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STRUCTURE OF NUCLEUS

• Study of the cell cycle has revealed that each cell has two phases in its cycle:

1. Interphase and 
2. Phase of cell division.

• In fact, interphase is the phase between two cell divisions. 
• This is much longer than the phase of cell division; the structure of the nucleus is studied in this interphase only.
• The electron microscopic studies of interphase nucleus have revealed that the nucleus may consists of following four parts:

1.  Nuclear Membrane: 

• It limits the nucleus externally and is also known as karyotheca. 
• It is bilayered, lipoproteins and trilaminar in nature. 
• Outer membrane is called ecto-karyotheca and the inner is endo-karyotheca. 
• The outer membrane is studded with ribosomes while the inner is free of that. 
• The two membranes have a thickness of 75-90 Å  And are apart from each other by a distance of 100-300Å. 
• This space is called perinuclear space.

• The nuclear membrane has many pores. 
• Its number may vary from 1000-10000 in a nucleus. 
• Each pore is about 400-1000 Å in diameter. 
• The number and size of pores may depend on the needs of the cell. 
• Each nuclear pore is fitted with a cylindrical structure called annulus. 
• The pore and the annulus both collectively form the pore complex or pore basket.

2. Nucleoplasm: 

• It is transparent semi fluid, homogenous, colloidal ground substance inside the nuclear membrane. 
• It is also called nuclear sap, karyolymph or karyoplasm. 
• Nuclear chromatin and nucleolus are embedded within nucleoplasm, chemically, it is formed of water, sugars, minerals (Mn2+, Mg2+, etc.), 
• Nucleotides, ribosomes, enzymes, DNA and RNA polymerases, mRNA, tRNA molecules etc. It is alkaline in nature (pH = 7.4  ).

Functions :

• Nucleoplasm forms the skeleton of nuclei and helps in maintaining their shape.
• The process of transcription takes place in the nucleoplasm in which different molecules of RNA are formed.
• It supports nuclear chromatin and the nucleolus.
• Ribosomal subunits are synthesized in the nucleoplasm.

3. Chromatin Net or thread : 

• Electron microscopic studies of well stained eukaryotic nuclei have revealed that presence of darkly stained network of long, fine and interwoven threads which is called chromatin net or thread. 
• It is also known as nuclear reticulatum. 
• It was first reported by Fleming in 1882. During the phase of cell division, the chromatin net is transformed into chromosomes due to high condensation of DNA molecules. 
• These chromosomes are rod like and have definite shape and size chracteristic of an organism.
• The chromatin is chemically nucleoprotein and formed of nucleic acid (DNA) and base proteins i.e., histones . 
• It may be classified in to two categories:

Heterochromatin : 

• It is made of comparatively thick regions which are darkly stained. 
• DNA strands in this chromatin are more condensed. 
• Transcriptionally, it is inactive and late replicative. 
• It does not contain active genes.

Euchromatin : 

• It is true chromatin and is formed of thick and less darkly stained areas. 
• It has loose, less condensed DNA which is transcriptionally, inactive and early replicating.