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  Section: Genetics » Physical Basis of Heredity » The Nucleus and the Chromosome
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Morphology of chromosomes

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
Number, size and shape of chromosomes
Morphology of chromosomes
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
Prokaryotic Nucleoids

Corresponding to different positions of centromere, chromosomes would be called (i) acrocentric or telocentric, having terminal centromere, (ii) submetacentric having sub-terminal centromere and (iii) metacentric having median centromere.

Besides centromere, which produces a primary constriction in chromosomes, secondary constrictions can also be observed in some chromosomes. Such a secondary constriction if present in the distal region of an arm would pinch off a small fragment called trabant or satellite. The satellite remains attached to rest of the body by a thread of chromatin. Secondary constrictions may be found in other regions also and are constant in their position, so that these constrictions can be used as useful markers. Secondary constrictions can be distinguished from primary constriction or centromere, because chromosome bends or shows angular deviation only at the position of centromere. Chromosomes having a satellite are marker chromosomes and are called SAT-chromosomes. The chromosome extremities or terminal regions on either side are called telomeres. If a chromosome breaks, the broken ends can fuse due to lack of telomeres. A chromosome, however, can not fuse at the telomeric ends, suggesting that a telomere has a polarity which prevents other segments from joining with it. Telomeres have been studied in great detail at the molecular level in recent years (see later in this section and in Chemistry of the Gene 2.  Synthesis, Modification and Repair of DNA).
Structure of a chromosome and chromatids.
Fig. 6.9. Structure of a chromosome and chromatids.

Detailed study of chromosome morphology reveals a coiled filament throughout the length of a chromosome. This filament is called chromonema (Vejdovsky, 1912). The chromonemata form the gene-bearing portions of the chromosomes. The chromonemata are embedded in the achromatic substance known as matrix (Fig. 6.9 A). Matrix is enclosed in a sheath or pellicle. Both matrix and sheath are non-genetic materials and appear only at metaphase when the nucleolus disappears. It is believed that nucleolar material and matrix are interchangeable i.e., when matrix disappears, nucleolus appears and vice versa.

It would be necessary here to make a distinction between chromonema and chromatid. While a chromatid is a half chromosome, two chromatids being connected at the centromere, the chromonema is a structure which is of a sub-chromatid nature and there can be more than one chromonemata in a chromatid (Fig. 6.9 B).

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