chromatin and histones Flashcards
what is chromatin and explain what its function
- Chromatin = nuclear complex of DNA and associated proteins that forms chromosomes within the nucleus of eukaryotic cells.
- Function: packaging long DNA molecules into more compact and denser shape (small volume) to fit into the nucleus and to protect the DNA structure and sequence.
- Chromatin regulates gene expressionand dna replication
- Allows processes such as mitosis, meisosis ,
what is chromatin made out of
dna + rna + histones
what are the 4 levels of packaging
1st level : - dna + histones = nucleosomes - increases DNA packaging 7-fold - 30nm 2nd level : - Nucleosomes pack themselves in fibers - Increases dna packaging 6-fold - 30nm 3rd level : - Fibers pack themselves into loosp and TADs and form chromatin - Increases DNA packaging 3-fold - 100-250nm 4th level - Only visible in th enucleus during mitosis - Represented by the mitotic chromosome - 1000-fold packaging - 700-1000nm
when does chromatin condensation begin and chromosomes become visible
prophase ( mitotic phase)
during the rest of the cycle only chromatin is visible not chromosomes
what happens during interphase
chromatin is organised into euchromatin and heterochromatin
what is euchromatin
dna is less condensed , 10nm fiber – “ beads on a string”
- Lightly staining areas of chromatin
- Rich in genes
- Euchromatin is accessible to the eznymes involved in dna transcription, replication or repair.
- ‘active’
what is heterochromatin
- Highly condensed, 30nm fiber
- Darkly staining areas of chromatin often associated with nuclear envelope
- Gene poor
- Constitutive or facultative
describe the two types of heterochromatin
Constitutive heterochromatin: invariably heterochromatin containing highly repetitive sequences of DNA which are genetically inactive and serves as structural element of the chromosome. Telomeres and centromeres !
- So telomerers and centromeres form the constitutive heterochromatin
- These regions remain condensed throughout the cell cycle and are not active transcribed.
- Facultative heterochromatin: regions on chromosomes which become heterochromatin in certain cells and tissues (heterocromatin that can become eucromatin in certain cells). Potential for expression at a particular point of the development eg, Inactivates X chromosome in female somatic cells.
- Plasticity – depending on the cell/tissue, these regions can either be facultative heterochromatin or euchromatin . enabled by choice of histone variants, modifications of DNA bases, and reversible posttranslational modifications (PTM) of histone tails.
explain in detail nucleosomes ( from nucleosomes to chromosome territories)
1) Histones assemble to form an octamer core :
- H2A, H2B, H3 and H4 ( 2 molecules of each)
- N-terminals tails stay outside the octamer core. Therefore they are exposed to posttranslational modifications.
- Histones are the main proteins in chromatin whose function is packaging DNA into nucleosomes.
2) 146bp of DNA wrapped around the histones core (1.7 turns of DNA).
3) The histone core contains central H3-H4 tetramer, and 2 flanking H2A-H2B dimers. Each core contains a common structural motif, histone fold. Facilitates the interactions between the individual histone cores. There is also the histone, H1, known as the linker histone. It doesn’t contain a fold motif because its function is to bind the linker DNA and facilitate the interaction between the different nucleosomes.
4) H1 protein wraps another 20 base pairs of “linker DNA” resulting in two full turns around the octamer. It helps to stabilize the zig-zagged 30 nm chromatin fiber.
5) Nucleosomes are joined by linker DNA (20bp) that runs between them. Long chain of nucleosomes gives the appearance of ”beads on a string” (10nm)
6) Nucleosomes tightly pack together into a fiber of 30nm (2nd level ). fibres called solenoids
nucleosomes are highly ? and explain why
highly dynamic. they are positionally maleable and can slide along the dna. they can fully or partially desassemble. they are often subject to post translational modifications. they are recycled and newly synthesised during processes like dna replication
what are chromatin loops
- Chromatin loop formation (looping): occurs when stretches of genomic sequence that lie on the same chromosome are in close physical proximity to each other.
- Cohesin: protein ring that binds to DNA and facilitates loops by allowing intereactions between distant regions in the same chromosome. E.g between promotors and enhancers that were distant, but due to the loop are close and as a consequence the gene is transcribed .
- Enhancers can interact with silencers and insulators aswell, facilating the expression or silencing of genes.
- Chromatin loops provide favourable environment to processes such as DNA replication, transcription, and repair
what are tads
- Organisation in Topologically Associating Domains (TADs): Highly conserved chromatin domains that shape functional chromosomal organization.
what are chromosome territories
- Chromosome territories: non-overlapping domains/regions of the nucleus occupied by uncondensed chromosomes. They are composed of TADs.
- It has been observed if we stain mitotic chromosomes during mitotic phase, then in the interphase phase, we will be able to see a regular distribution of the chromosomes within the nucleus.
- Visible during cell INTERPHASE,cells in the mitotic phase, instead of seeing territories we would see mitotic chromosomes
when are chromosome territories visible ?
during interphase
what enables us to see chromosome territories
stain mitotic chromosomes during mitotic phase
