EUKARYOTIC DNA PACKAGING

 DNA is the genetic material. It is a large biomolecule. Eukaryotic DNA is present in the nucleus. It has millions of base pairs. The total length of DNA is 1.8 m in a cell. Such a large molecule is to be packed in the nucleus of an 8 micrometer in size. 

Metaphase DNA is packed into chromosomes that are visible through the light microscope. This packaging occurs approximately 10,000 times. The original thickness of the DNA strand is the two-nanometer.  The thread of DNA is packed into a 1400 nanometer thread which is the breadth of the chromosome. This packaging requires multiple types of proteins that are associated with DNA molecules.

 The first level of Packaging of DNA: Nucleosome 

 The 11-nanometer structure is known as a nucleosome.  This structure requires a special type of protein those are known as histone proteins. 

There are five types of histone proteins which are named H1, H2A, H2B, H3, and H4. The histone proteins are positively charged due to their composition of amino acids. The amino acids present in histone proteins are lysine and arginine, these give histone proteins a positive charge.

 Eight histone proteins form the core of the nucleosome. The core of the nucleosome is octamer which has two subunits each of H2A, H2B, H3, and H4. 

146 base pair of DNA coil around the histone octamer, H1 histone then clip both the ends of DNA to the histone octamer. This further takes 20 base pairs of DNA.

 

The DNA between the two histone octamer is called linker DNA this reduces the length of DNA from 1m to 14 cm which reduces the size of DNA to 1/7. The Packaging of DNA into nucleosomes, that results in the formation of a string of beads. This is visible under the electron microscope. 

30 nm fiber

 Even after this level of packaging DNA is too large to be packed into the nucleus of the cell for this the histone octamer is packed into a solenoid or zig-zag structure of the 30-nanometer fiber.

 In the solenoid structure, the linker DNA is present in the center. It is a compact structure.

Zig-zag 

In this folding, the 30 nm fiber is formed by the zig-zag arrangement of nucleosomes. Here the linker DNA is outside the center. 

CHROMATIN

 This 30-nanometer fiber is then packed into 300nm and the 300 nm fiber coil around proteins to form 700 nm fiber. This is called chromatin.

 The 700-nanometer fiber is then packed into a chromosome that has 1400 nanometers this thickness appears in the metaphase chromosome.

SIGNIFICANCE OF DNA PACKAGING 

Interphase DNA is diffusely distributed and in form of chromatin. This is in an irregular shape.

This allows a huge amount of DNA to be packed in the small nucleus of a cell.

DNA packaging in chromosomes allows the distribution of genetic material equally to the daughter nuclei during cell division.

The numerical and structural study is possible of the metaphase chromosome.

Karyotype and banding pattern study is done on metaphase Chromosomes, this helps in identifying any genetic disorder in feotus. This study is not on unpacked DNA.