Nucleolus & Chromatin

Nucleolus:

  • Fontana (1781) discovered the presence of round oval bodies called Nucleoli (Nucleolus-singular) embedded in the nucleoplasm.
  • It does not have a membrane.
  • Nucleoli are associated with the nuclear organizing portion of the chromosomes.
  • The size of the nucleoli is related to the activities of the cell. Cells that are actively involved in protein synthesis have a large nucleoli and vice-versa.

Chemically, nucleolus is made up of proteins and RNA. The important function of the nucleolus is the synthesis of ribosomal RNA and protein. Hence, it is a major site of active ribosome sysnthesis.

It is seen distinctly only in the interphase of a cell. During the division of the nucleus, the nucleolus disappears and a new nucleolus develops in each daughter nucleus.

Chromatin reticulum:

The nucleoplasm contains many thread-like, coiled and elongated structures made of nucleoprotein fibres. These thread – like structures are known as the chromatin substances or chromatin fibres or chromatin reticulum (chrome-colour).

  • It is seen distinctly in the interphase nucleus.
  • Chemically it is made up of DNA, some basic proteins called histones, non-histone proteins and RNA (and hence called nucleoprotein).
  • During metaphase, the chromatin network is condensed to form thick ribbon- like bodies called chromosomes.

  • At certain places, the chromatin network remains condensed as darkly which take readily the basic stains called heterochromatin and the less stained region is called euchromatin.
  • Hetero-chromatin is usually an inactive region and euchromatin is the active region.

Chromosome

  • Chromosomes are clearly visible (through light microscope) only in the metaphase stage of cell cycle.
  • Each chromosome has two arms- a long arm (q) and a short arm (p).
  • In metaphase each arm splits longitudinally into two halves called chromatids.
  • Each chromosome has a constriction called primary constriction.
  • The clear unstainable zone at the region of primary constriction is called kinetochore or centromere.
  • Occasionally, chromosomes contain additional constriction other than the primary constriction, called secondary constriction.
  • The small portion located beyond the secondary constriction is called satellite.
  • Chromosomes with satellite are called SAT (Sine Acido Thymonucleinico) chromosomes.

Image result for chromosome sat Image result for chromosome types

Based on the position of the centromere, chromosomes are of four types.

  • Metacentric – The centromere is near the middle of the chromosome. The two arms are nearly equal.
  • Submetacentric – The centromere is situated some distance away from the middle. One arm will be shorter than the outer.
  • Acrocentric – The centromere is situated near the end of the chromosome.
  • Telocentric – The centromere is situated at the tip of the chromosome

Functions.

  • They control the heredity.
  • They control the metabolism of an individual.
  • Heterochromatin helps in the formation of nucleolus.
  • Chromosomes control the differentiation of different characteristics of an organism.
  • Changes in the number, position and structure of chromosomes lead to the formation of new species.

 

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