P.Oudet named Nucleosome
R. Korenberg-gave the Nucleosome Model
Nucleosome –
- It is a circular structure of 10 nm diameter which is made up of 146 bp.
- In this, DNA is wrapped on Octamer unit of 8 Histone by taking one complete round and another 3/4.
- Two octaves of H2A, H2B, H3 and H4 histone are present in an octamer unit.
- This entire structure is called Nucleosome.
- A histone H1 is present between two nucleosome units, which are linked by linker DNA, this linker DNA is bound to 55 BP long DNA.
Hisone core
- It is made up of 8 histone proteins.
- In which two-two molecules of four types of Histone (H2A, H2B, H3 and H4) are present, which are Positively charged.
- It is an alkaline protein. Which is made up of high amounts of Lysine and Arginine.
- Arginine in H3 and H4 and Lysine in H2A, H2B, are present in high amount.
Organization of Chromatin in Nucleosome
- Chromatin is formed by recycling the nucleosome unit
- Each nucleosome has 200 bp of long DNA attached to Histone’s Octamer unit, in which a nucleosome unit is formed and two molecules of each nucleosome unit are joined by a Histone H1.
- Thus, many nucleosome units are arranged in a linear sequence to form chromatin threads, with which some proteins are also attached.
- This chromatin condenses to form the heterochromatin portion which is covered by the nucleus of the eukaryotic cell.
This chromatin portion is further condensed to form chromatid and chromosome
Nucleosome Solenoid model
- Finch and Clug gave it in 1976
- It consists of a 10nm diameter nucleosome unit producing chromatin fiber.
- This 10nm filament is folded again to make a thicker filament of 300 A° (30nm) called Solenoid.
- This 30nm filaments merge again to form a chromatin of 7000 A° which is called chromatid.
- These 2 chromatids form a chromosome of 14000A°.
