Chromosomes: structure and functions


  • Karl Wilhelm von Nägeli in 1842 discovered a thread like structure which was called chromosomes.
  • They are the filamentous bodies typically present in the nucleus and become visible during cell division.
  • They are the carriers of hereditary materials called genes.
  • Most of the chromosomes on a cell are called heterosome and only one to two sex chromosomes are present.

Structure of chromosome

  • Each chromosome consists of two chromatids held together at a point along their length.
  • The point at which the two chromatids of a chromosome are held together appears as a constriction in the chromosomes called centromere.
  • The chromosomes consist of various structures like centromere, secondary constriction, nuclear organisers, telomeres, satellites and matrix.

Explain the structure of chromosome with diagram.

   Image source: toppr

1. Chromonema

  • Metaphasic chromosomes consist of two sub-units called chromatids.
  • The chromatids consist of filamentous structure called the chromonema.
  • The chromonemata form the gene bearing portion of the chromosomes.
  • They are embedded in achromatic substance known as matrix.
  • Matrix is enclosed in a sheet.
  • The sheet is called pellicle. Pellicle is not uniformly formed.
  • The chromonema may contain threads. The threads remain coiled with each other.
  • The coil may be of two types: paranemic coils and plectonemic coils.
  • When the thread separate easily that is called paranemic coils and when the thread cannot separate easily that is called plectonomic coils.

2. Centromere

  • The constricted region of chromosome is called centromere.
  • It consists of small granules for that centromere is also called kinetochore or primary constriction.
  • Position of centromere is constant for particular chromosome.
  • Depending upon the position of centromere, chromosomes are categorized as:
  1. Metacentric (V-shaped): This centromere is located at or near the midpoint so that the arm ratio is 1:1.
  2. Sub-metacentric: When the centromere is located slightly away from the midpoint so that the two arms are unequal, then the chromosome is called sub-metacentric.
  3. Acrocentric: Rod shaped chromosome where centromere is towards one end sub-terminal. So the chromosome has a short and a long arm.
  4. Telocentric: Rod shaped chromosome with centromere at one end during anaphase.

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3. Secondary constriction:

  • In addition to centromere as primary constriction, there is another constriction in the specific region of the chromosome; called secondary constriction.
  • The location of secondary constriction is specific in its site for particular chromosome.
  • Thus, it helps in identification of chromosomes.

5. Nucleolar organizer

  • Normally in each diploid sets of chromosomes, two homologous chromosomes have additional constriction, i.e.called nucleolar organizer.
  • They are called so because they are necessary for nucleolus.

6. Telomere

  • The tips of the chromosomes are called telomeres.
  • The telomeres differ from rest part of chromosome on structure and composition.
  • They have unique non-sticking property with other parts of chromosomes and other telomere.
  • Telomeres are region for the attachment to the nuclear envelope.

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7. Satellite

  • They are round, elongated or knob-like appendages with the chromosomes.
  • Chromosomes bearing satellites at their terminal points are called satellite chromosome.

Functions of chromosomes

  1. These are the vital component of cell.
  2. These control almost all cellular activities at physiological levels.
  3. These maintain the identity of species.
  4. These help in determination of sex of species.
  5. These act as vehicle of hereditary characters from one generation to another.
  6. With the help of chemical constituents DNA and RNA synthesize protein and enzymes.
  7. Some species of chromosomes synthesize yolk in oocytes of many vertebrates.




Chromosomes: structure and functions