Chromosome Ultra Structure

The main genetic material of the cell is DNA. In eukaryotic cells the DNA is complexed with proteins to form chromatin fibres. At the time of cell division the chromatin becomes condensed into chromosomes.

Chromosome consists of two symmetrical chromatids. The two chromatids are attached to each other by a single centromere. The chromatids become separated at anaphase stage of mitosis. Therefore anaphase chromosome has only one chromatid, while metaphase chromosome has two. Centromere is the region of the chromosome that becomes attached to the mitotic spindle. The centromere lies within a thinner segment of the chromosome is called primary constriction  The centromere contains kinetochores (One kinetochore for each chromatid) . Kinetochores are disc shaped structures formed of DNA and proteins. During mitosis microtubules become attached to the kinetochore and provide force for the chromosome movement. (Centromere vs Kinetochore)

Chromosome Ultra Structure

The chromosome may contain a constriction site along the chromosome other than the primary constriction known as secondary constriction. Secondary constriction contain the gene coding for ribosomal RNA and that induce the formation of nucleoli. Satellites are round bodies separated from the rest of the chromosome by a secondary constriction. The chromosome with the satellite are designated as sat chromosomes. The tip of chromosome is called telomere. Telomeres promote chromosomal stability. They contain the ends of the DNA molecule contained in the chromatid.

Chemical nature of Chromosomes: Chromosomes are composed of nucleoproteins

• Euchromatin: Some part of chromosomes take less stain and appear lighter in colour. These areas are called 'euchromatin'.
  
  
• Heterochromatin: When chromosomes are stained with acetocarmine or acetoorcein, some parts take more stain and appear darker. These areas are called 'heterochromatin'.