Chromatin Structure and Function

Chromatin Structure and Function
Contents:
INTRODUCTION
HISTORY
Chemical composition
Replication of Chromatin
Packing of DNA
Functions of Chromatin

1. Welcome Ghodake Balkrushna D . 1 st Year M.Sc. Agril.Biotechnology Marathwada Agriculture University, Parbhani Maharashtra

2. Chromatin Structure and Function

3. Contents INTRODUCTION HISTORY Chemical composition Replication of Chromatin Packing of DNA DNA Functions of Chromatin

4. I ntroduction Chromatin: The complexes between eukaryotic DNA and proteins are called chromatin Or Substance of which chromosomes are composed. The chromatin was discovered by ‘Walther Flemming ’ in 1878. It typically contains about twice as much protein as DNA Originally named because of the readiness with which it stains with certain dyes.

5. History: 1878 – Chromatin discovered by ‘Walther Flemming ’ 1884 – Histones discovered by ‘Albrecht Kossel’ 1965 – Folded- Fibre Model prposed by ‘ Dupraw ’ 1974 – Nucleosome-Solenoid Model was proposed by ‘Kornberg and Thomas’ 1974 – Nucleosome is a basic structural unit of chromatin described by ‘Kornberg’.

6. Chemical composition: DNA : 30-40 % Histones (80%) Protein : 50-65 % RNA : 1-10 % Non-histones (20%) DNA + Histones = Chromatin

7. Chromatin consists of: • Histones ( H 1 ,H 2 a,H 2 b,H 3 and H 4 ) • Non-histone proteins • RNA (small amount)

8. Ave. M.W. : 10000 – 30000 Histones are highly heterogeneous class of proteins. The histones H2a, H2b, H3 and H4 are involved in the structural organization of chromatin fibers. The H1 histone holds together the folded fibers. Histone Type M. W. (Dalton) A. A. content H 1 23000 Lysine-rich H 2 a 14000 Slightly lysine-rich H 2 b 13800 Slightly lysine-rich H 3 15300 Arginine-rich H 4 11300 Arginine-rich

9. Types of Chromatin Heterochromatin (Inactive): It is a highly condensed form, it takes up deep stain during interphase and prophase, while lightly during metaphase. T hat  can silence gene transcription It constitutes telomeres, pericentric regions and areas rich in repetitive sequences A) Constitutive heterochromatin: Always condensed and thus inactive (found near centromeres and telomeres)

10. B ) Facultative heterochromatin: At times condensed and at others Uncondensed and actively transcribed appearing as euchromatin 2. Euchromatin: It is less condensed It takes up little stain during prophase, but deeply stained during metaphase It contains most actively transcribed genes

11. Difference between heterochromatin & euchromatin : Heterochromatin Euchromatin Represent darkly stained regions Lightly stained regions Contains few inactive genes Contains lot of active genes Covers small region of chromosome Larger region of chromosome Usually found near centromere & telomere Found in the middle of chromosome between centromere & telomere 2 types:- constitute & facultative Only 1 type Late replicating Normal replicating Usually no active part in transcription and 8.30nm fibre Plays active roles in transcription and 8.3-8nm fibre

12. Replication of Chromatin: During interphase, chromatin replicates,resulting in formation of two copies of each chromatin. In M-phase, chromatin condensation leads to the formation of metaphase chromosome consisting of two identical sister chromatids. Sister chromatids are held together at centromere, which is seen as constricted chromosomal region.

13. Chromatin : Nucleosomes are packed to form chromatin fibers and chromosome.

14. Packing of DNA DNA in eukaryotes

15. Nucleosome is simplest packaging structure of DNA that is found in all eukaryotic chromosomes. Core DNA is the DNA that is actually associated with the histone octamer . linker DNA- DNA that is between each histone octamer

16. Chromatin is comprised of histones and DNA:  200 bp  of DNA wraps around the 8 core histones to form the basic chromatin unit, the nucleosome.  1 ) Unpackaged DNA 2 ) DNA wrapped around  histone  octamers to form  nucleosomes 3 ) Nucleosomes compacted into a chromatin fiber

17. 147 bp  of DNA wraps around the 8 core histones to form the basic chromatin unit, the nucleosome.

18. Functions of Chromatin: To package the DNA into small volume efficiently The packaging of DNA into chromatin allows for mitosis and meiosis To Prevent chromosome breakage To Control gene expression and gene regulation