density
• Present in Eukaryotes only
• It contains Sticky regions
• No or little transcriptional activity
8. CONSTITUTIVE HETEROCHROMATIN
• These are regions of DNA found throughout
the chromosomes of eukaryotes.
• Heterochromatin is found at the peri-
centromeric regions of chromosomes but is
also found at the telomeres and throughout the
chromosomes.
• It has structural function and made up of
satellite DNA
9. FACULTATIVE HETEROCHROMATIN
• Consists of euchromatin that takes on the
staining and compactness characteristics of
heterochromatin during same phase of
development.
• The inactive x-chromosomes is made up of
facultative heterochromatin.
• It may convert to euchromatin depending upon
requirement.
10. Euchromatin
• Loosely packed form of chromatin.
• Active during the transcription.
• Contains around 90% of the entire human
genome. Housekeeping genes are one of the
forms of euchromatin.
• It contains low DNA densitty
• Found in Prokaryotes as well as eukaryotes
• Early replicative
• Regions are not sticky
11. Euchromatin Structure
• Before understanding the structure of
euchromatin, we should comprehend the different
ways in which DNA is packaged in cells.
• Briefly, euchromatin (also known as the beads-
on-a-string structure) is composed of DNA
helices that are condensed at intervals into
nucleosomes.
• Nucleosomes are the basic unit of chromatin and
they consist in packaged complexes containing
histone proteins around which DNA is wrapped,
i.e. nucleosomes are made of DNA coiled
around histones.
12. • The DNA that connects nucleosomes is known as the
linker DNA.
• Heterochromatin is euchromatin that has been more
densely packed into 30-nm fibers.
• During interphase, heterochromatin is packaged into
denser structures—active chromosomes, which are
further condensed into denser structures during
mitosis and meiosis—metaphase chromosomes.
• An image of the different chromatin structures can be
seen here:
13.
14. • Despite being actively researched, the structure of
chromatin is still poorly understood although it seems
that the cycle in which the cell is at a certain time
determines the structure of chromatin.
• In this conformation, euchromatin is loose and
consequently leaves the linker DNA exposed so that it
can be transcribed; this way, RNA and DNA
polymerases as well as other proteins can access the
DNA. Because of its loose structure, euchromatin is
difficult to see under a microscope and appears
faintly when stained—in contrast to the easily visible
heterochromatin, which is densely packed.
15. • It has been hypothesized that the regulation of the
chromatin structure is a way to control gene expression.
It is believed that the euchromatic structure is present
when genes are turned on, that is, when they are being
actively transcribed, while the heterochromatic
structure is present when genes are turned off or
inactive.
• Euchromatin is present in transcriptionally active cells
because of the accessibility to the DNA, folding into
heterochromatin may be a way to regulate transcription
by preventing the access of RNA polymerases and other
regulatory proteins to the DNA.
• In this line, housekeeping genes, for instance, are
always in the euchromatic conformation because they
need to be constantly replicated and transcribed to keep
the functional activity and survival of the cells.
16. DNA PACKAGING
• Electronic micrographs show unfolded
chromatin and they look like beads on a string.
• These beads are referred to as nucleosomes
(The basic unit of DNA packing) and the string
is DNA.
18. • The nucleosome is a piece of DNA wound around a protein
core.
• The DNA histone association remains intact throughout the
cell cycle.
• Histone only leave the DNA very briefly during DNA
replication.
• Enzymes that add acetyl groups to histones are
called histone acetyltransferases (HATs) while those that
remove acetyl groups are called histone deacetylases
(HDACs).
• Enzymes that add methyl groups are called histone
methyltransferases (HMTs).
• Activity of these enzymes affects whether or not regions of
DNA are tightly packed, and unable to transcribe, or are
loosely packed and therefore, highly transcribed.
20. DNA packaging
DNA packaging is the process of tightly packing up the
DNA molecule to fit into the nucleus of a cell.
Why is DNA packaging required?
• The length of the DNA is around 3 meters which needs
to be accommodated within the nucleus, which is only a
few micrometres in diameter. In order to fit the DNA
molecules into the nucleus, it needs to be packed into
an extremely compressed and compact structure called
chromatin.
• During the initial stages of DNA packaging, the DNA is
reduced to an 11 nm fibre that denotes approximately
5-6 folds of compaction. This is achieved through a
nucleosome order of packaging.
21. Orders of DNA packaging
• First order of DNA packaging – Nucleosome.
• Second order of DNA packaging – Solenoid
fibre.
• Third order of DNA packaging – Scaffold loop
Chromatids Chromosome.
22. Solenoid fibre.
•Nucleosome and linker DNA together
constitute chromatosome.
•Nucleosome chain gives a ‘beads on
string’ appearance under an electron
microscope.
•The nucleosomal organisation in about
10 nm in thickness, which gets further
condensed and coiled to produce a
solenoid of 30 nm diameter.
•This solenoid structure undergoes further
coiling to produce a chromatin fibre of
30-80 nm and then, a chromatid of 700
nm.
•Chromatin is held over a scaffold of non-
histone chromosomal or NHC proteins.
