University:
California State University, Northridge
Course:
BIOL 360 | Genetics
Academic year:

2007
Views:

65

Pages:

15
Author:

verykoshinodat6y

DNA: The Genetic Material Chromatin Structure Chapter 10 Chromatin Structure • Histones organize DNA, condense it and prepare it for further condensation by nonhistone proteins. – This compaction is necessary to fit large amounts of DNA (2m/6.5ft in humans) into the nucleus of a cell. • Nonhistone is a general name for other proteins associated with DNA. – This is a big group, with some structural proteins, and some that bind only transiently. – Nonhistone proteins vary widely, even in different cells from the same organism. – Most have a net (-) charge, and bind by attaching to histones. Chromatin Structure • Both histones and nonhistones are involved in physical structure of the chromosome. • Histones are abundant, small proteins with a net (+) charge. – The five main types are H1, H2A, H2B, H3, and H4. – By weight, chromosomes have equal amounts of DNA and histones. • Histones are highly conserved between species – H1 less than the others. Fig. 10.21 A possible nucleosome structure 24 Chromatin Structure • Chromatin formation involves histones and DNA condensation so it will fit into the cell, making a 10-nm fiber • Chromatin formation has two components: – Two molecules each of histones H2A, H2B, H3, and H4 associate to form a nucleosome core – and DNA wraps around it 1-3 or 4 times for a 7-fold condensation factor. • Nucleosome cores are about 11 nm in diameter. – H1 further condenses the DNA by connecting nucleosomes to create chromatin with a diameter of 30nm, for an additional 6-fold condensation. • The solenoid model proposes that the nucleosomes form a spiral with 6 nucleosomes per turn Fig. 10.21 Nucleosomes connected together by linker DNA and H1 histone to produce the “beads-on-a-string” extended form of chromatin H1 Fig. 10.21 Packaging of nucleosomes into the 30-nm chromatin fiber H2A, H2B, H3, and H4 Chromatin Structure • Beyond the 30-nm filament stage, electron microscopy shows 30–90 loops of DNA attached to a protein scaffold • SARs (scaffold-associated regions) bind nonhistone proteins to form loops that radiate out in spiral fashion • Fully condensed chromosome is 10,000fold shorter and 400-fold thicker than DNA alone. Fig. 10.24 Model for the organization of 30-nm chromatin fiber into looped domains that are anchored to a nonhistone protein chromosome scaffold The many different orders of chromatin packing that give rise to the highly condensed metaphase chromosome 29 Euchromatin & Heterochromatin • Staining of chromatin reveals two forms: – Euchromatin: condenses and decondenses with the cell cycle. – It is actively transcribed, and lacks repetitive sequences. – Euchromatin accounts for most of the genome in active cells. – Heterochromatin remains condensed throughout the cell cycle. – It replicates later than euchromatin – is transcriptionally inactive. Figure 10.20 • There are two types of heterochromatin – Constitutive heterochromatin • Regions that are always heterochromatic • Permanently inactive with regard to transcription • occurs at the same sites in both homologous chromosomes consists mostly of repetitive DNA – (e.g., centromeres). – Facultative heterochromatin • Regions that can interconvert between euchromatin and heterochromatin • varies between cell types or developmental stages, or even between homologous chromosomes. – Example: Barr body Compaction level in euchromatin During interphase most chromosomal regions are euchromatic Compaction level in heterochromatin Centromeric and Telomeric DNA • eukaryotic chromosomal regions with special functions. – Centromeres are the site of the kinetochore, where spindle fibers attach during mitosis and meiosis. • They are required for accurate segregation of chromatids. – Telomeres are located at the ends of chromosomes, and needed for chromosomal replication and stability. • Generally composed of heterochromatin, they interact with both the nuclear envelope and each other. • All telomeres in a species have the same sequence. • Both are Heterochromatin Unique-Sequence and RepetitiveSequence DNA • Prokaryotes have mostly unique-sequence DNA • Eukaryotes have a mix of unique and repetitive sequences. – Unique-sequence DNA includes most of the genes that encode proteins – as well as other chromosomal regions. • Human DNA contains about 65% unique sequences Dispersed Repetitive Sequences • There are two types of interspersion patterns found in all eukaryotic organisms: – LINEs (long interspersed repeated sequences) with sequences of 5 kb or more. The common example in mammals is LINE-1, with sequences up to 7kb in length, that can act as transposons. – SINEs (short interspersed repeated sequences) with sequences of 100–500bp. An example is the Alu repeats found in some primates, including humans, where these repeats of 200–300bp make up 9% of the genome.

DNA: The Genetic Material. Chromatin Structure

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