30nm fiber or filament (Second level of chromatin organization)
The second level of packing is the 30 nm fiber which provides an approximately 100-fold compaction of the DNA
- Formed by the helical supercoiling of the string of nucleosomes (10nm diameter)
- Found in both interphase chromatin and mitotic chromosomes.
- This structure increases the packing ratio to about 40.
- Histone H1 is required for formation and stabilization of the 30 nm fiber.
When chromatin is examined in the electron microscope, two types of fibers are seen: the 10 nm fiber and 30 nm fiber
- When chromatin is visualized in conditions of greater ionic strength the 30 nm fiber is obtained.
- The 10 nm fibril structure is obtained under conditions of low ionic strength and does not require the presence of histone HI.
The 30 nm and 10 nm fibers can be reversibly converted by changing the ionic strength.
(10nm nucleosome structure coiled into 30nm fiber at higher ionic strength and in the presence of HI).
Various models have been proposed to explain how nucleosomes fold to form the 30nm fiber
Two major models for the 30nm fiber structure
1) Solenoid model of 30nm fiber
2) zig – zag model of 30nm fiber
- In the solenoid structure, the nucleosomes fold up and are stacked, forming a helix.
- There are approximately 6 nucleosomes in each turn of the helix.
- Solenoid model shows that 10nm fiber coils around a central axis of symmetry with nucleosome which is packed face-to-face.
- Linker DNA must be bent to connect the neighboring nucleosomes in a solenoid model.
- In solenoid model consecutive nucleosomes are next to each other in the 30nm fiber (folds into a simple one-start helix)
- When DNA is compacted into the solenoid structure it is not transcriptionally active.
In solenoid model the linker DNA does not pass through the central axis of the superhelix. Here the sides, entry and exit points of the nucleosomes are relatively inaccessible to enzymes.
- The histone N terminal tails are also involved in the formation of 30nm fiber.
- N-terminal tails of the core histones which protrude out from the nucleosome core allow adjacent nucleosomes to interact to form the tightly packed solenoid structure.
Zig –Zag model
- Nucleosomes are arranged as a zigzag such that two rows of nucleosomes form.
- Straight linker DNA will connect the successive nucleosomes and they will lie on opposite sides of the 30nm fiber.
- In zig – zag model alternative nucleosomes become interacting partners.
- In zig zag model linker DNA frequently passes through the central axis of the fiber and the sides and even the entry and exit points are more accessible to enzymes.
The average length of linker DNA varies between species. So the form of 30nm fiber may not always be the same. Longer linker DNA prefers zig – zag 30nm fiber conformation (because this conformation requires the linker DNA to pass through the central axis of the fiber in a relatively straight form)