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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