How is DNA fitted into the nucleus?
By packaging it into chromatin
What is chromatin composed of?
Chromatin is composed of small basic proteins known as Histones.
What are the two main types of histone?
1. Core histones
2. Linker histones
Outline the structure of core histones.
- Globular domain with a flexible N-terminal tail
- Tail region is rich in Arginine and Lysine
What are the 4 main histones and what is the size of the globular domain?
- H2A (25-129)
- H2B (32-125)
- H3 (40-135)
- H4 (32-102)
How do histones bind DNA?
DNA binding is by the core histones to form repeating units known as nucleosomes around which, DNA wraps twice.
What is the structure of a nucleosome?
- 147bp of DNA is wrapped twice around an octamer of histone proteins
- The octamer consists of a central H3/H4 tetramer with 2 flanking H2A/H2B dimers
How are the histones orientated in the nucleosome?
- Globular heads are in the middle
- N terminal tails project outward
Outline the arrangement of the nucleosome
- DNA directly passes from one nucleosome to the next to form a 10nm fibre (beads on a string arrangement)
- Linker histones such as H1 bind to the DNA between nucleosomes (this is at both the entry and exit of the DNA to the histone)
- This aids the formation of the 30nm fibre which is thought to have a solenoid-esque structure
What are the two main methods of chromatin structure control?
1. Post translational modification of histones
2. ATP-dependant chromatin remodelling
What are the enzymes involved in acetylation and what are the ways these effect the process?
- HATs (Histone Acetyl Transferases) mediate the acetylation of lysines on the N-terminal tail
- HDACs (Histone Deacetylases) these readily reverse the acetylation via hydrolysis
What are the 4 main ways by which post translational modification can occur?
How are HATs recruited?
- Activators recruit HATs to specific promoters
- This is due to the fact that many HATs contain a specific subunit which interacts with the activators
Outline the two main ideas as to how Acetylation of hitstones occurs.
- Histones have a positive charge and the DNA has a negative charge so they attract and hold the structure together.
- By acetylating the N-terminal lysines we make the histone less positive and so weaken the attraction
- Acetylation is a signal for the recruitment of proteins (ones with a bromodomain) to chromatin
Outline the process of histone of methylation
- Occurs on lysine or arginine residues and is catalysed by HMTase many of which have set catalytic domains
- Depending on the context this can either be repression (H3 Lys9 & H3 Lys27) or activating (H3 Lys4 & H3 Lys36)
What are the properties of histone methylation?
- Difficult to reverse via hydrolysis
- Does not affect the charge of the N-terminal tail
- Methyl lysine residues are bound by proteins with Chromodomains.
In what type of proteins are chromodomains found?
- Proteins which are found in complexes which regulate chromosome function.