Chromatin Structure And Histones Code Flashcards
What does chromatin consist of?
DNA + Histones + RNA
Give the definition of chromatin
Nuclear complex of DNA and associated proteins that forms chromosomes within the nucleus of eukaryotic cells
Give the general function of chromatin
Packaging long DNA molecules into more compact and smaller volume to fit into the nucleus and to protect the DNA structure and sequence
What are the four levels of chromatin packaging, how much do they increase packaging and what is the rough size of them
1- nucleosomes- 7fold - 10nm
2- nucleosomes packed into fibres- 6fold- 30nm
3-fibres pack into loops and TADs to form chromatin - 3fold- 100-250nm
4- mitotic chromosome - 700-1000nm
When does chromatin condensation happen?
Prophase
Give the structure and function of euchromatin
Low compactivity (10nm fibre) - beads on a string
Gene rich
Enzyme accessible
Involved in DNA transcription, replication or repair
Give the structure and function of heterochromatin
Highly condensed (30nm) fibre
Darkly staining areas of chromatin often associated with nuclear envelope
Gene poor
Constitutive or facultative
What is constitutive heterochromatin?
Contains highly repetitive sequences of DNA which are genetically inactive and serve as a structural element (in the telomeres and centromeres)- always switched off
What is facultative heterochromatin?
Regions on chromosomes which become heterochromatin in certain cells and tissues- can be turned on/off at any given point in the organisms development or in a certain tissue
How does chromatin show plasticity?
A choice of histone variants, modifications of DNA bases and reversible posttranslational modifications of histone tails
Give the definition of a nucleosome
A little DNA wrapped around histones
What does an octamer core consist of?
H2A, H2B, H3 AND H4 (two of each)
Where are the N-terminal tails in relation to the octamer core?
Outside
How much DNA wraps around the histone cores?
146 base pairs or 1.7 turns of DNA
Where is the H1 protein found?
Wrapped around another 20 base pairs of linking DNA
What is The purpose of a H1 protein?
Stabilises the zigzagged 30nm chromatin fibre?
What happens to the histones each time a polymerase reaches it?
They have to be removed and replaced behind it (having been recycled and resynthesised)
When does a chromatin loop happen?
When stretch’s of genomic sequences that lie on the same chromosome are in close physical proximity to each other
What is cohesion in relation to chromatin loops?
A protein ring that binds to DNA and facilitates loops
What do chromatin loops exist?
They provide a favourable environment for process such as DNA replication, transcription and repair
What is a topologically associated domain (TAD)?
Highly conserved chromatin domain that shape functional chromosomal organisation.
What do TADs do?
We don’t know, but their disruption leads to disease so they must do something
What is a chromosome territory?
A non overlapping domain of the nucleus occupied by uncondensed chromosomes- composed of TADs
What is epigenetics?
Heritable and reversible changes in gene expression which don’t involve a change in the DNA sequence. Results from external or environmental factors or as a part of a development programme
