5. Cell Structure And Organization - part 13 - Nucleus

 


5. Cell Structure And Organization - part 13 - Nucleus


Nucleus :

  • Structure of nucleus of a eukaryotic cell becomes distinct in a non-dividing cell or during interphase. 
  • Such an interphase nucleus is made up of - 
  1. nuclear envelope
  2. nucleoplasm
  3. nucleolus and 
  4. chromatin network.

1. Nuclear envelope :
  • It  is a double walled delimiting membrane of nucleus. 
  • Two membranes are separated from each other by perinuclear space (10 to 50nm). 
  • Outer membrane is connected with endoplasmic reticulum at places. 
  • It also harbours ribosomes on it. 
  • The inner membrane is lined by nuclear lamina- a network of protein fibres that helps in maintaining shape of the nucleus. 
  • The two membranes along with perinuclear space help in separating nucleoplasm from cytoplasm.
  • However, nuclear membrane is not continuous.
  • At places, there are small openings called nucleopores. 
  • The nucleopores are guarded by pore complexes which regulate flow of substances from nucleus to cytoplasm and in reverse direction.

2. The nucleoplasm or karyolymph :

  • It contains various substances like nucleic acids, protein molecules, minerals and salts.
  • It contains chromatin network and nucleolus. 
3. Nucleolus :
  • It is another component which is not bound by cell membrane.
  • Nucleolus is made up of rRNA and ribosomal proteins and it is best known as the site of ribosome biogenesis. 
  • Depending on synthetic activity of a cell, there are one or more nucleoli present in the nucleoplasm.
  •  For ex: cells of oocyte contain large nucleolus whereas sperm cells contain small inconspicuous one. 
  • They appear as dense spherical bodies present near chromatin network. 
  • They produce rRNA and ribosomal proteins which are then transported to cytoplasm and are assembled together to form ribosomes.
4. Chromatin material :


  • Nucleus contains genetic information in the form of chromosomes which are nothing but DNA molecules associated with proteins.
  • In a nondividing cell, the chromosomes appear as thread like network and cannot be identified individually. This network is called chromatin material. 
  • The chromatin material contains - 
  1. DNA
  2. histone
  3. non-histone proteins and
  4. RNA. 
  • In some regions of chromatin, DNA is more and is genetically active called euchromatin. 
  • Some regions that contain more of proteins and less DNA and are genetically inert, are called heterochromatin.
  • When the cell prepares to divide, the chromosomes coil and get condensed. 
  • At metaphase stage, they become distinct and can be clearly identified. 
  • Every species of living organism has specific number of chromosomes like normal human cell has 46.


Nucleus thus is the master cell organelle Because -
  • The nucleus contains entire genetic information, hence play important role in heredity and variation.
  •  It is the site for synthesis of DNA, RNA and ribosomes. 
  • It plays important role in protein synthesis. 
  • Chromosome number being constant for a species, it is important in phylogenetic studies. 

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