- 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.
- Based on the position of the centromere, the chromosomes can be classified into four types
- 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