Chromatin Structure and Function
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This document discusses the structure and function of chromatin. It begins with a history of chromatin discovery from 1878-1974. Chromatin is composed of DNA, histones, and non-histone proteins. There are two types of chromatin - heterochromatin, which is condensed and inactive, and euchromatin, which is less condensed and contains actively transcribed genes. Chromatin replicates during interphase and condenses further during mitosis. DNA is packaged into nucleosomes, which involve 147 base pairs of DNA wrapped around an octamer of histone proteins, and these compact to form chromatin fibers and chromosomes. The functions of chromatin include efficient DNA packaging, facilitating cell division, preventing chromosome breakage, and regulating gene expression.
Chromatin Structure and Function
- 1. Ghodake Balkrushna D. 1st Year M.Sc. Agril.Biotechnology Marathwada Agriculture University, Parbhani Maharashtra
- 3. CONTENTS INTRODUCTION HISTORY Chemical composition Replication of Chromatin Packing of DNADNA Functions of Chromatin
- 4. Introduction 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 (H1,H2a,H2b,H3 and H4) • 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 H1 23000 Lysine-rich H2a 14000 Slightly lysine-rich H2b 13800 Slightly lysine-rich H3 15300 Arginine-rich H4 11300 Arginine-rich
- 9. Types of Chromatin 1. Heterochromatin (Inactive): It is a highly condensed form, it takes up deep stain during interphase and prophase, while lightly during metaphase. That 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
- 12. 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
- 13. 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.
- 15. Chromatin: Nucleosomes are packed to form chromatin fibers and chromosome.
- 16. Packing of DNADNA in eukaryotes
- 17. 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
- 18. • 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
- 19. • 147 bp of DNA wraps around the 8 core histones to form the basic chromatin unit, the nucleosome.
- 20. 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