Chromosome term was given by Waldeyer.

  • Chromosomes are seen during the Metaphase stage of cell division.
  • Chromatin reticulum seen during the interphase coils to form chromosomes during cell division.

Chromatin shows 2 types of coiling behavior:

  • Euchromatin: loosely coiled chromatin. It is transcriptionally active.
  • Heterochromatin: densely coiled chromatin. It is transcriptionally inactive and consists of repetitive DNA. It is present in centromere, telomere & satellite.

Packaging of DNA in chromatin:

Chromatin is made of DNA and histone proteins, in the ratio 1:1.

Histone proteins are basic proteins, rich in Lysine and Arginine.

  • They are of 5 types H1, H2A, H2B, H3 & H4.

Chromatin consists of repeating units called Nucleosomes.

  • Nucleosomes appear like ‘beads on a string’.
  • Kornberg proposed the Nucleosome model.
  • A nucleosome consists of the core particle and a linker DNA.
  • Core particle is an octamer of 4 histone proteins; H2A, H2B, H3 & H4 each present twice. DNA strand of 146 bp surrounds the octamer. It makes 1¾ coil around the octamer.
  • Linker DNA; is a DNA strand of about 8 to 114 bp that joins 2 core particles & completes the 2nd coil around the octamer. H1histone protein stabilizes the 2 coils around the octamer.


Diameter of the nucleosome is 11nm.

Nucleosomes are coiled in the form of a solenoid, having 6 nucleosomes per coil. They form the chromatin thread having a diameter of 30nm.

The chromatin thread further coils with the help of Non-histone chromosomal proteins.

Non-histone chromosomal proteins form a central scaffold around which the solenoid forms loops. It further coils to form chromosomes.

  • Non-histone chromosomal proteins are rich in Tryptophan & Tyrosine.

choromsome formation

Chromosome structure:

A chromosome consists of two chromatids attached to each other at the centromere or the primary constriction.

Lying on the either side of the centromere are disc-like structures, the kinetochore to which spindle fibers attach during cell division.

Specialized ends of chromosomes are Telomeres.

  • Telomeres help the chromosomes to maintain their identity.
  • Telomeres are tandem repeats of ‘TTAGGG‘.

chromosome structure

Satellite chromosomes / SAT-chromosomes:
  • In some chromosomes besides the primary constriction, a secondary constriction is also present.
  • Also referred as the Nucleolar organizer (NOR). They are associated with the nucleolus and help in the formation of nucleolus at the end of cell division.
  • In man chromosome 13, 14, 15, 21, 22 have the NOR.
  • Satellite is the region outer to the secondary constriction & is usually spherical.
  • SAT represents Sine Acid Thymidine i.e., this region is without Thymidilic Acid.
Chromosomes are divided into different types according to the position of centromere:
  • Metacentric: the centromere is present at the center, dividing the chromatid into 2 equal arms.
  • Sub-metacentric: the centromere is present slightly away from the center, dividing the chromatid into 2 unequal arms.
  • Acrocentric: the centromere is present slightly away from the tip, dividing the chromatid into 2 unequal arms.
  • Telocentric: the centromere is at the tip.

types of chromosomes


Complete set of chromosomes in a species or in an individual organism.

  • Karyotypes describe the chromosome number in an organism and how these chromosomes look like under a light microscope.
  • Length of a chromosome, the position of the centromere, its banding pattern, differences between the sex chromosomes and autosomes and any other physical characteristics are considered.


Idiogram is a diagrammatic representation of a karyotype of a species.


Genome: is the complete set of chromosomes/genes in an organism.

Autosomes: Chromosomes other than sex chromosomes (X & Y) are autosomes.

  • Autosomes appear in pairs.

Homologous chromosomes are chromosomes having the same number, type, and arrangement of genes. But one is paternal and one is maternal.

Also, read DNA-structure-Mnemonic

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