3. INTRODUCTION
• The term Heterochromatin and Euchromatin was coined by Emil Heitz in 1928.
• Heterochromatin and Euchromatin are the parts of the chromatin.
• DNA protein complex found in the eukaryotes.
• These were take part in the protection of DNA inside the nucleus.
4. INTER-PHASE CHROMATIN
• Interphase chromatin refers to the overall structure of chromatin during the interphase
stage of the cell cycle, which includes G1 (gap phase 1), S (synthesis phase), and G2 (gap
phase 2). During interphase, chromatin appears as a diffuse, granular network throughout
the nucleus. It is not condensed into visible chromosomes and is actively involved in
various cellular processes such as transcription, DNA replication, and repair. Interphase
chromatin is the least condensed form of chromatin.
• Chromatin is composed of DNA and proteins, mostly basic proteins called histones
• That help chromatin fold so it can pack into the tiny volume of a cell’s nucleus.
• The two basic types of chromatin are
1. Euchromatin
2. Heterochromatin
5. EUCHROMATIN
• The chromatin fibres in this region are loosely coiled .
• Euchromatin undergoes the normal process of condensation and de
condensation in the cell cycle.
• Euchromatin constitutes the majority of the chromosomal material
and is where most transcription takes place.
6. HETEROCHROMATIN
• The chromatin fibres in this region are more tightly folded
• Heterochromatin remains in a highly condensed state throughout the cell
cycle, even during interphase.
• All chromosomes have heterochromatin at the centromeres and telomeres.
• In addition to remaining condensed throughout the cell cycle,
heterochromatin is characterized by a general lack of transcription.
7. HISTONES
Most eukaryotic cells contain five different kinds of histones: H1, H2A, H2B,
H3,and H4.
These are extremely abundant proteins; the mass of histones in eukaryotic
nuclei is equal to the mass of DNA.
8. NUCLEOSOMES
• The total length of human DNA, if stretched out, would be about 2 m, and this all
has to fit into a nucleus only about 10 mm in diameter.
• In fact, if you laid all the DNA molecules in your body end to end, they would
reach to the sun and back hundreds of times.
• DNA folding must occur in your body and in all other living things. We will see
that eukaryotic chromatin is indeed folded in several ways.
• Eukaryotic DNA combines with basic protein molecules called histones to form
structures known as nucleosomes. These structures contain four pairs of core
histones (H2A, H2B, H3, and H4) which wrapped a stretch of about 146 bp of
DNA.
9. 30-NM FIBER
• Nucleosomes fold on themselves to form a dense, tightly packed
structure that makes up a fiber with a diameter of about 30 nm .
• Two different models have been proposed for the 30-nm fiber:
• A solenoid model, in which a linear array of nucleosomes are coiled,
• Helix model, in which nucleosomes are arranged in a zigzag ribbon that
twists or supercoils. Recent evidence supports the helix model
10. CONCLUSION
• From these of the chromatin information and structures and types Euchromatin,
Constitutive heterochromatin and Intercalary heterochromatin, presumably the
only chromatin involved in transcription is Euchromatin.
• Constitutive heterochromatin surrounds the centromere and is rich in satellite
DNA. Intercalary heterochromatin is dispersed. Thus it becomes apparent that
the Eukaryotic chromosome is a relatively complex structure.
11. REFERENCES
• Authored by G.S Miglani (2002), “Advance Genetics”, Narosa Publications. Vol 1
Page 520.
• Authored by Van Steensel B (2011), “Chromatin constructing the big picture”. Vol
1 Page 155.
• Authored by Elagin S.C (1996),“Heterochromatin and gene regulation in
drosophila”. Vol 1 Page 193-202.
