Chromatin Flashcards
What proteins is DNA wrapped around in chromatin
Histone proteins
What is a nucleosome composed of
8 histone proteins: 2x H3, 2x H4 (tetramer) + 2x H2A, 2x H2B (dimers) = histone octamer
What is the charge of core histones and why
Positively charged (20-25% lysine & arginine) to interact with negatively-charged DNA
How much DNA wraps around the nucleosome core
147 bp, wrapping nearly twice around
What protrudes from the nucleosomes and is important for modification
Histone tails
What is the role of the H1 histone
Binds linker DNA between nucleosomes and helps compact chromatin
Is H1 present on all nucleosomes
No, it is not associated with every nucleosome
What is the first level of chromatin structure
10nm fibre (“beads on a string”) - nucleosome cores + linker DNA
What is the second level of chromatin compaction
30nm fibre – structure unclear (solenoid/zigzag); provides ~50x compaction
What is the third level of compaction
Chromatin loops anchored to the nuclear matrix; forms metaphase chromosomes
Where are chromosomes during interphase
In discrete chromosome territories, anchored to the nuclear matrix
What technique is used to visualise DNA/RNA spatial patterns
FISH – Fluorescence In Situ Hybridisation
What is heterochromatin
Densely staining, gene-poor, transcriptionally silent, often repetitive regions
Where is heterochromatin found
Pericentromere and distal arms (“knobs”)
Where are most genes located
Euchromatin, which is loosely packed and transcriptionally active
What causes Position Effect Variegation (PEV) in Drosophila
Inversions place genes near heterochromatin, silencing them
What are Su(var) and E(var) genes
Suppressors and enhancers of variegation – affect heterochromatin spreading
What does acetylation of histone tails do
Neutralises positive charge → relaxes chromatin → activates transcription
What enzymes acetylate and deactylate histones
HATs: Histone Acetyl Transferases – activate expression
HDACs: Histone Deacetylases – repress expression
How is heterochromatin maintained through DNA replication
H3-H4 tetramers are redistributed and reader/writer complexes restore marks
What is epigenetics
Heritable changes in gene expression independent of DNA sequence, often self-reinforcing
Why can’t most transcription factors bind tightly packed DNA
Because DNA is occluded by histones
What are ‘pioneer’ transcription factors
TFs that can bind to histone-bound DNA and recruit chromatin remodelers
What do chromatin remodelling proteins do
Move or remove nucleosomes to make DNA accessible
