STRUCTURE OF NUCLEUS
- Study of the cell cycle has revealed that each cell has two phases in its cycle:
- Interphase and
- 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.
