Chromosomal organisation is necessary process occur in all eukaryotic cells

1. EUKARYOTIC CHROMOSOMAL
ORGANIZATION
Mrs. Praveen Garg
VITS College, Satna

2. INTRODUCTION
• Eukaryotic chromosome consist of one linear, unbroken double
stranded DNA molecule.
• It is occur in nucleus. It consist of DNA and protein molecules.
This protein and DNA complex is known as chromatin.
• The total amount of DNA in eukaryotes is greater than bacterial
cells.
• It is highly compact structure.
• Eukaryotic chromosome structure refers to the levels of
packaging from the raw DNA molecules to
the chromosomal structures seen
during metaphase in mitosis or meiosis.

4. CHROMATIN
• Chromatin is a substance within a chromosome consisting of
DNA and protein.
• The major proteins in chromatin are histones, which help in the
packaging of DNA in a compact form.
• During cell division, chromatin fibres condensed into
chromosome, which can be easily visible in anaphase.
• There are mainly two levels of chromatin organization:
• Euchromatin: DNA wraps around histone proteins,
forming nucleosomes complex and also called "beads on a string"
structure.
• Heterochromatin: Multiple histones wrap into a 30-
nanometer fibre consisting of nucleosome arrays in their most
compact form.

6. HETEROCHROMATIN
• Heterochromatin is a tightly packed form of DNA or more
condensed.
• Because it is tightly packed, therefore it can not be transcribed by
RNA polymerase.
• Heterochromatin appears as small, darkly staining (dark band),
irregular particles scattered throughout the nucleus .
• Heterochromatin is usually localized to the periphery of
the nucleus.
• Heterochromatin mainly consists of genetically inactive satellite
sequences.
• Centromeres and telomeres are made up of heterochromatin.
• It is highly rich in AT sequence.

7. Heterochromatin is classified in to two groups.
1. Constitutive
2. Facultative
 Constitutive heterochromatin: It remains permanently in
the heterochromatic stage. it does not revert to the
euchromatin stage.
 Facultative heterochromatin: It can revert into
euchromatin that takes on the light staining.

8. • Euchromatin is a packed form of chromatin (DNA, RNA,
and protein) that is enriched in genes, and undergoes
active transcription.
• Euchromatin is the most active portion of the genome within
the cell nucleus.
• 92% of the human genome is euchromatic and highly rich in GC
content.
• The structure of euchromatin is an unfolded set of beads along a
string, whereas these beads represent nucleosomes.
• Euchromatin appears as light-colored bands after staining and
observed under an optical microscope.
• Euchromatin participates in the active transcription of DNA
to mRNA products. Not all euchromatin is necessarily
transcribed.
EUCHROMATIN

9. PROTEINS
• Chromatin consist of
protein apart from DNA.
• Mainly two type of protein
present with chromatin.
 Histone protein
 Non Histone protein

10. HISTONE
• Histones are highly basic
proteins found in eukaryotic cells.
• Its packaged with DNA
structural units called nucleosomes.
• Histones are abundant in lysine and arginine.
• Histone are the main protein components of chromatin, acting
as spools around which DNA winds, and allow to replication
and play a role in gene regulation.
• Without histones, the unwound DNA in chromosomes would be
very long (a length to width ratio of more than 10 million to 1
in human DNA).
• Histone protein consist of five units such as H1/H5, H2A, H2B,
H3, H4.

11. • H2A, H2B, H3 and H4 - Core
histones.
• H1/H5 - linker histones.
• The core histones all exist
as dimers, the resulting four
distinct dimers then come
together to form one
octameric nucleosome core,
(a solenoid (DNA)-like
particle).
• The linker histone H1 binds
the nucleosome at the entry
and exit sites of the DNA, thus
locking the DNA into place.
• It is responsible for eukaryotic
chromosomal organisation.

12. NON HISTONE
• Non histone proteins are acidic in nature.
• The non‐histone chromatin proteins are a heterogeneous group
of proteins.
• It act as a part of large multi subunit complexes, playing
important roles in regulating many processes such as
nucleosome remodeling, DNA replication, RNA synthesis and
processing, nuclear transport.
• Non-histone may interact with the nucleosomal structure of
chromatin and to produce specific regulatory effects.
• It is higher molecular weight component.
• Example: DNA polymerase

13. Core proteins are separated
from one another by linker
DNA.
Non histone protein
associated with the linker
DNA and few appear to bind
directly to core particles.
It helps in chromosal
packaging.

14. • Nucleosome model is a scientific model which explains the
organization of DNA and associated proteins in the
chromosome.
• It also explains the exact mechanism of the folding of the
DNA in the nucleus.
• The model was proposed by Kornberg in 1974 and is the
most accepted model of chromatin organization.
• It was confirmed and christened by P. Oudet et al., (1975).
• The packaging of DNA into nucleosomes shortens the fiber
length about sevenfold.
NUCLEOSOME MODEL

15. • A nucleosome is a section of DNA
that is wrapped around a core of
proteins.
• The nucleosome is the
fundamental subunit of
chromatin.
• Each nucleosome is composed of
a less than two turns of DNA
wrapped around a set of eight
proteins called histones, which
are known as a histone octamer.
• DNA and histones are packed
together to be nucleosome,
nucleosome form a pack which
are called chromatin, two
chromatin form a chromosome.

16. • Histones undergo posttranslational epigenetic
modifications that alter their interaction with DNA and
nuclear proteins.
• The H3 and H4 histones can be modified at several places.
• The core of the histones H2A and H2B can also be modified.
• Modifications of the histone
include methylation, acetylation, phosphorylation, and ADP-
ribosylation (Me: methyl, P: phosphate, Ac: acetyl).
• Histone modifications act in diverse biological processes such
as gene regulation, DNA repair, chromosome condensation
(mitosis) and spermatogenesis (meiosis).
HISTONE MODIFICATION

17. • Histones can be displaced
by chromatin
remodeling complexes,
thereby transcription and
translation process
continue by DNA
polymerases and other
enzymes.
• These processes are
reversible, so modified or
remodeled chromatin can
be returned to its compact
state after transcription
and/or replication are
complete.

19. Histone Acetylation/Deacetylation
• Histone acetylation occur by the addition of an acetyl group from
acetyl coenzyme A.
• Enzymes used for the acetylation are called
histone acetyltransferases (HATs).
• The histone acetylation is involved in the regulation of many
cellular processes such as chromatin dynamics and transcription,
gene silencing, cell cycle progression, apoptosis, differentiation,
DNA replication, DNA repair etc.
• Histone deacetylaces (HDACs) catalyze the hydrolytic removal of
acetyl groups from histone lysine residues.
• Histone acetylation/deacetylation impacts chromatin structure
and gene expression.
• Acetylation of lysine residues leads to a transcriptionally active
chromatin structure (euchromatin) and deacetylation leads to an
inactive, condensed chromatin structure (heterochromatin).

21. Histone Methylation/Demethylation
• Histone methylation is defined as the transfer of one, two, or
three methyl groups to lysine or arginine residues of histone
proteins by histone methyltransferases (HMTs).
• In the cell nucleus, when histone methylation occurs, specific
genes within the DNA complexed with the histone may be
activated or silenced.
• Arginine methylation of histones H3 and H4 promotes
transcriptional activation and is mediated by a protein arginine
methyltransferases (PRMTs).
• Histone demethylation is the removal of methyl groups in
modified histone proteins via histone demethylases.
• Histone methylation and demethylation are epigenetic
modifications that have the power to reduce or booster gene
expression, especially as a result of altering chromatin structure.

22. • Histone phosphorylation occur by the
addition of phosphate group and
dephosphorylation occur by removal of
phosphate group.
• Histones can be phosphorylated by protein
kinases and dephosphorylated
by phosphatases.
• Histone phosphorylation can occur on
serine, threonine and tyrosine residues.
• Histone phosphorylation plays main roles
in chromatin remodeling.
• Phosphorylation of H2A is an important
histone modification that plays a major
role in DNA damage response,
transcription regulation, mitosis and
apoptosis, this modification takes place on
serine residue.
Histone Phosphorylation

23. THANK
YOU