1 - DNA and Chromosome structure Flashcards
What does chromatin facilitate
- Packing and protection of DNA
- Regulation of gene expression
Nucleosome
DNA coiled around a core of histone proteins
Histone core
Octamer (8 protein complex) - 2 of each subunits
Subunits of histone core
- H2A
- H2B
- H3
- H4
Role of H1
Stabilises internucelosome region. binds DNA at nucleosome entry and exit points
Heterochromatin
Tightly condensed, no transcription, recombination, replication, repair
Constitutive heterochromatin
Remains condensed throughout cell cycle. Not transcribed
Facultative heterochromatin
Can be unpacked when needed and become transcriptionally active
Open chromatin
DNA is accessible and transcriptionally active
Closed chromatin
DNA is not accessible, gene expression is repressed
Topoisomerase
reduces the torsional stress, prevents tangling
Chromatin conformation
DNA –> nucleosome –> Chromatin –> loop –> domain –> compartment –> chromosome
Cohesins in interphase
Maintain chromatin conformation by bringing together distant regions of DNA. Facilitate gene expression
When is chromatin less condensed
Interphase. Extremely condensed during metaphase and anaphase, allowing segregation of chromosomes
Chromosome territories
Arrangement of chromosomes within nucleus. Within a chromosome territory, positions at edge are expressed and positions in centre are repressed
where is Euchromatin found and give example
Active X chromosome, towards centre of nucleus
where is Heterochromatin found and give example
inactivated X chromosome, periphery of nucleus
Do chromosome territories vary
Yes, within cell type and disease state
Examples of post translational modification
- Acetylation
- Methylation
- Phosphorylation
- Ubiquitination
Histone lys acetylation
- Negative charge modification (histones become less positive)
- DNA is less tightly wound around histones (starts to unravel, allowing transcriptional machinery to access DNA)
Enzymes involved in histone modification
- Histone acetyl transferase (HAT) adds acetyl groups
- Histone deacetylase (HDAC) removes acetyl groups
- Histone methyl transferase (HMT) adds methyl groups
Chromodomain
Recognises methylated lysines
Chromoshadow domain
Aggregates with other chromoshadow domains, bringing nucleosomes close together
Hinge domain
Joins chromodomain and chromoshadow domain together
Boundary elements
Prevent condensation spreading to active chromatin regions
Mechanism of constitutive heterochromatin formation
- A HP1 chromodomain binds to H3K9me3
- HP1 chromoshadow domains self aggregate resulting in condensation
- HP1 chromoshadow domain interacts with HMT tri-methylates adjacent H3K9s, causing chain reaction
H3K9me3
maintains closed chromatin conformation
