Chromatin Structure & Histone Code Flashcards
What are histones?
Proteins responsible for the first level of packaging DNA
What are nucleosomes?
- DNA wrapped around 8 histones
- Octamer core made of two molecules of H2A, H2B, H3 + H4
- the N terminals stay outside the core
What are the 4 levels of chromatin folding?
L1: Nucleosomes: 146bp of DNA wrapped around the core
L2: 30nm fibres: nucleosomes pack themselves into fibres
L3: Chromatin loops: fibres pack themselves into loops and topologically associated domains (TADs) to form chromatin
L4: Chromosome territories: represented as the mitotic chromosome
What are the components of DNA?
- Histones
- DNA
- Non coding RNA
- Non histone proteins
What does compaction of histones involve?
- Linker histones
- Interaction of N tails with other nucleosomes
- Binding of packing proteins to histone tails
What is the difference between euchromatin and heterochromatin?
Euchromatin:
- lightly staining areas of chromatin
- rich in DNA
- made of nucleosomes
Heterochromatin:
- darker staining area
- few genes
- dense
What is facultative heterochromatin?
Regions on chromosomes that become heterochromatic in certain cells and tissues
- has potential to be expressed
What is constitutive heterochromatin?
Highly repetitive regions of DNA that are genetically inactive
- serves as structural elements at telomeres and centromeres
What determines if a nucleosome is hetero or euch?
- chemical modifications of lysine residues on histone tails
What is epigenetics?
Heritable and reversible changes in gene expression which do not involve a change in DNA sequence
What are the 4 types of modifications on gene expression?
- DNA methylation
- Covalent histone modification
- ATP dependent chromatin remodelling
- Noncoding RNAs
What is DNA methylation?
- tails can be methylated by histone methyl transferases (HMTs) and demethylated by histone demethylases (HDMs)
- causes chromatin condensation
What is DNA acetylation?
- acetylated by histone acetyl transferases (HATs) and de-acetylated by histone de-acetylases (HDACs)
- usually associated w’ relaxed chromatin and gene expression
What happens in the phosphorylation of DNA?
- Protein kinases and dephosphorylated by proteases
What is the histone code?
combinations of post-translational modifications on the same or different histone tails
