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  Section: Genetics » Physical Basis of Heredity » The Nucleus and the Chromosome
 
 
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Euchromatin and heterochromatin

 
     
 
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Physical Basis of Heredity 1.  The Nucleus and the Chromosome
The Nucleus 
Significance of nucleus : Hammerling's experiment
Number, shape and size of nucleus
Nucleus in prokaryotes and eukaryotes
Nuclear envelope
Nuclear pore complex and nucleocytoplasmic traffic
Nucleolus
Chromosomes
Number, size and shape of chromosomes
Morphology of chromosomes
Karyotypes
Euchromatin and heterochromatin
Constitutive and facultative heterochromatin
Single-stranded and multi-stranded hypotheses for chromosomes
Chemical composition of chromosomes
Infrastructure of chromosomes
Function of chromosomes
Special types of chromosomes 
Lampbrush chromosomes
Salivary gland chromosomes
B-Chromosomes
Prokaryotic Nucleoids


When chromosomes are stained with stains like acetocarmine or feulgen (basic fuchsin) at prophase, a linear differentiation into regions having dark stain and those having light stain becomes conspicuous. In 1930's and 1940's Emil Heitz and other cytologists studied this aspect. The darkly stained regions were called heterochromatic and light regions were called euchromatic (Fig. 6.11).

Heterochromatic regions are constituted into three structures namely chromomeres, chromocentres and knobs. Chromomeres are regular features of all prophase chromosomes, large enough to reveal them, but their number, size, distribution and arrangement are specific for a particular species at a particular stage of development.
 
Chromosomes showing euchromatin and hetero-chromatin. (A) Early prophase (B) Late prophase (redrawn from Stebbins : Chromosomal Evolution in Higher Plants).
Fig. 6.11. Chromosomes showing euchromatin and hetero-chromatin. (A) Early prophase (B) Late prophase (redrawn from Stebbins : Chromosomal Evolution in Higher Plants).

Chromocentres are heterochromatic regions of varying size which occur near the centromeres in proximal regions of chromosome arms. At mid-prophase, many chromocentres can be resolved into strings of chromomeres, which are larger than chromomeres found in distal regions. In some dipteran salivary glands, the chromocentres of different chromosomes fuse to form a large chromocentre. The relative distribution of chromocentres are sometimes considered to be of considerable evolutionary value.Knobs are spherical heterochromatin bodies which may have a diameter equal to the chromosome width but may reach a size having a diameter which is several times the width of the chromosome. Very distinct chromosome knobs can be observed in maize at pachytene stage. Knobs are valuable chromosome markers for distinguishing chromosomes of related species and races.
 
     
 
 
     




     
 
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