Class 11 Biology Cell : The Unit of Life Nucleus


  • Nucleus as a cell organelle was first described by Robert Brown.
  • Nucleus was named chromatin by Flemming.
  • The interphase nucleus has highly extended and elaborate nucleoprotein fibres called chromatin, nuclear matrix and one or more spherical bodies called nucleoli.
  • The nuclear envelope consists of two parallel membranes with a space between them called the perinuclear space.
  • At a number of places, the nuclear envelope is interrupted by minute pores, which are formed by the fusion of its two membranes.
  • The nuclear matrix or the nucleoplasm contains nucleolus and chromatin.
  • The nucleoli are spherical structures present in the nucleoplasm.
  • Nucleolus is the site of active ribosomal RNA synthesis.
  • A loose and indistinct network of nucleoprotein fibres is called chromatin.
  • During different stages of cell division, cells show structured chromosomes in place of the nucleus.
  • Chromatin contains DNA and some basic proteins called histones, some non-histone proteins and also RNA.
  • Every chromosome essentially has a primary constriction or the centromere on the sides of which disc shaped structures called kinetochore are present.


Fig. Nucleus

  • Based on the position of the centromere, the chromosomes can be classified into four types
  • Metacentric
  • Submetacentric
  • Acrocentric
  • Telocentric
  • The metacentric chromosome has middle centromere forming two equal arms of the chromosome.
  • The sub-metacentric chromosome has centromere nearer to one end of the chromosome resulting into one shorter arm and one longer arm.
  • In case of acrocentric chromosome, the centromere is situated close to its end forming one extremely short and one very long arm, whereas the telocentric chromosome has a terminal centromere.
  • Sometimes a few chromosomes have non-staining secondary constrictions, which gives the appearance of a small fragment called the satellite.


Fig. Different types of chromosome

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