Comprehensive Overview of Eukaryotic Genome Structure and Chromatin Organization

Comprehensive Overview of Eukaryotic Genome Structure and Chromatin Organization

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  EUKARYOTIC GENOME PRESENTED BY Dr.Shri Ram Saini P- 2766 Ph.D. Scholar Division of Animal Genetics, ICAR-IVRI, Izzatnagar, Bareilly (India)
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  What Is theGenome? 1 4 2
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  • Repetitive DNAshows faster renaturation/reassociation than nonrepetitive DNA. • Prokaryotes contain mainly nonrepetitive DNA, while lower eukaryotes have mostly nonrepetitive DNA. • In higher eukaryotes, repetitive DNA may constitute up to ~50% of the genome; in some plants and amphibians, up to ~80%. • The length of the repetitive DNA component generally increases with overall genome size. • Repetitive DNA is broadly classified into moderately repetitive and highly repetitive DNA based on renaturation kinetics. Repetitive Sequences
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  • Consist ofvery short DNA sequences (typically <100 bp) repeated many times, often in tandem arrays. • Separated from bulk DNA by CsCl density-gradient ultracentrifugation, based on differences in buoyant density. Highly Repetitive Sequences
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  Tandemly Repeated DNA •Repeat units are arranged next to each other in continuous arrays. • Variable Number Tandem Repeats (VNTRs) • Minisatellites • Microsatellite s
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  • Genome-wide (interspersed)repeats – Repeat units are scattered throughout the genome in a relatively random pattern. • Short Interspersed Nuclear Elements (SINEs) and Long Interspersed Nuclear Elements (LINEs) are dispersed throughout the genome rather than arranged as tandem repeats. Interspersed repeats
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  Chromatin • Complex combinationof DNA and proteins that makes up chromosomes. • The major proteins associated with chromatin are histone proteins, although several other chromosomal proteins also play important roles. EUKARYOTIC GENOME COMPOSITION • DNA (20–40%): It is the most important chemical constituent of chromatin. • RNA (5–10%): Associated with chromatin in the form of rRNA, mRNA & tRNA • Proteins (55–60%) 1. Histone Proteins • Constitute about 60% of the total chromatin proteins & Present in an almost 1:1 ratio with DNA. Five types of histones: H1 H2A H2B H3 H4 • 2. Non-Histone Proteins • Constitute about 20% of the total chromatin proteins. Chemical Composition of Chromatin
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  Models of ChromatinOrganization • Multi-strand model • Folded fiber model •Nucleosome model (R. D. Kornberg & O. Thomas, 1974 – widely accepted) Nucleosome Model: DNA is tightly bound to histone proteins, forming a repeating array of DNA–protein particles called nucleosomes. Three Levels of Organization 1. Nucleosome Level (Beads-on-a-String) •DNA wraps around histone proteins to form nucleosomes. •Produces the "beads-on-a-string" structure (10 nm fiber). 2. 30 nm Chromatin Fiber (Solenoid Structure) •Nucleosome arrays coil together to form a 30 nm condensed chromatin fiber. 3. Higher-Order Chromatin Packaging •Chromatin folds into loops, domains, and scaffold structures. •These higher levels of packaging condense into the metaphase chromosome. Ultrastructure and Organization of Chromatin
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  Chromatin Modifications Chromatin modificationsaffect the availability of genes for transcription: • The physical state of DNA in or near a gene is important in helping control whether the gene is available for transcription. • Genes of heterochromatin (highly condensed) are usually not expressed because transcription proteins cannot reach the DNA. • Histone methylation • Histone acetylation