The Nucleosome: Genomic Organization

INTRODUCTION The human somatic cell contains 46 (23 pairs) chromosomes. The total length of the DNA in a cell is about 2 meters. The question is how is all this packed into a single nucleus of 10 microns diameter ?????

INTRODUCTIONHow The expression level of different genes vary with spatial and temporal distribution ???????

INTRODUCTION The eukaryotic genome is organized into repeating disc shaped nucleosomal units composed of histones and their associated DNA. Nucleosomes are recognized highly dynamic units through which the eukaryotic genome can be regulated.

Levels of packing of genome

Genomic regulation

Structure of the Core (H2A,H2B,H3,H4) and Linker (H1) Histones

string-of-beads conformation (11nm fibre)

Nucleosome strings. Chambon and colleagues used trypsin to remove histone H1 from chromatin isolated from chicken red blood cells, revealing a beads-on-a-string structure. The bar represents 500 nm.

Models that are proposed for 30 nm fibre Two start helix (zigzag model)

One start helix (solenoid model)

30 nm fibre ( Two start helix ) No. of nucleosomes= 22 The two stacks of nucleosome are connected by three esentially straight linkers which are repeated in a zig-zag arrangement

Evidences for the solenoid model (one start helix)Showing the presence of central hole.

The number of regions that deform the externel ring ranges from 5 to 7.

Evidences for the solenoid model (one start helix) Relationship between fiber diameter and linker length.

The average diameter dimensions shown are calculated from chromatin arrays folded in 1.6 mM MgCl2.

Evidences for the solenoid model (one start helix)Comparison of the model and raw EM images of folded chromatin fibres. Both the model and the cryoEM images contain 22 nucleosomes, and matching tilts are shown.

30 nm fibre ( One start helix) No. of nucleosomes=22 The dyad axis of the nucleosomes intersects the fibre axis approximately orthogonally, placing the linker histone in the interior of the fibre.

Radial loop model (300 nm fibre) The histones were removed from the isolated mitotic chromosome gently by competition with an excess of the polyanions dextran sulfate and heparin. After removing the histones, the DNA remains highly organized by thechromosome scaffold (drawn in red), keeping the size and shape of the original chromosomes.

The most abundant proteins of the chromosome scaold are DNA topoisomerase II (topo II) and ScII, which is a member of the SMC (stable maintenance of chromosomes) family.

Radial loop model Schematic depiction of the excision of DNA loops by nuclear matrix-bound topoisomerase II. SfiI fragments are shown in bold. Topoisomerase II cleavage sites by arrows. Hybridization probes by arrowheads. The resulting DNA fragments are separated by PFGE and truncated fragments are detected by hybridization (below).