Eukaryotic Chromosome Organization

Eukaryotic Chromosomal Organization:

Ø      Fundamental difference between prokaryotic and Eukaryotic genome organization is prokaryotes have a single type of chromosome but Eukaryotes have two sets of chromosomes making the cell has diploid.

Ø      To know the structure of Eukaryotic chromosome, Metaphase chromosome is preferred.

Ø      They are easily observed under microscope after staining.

Ø      The number of human chromosomes is 46 and the length of human genome is 5.5X10⁹ base pairs.

Ø      When compared with prokaryotic cell, Eukaryotic cell is having the large amount of DNA in its nucleus.

Ø      The DNA is found associated with proteins.

Ø      Chromatin is the complex basis of DNA and protein that makes up chromosomes consists of Linear unbroken double stranded DNA. Chromatin is of two types

a.     Euchromatin, and

b.     Heterochromatin

a. Euchromatin

·        Euchromatin is a type of chromatin which is genetically active. It consists of all functional genes which are expressive.

·        This type of chromatin stains very lightly.

b. Heterochromatin:

·        It is genetically inactive, the genes are not expressive. Heterochromatin stains darkly because the chromatin region here is highly condensed.

·        Histones are the chief protein components of chromatin they act as spools around which DNA winds and play a role gene regulation.

Ø      Five major histone classes are present which are associated with eukaryotic genome i.e., H₁, H₂A, H₂B, H₃ & H₄.

Ø      These are basic proteins which have a positive charge at normal pH which facilitates the binding of negatively charged DNA

Nucleosome:

Ø      The simplest level of packing of DNA which is found winding around Histones is known as Nucleosome.

Ø      There are two molecules of each of four types of Histones namely H₂A, H₂B, H₃ & H₄. this give rise to a complex of 6 proteins named as “Histone octomer”

Ø      This Histone octomer is flattened cylindrical particle of about 11 nm and thickness of nucleosome is 5.7nm.

Ø      The H1 protein is present only in single set. These nucleosomes are attached to each other by means of a thin naked DNA which is known as Linker DNA (the H₁ is associated with Linker DNA)

Ø      Higher order organization is seen further where this nucleofilament has the appearance of beats on a string at 11 nm

Ø      The complexity in the organization may be represented further by the packing of chromosomes in a highly compact fashion giving rise to 30 nm, then 300 nm, 700 nm and finally 1400 nms in thickness which are able to see as rod like chromosome at metaphase of cell division.