cell nucleus; chromatin structure Flashcards
what is the chromatin?
2m of DNA packed, wrapped around histones in cell nucleus of 10-20 micrometres but still accessible for all nuclear functions (replication, transcription, repair)
early models of interphase
- Rabl’s model (1885); threads that each represents an individual chromosome territory
- Coming’s model (1968); attachment sites for chromosome, non-territorial
electron micrograph
can visualise nuclear, nucleolus, heterochromatin and euchromatin
FISH
visualise location of genes & chromosomes in nucleus
limitations of electron micrograph and FISH
- sensitivity; how small a single copy genomic region can be visualised
- resolution; minimum distance between two genomic regions that can be distinguished as two separate signals (super resolution)
- single cell analysis; snapshot only of what is happening in a cell population
gene locations
- genes have preferential locations at surface of chromosome terrritory
- can dynamically loop out in response to transcriptional activation
- correlates with gene density
structure of nucleosome core particle at 2.8A resolution in 1997;
- revealed by X-ray crystallography of nucleosome crystals
- 1.65 turn, 147 bp DNA
- DNA show up white, histones H2A, H2B H3 & H4 are yellow, red, blue & green
what are the structure states of the nucleosome?
- tetrasome
- hexasome
- hemisome
- lexasome
tetrasome
H3-H4 tetramer; missing H2A and H2B
hexasome
H3-H4 tetramer + 1 x H2A - H2B hetradimer
- intermediate states during nucleosome assembly and during transcription
hemisome
half nucleosome
lexasome
transcriptionally poised nucleosome
transcriptionally silent
highly packaged chromatin basedon 30nm fibre
transcriptionally active
more open chromatin
models of 30nm fibre from electron micrography and crystal studies
- solenoid model
- zigzag model
solenoid model
coiled around a central axis of symmetry, 6-7 nucleosomes per turn.
- nucleosomes follow one another along same helical path
zigzag model
two rows of two nucleosomes with linker DNA crisscrossing between
recent progress in understanding of 3D chromosomes folding looks at:
- library of ligation products representing chromatin structure
- frequency at which 2 genetic loci are in close proximity
- one to one between specific genomic regions