Waldeyar (1988) introduced the name chromosome (Chroma-color, Soma-body) for first time which is deeply staining bodies of the cell.
These are thread like bodies carrying hereditary material. They are capable of self reproduction and play important role in heredity. Their number is constant in each species of plants and animals.
A typical chromosome has an elongate cylindrical body tapering at both ends which consist of following parts.
(A) External parts:
The tapering elongated parts on either side of centromere are called arms. They may be of equal or unequal length.
The chromosome has non staining constricted region called centromere or primary constriction. The position of centromere is constant for a particular chromosome. It is functional during cell division.
The tip or end portion of chromosome is called telomere. It has unique property by which it prevents the ends of chromosomes from sticking to each other.
(4) Secondary constriction:
In addition to primary constriction, chromosome may show one or more secondary constrictions.
Their positions are also constant for a particular chromosome and therefore useful for identification of chromosome.
A mass of heterochromatic material attached to one of the arm of chromosome is called satellite. A chromosome having this body is called SAT-chromosome.
(B) Internal parts:
The matrix encloses the thread like coiled filaments called chromonemata. They are two in number and coiled over each other.
However, their coiling is not in the same direction throughout the length but changes at primary constriction. They are present in the form of coils i.e. major and minor coils.
These are small granules present in centromeric region to which chromonemata are attached and direction of coiling is changed.
Chromosome is having mass of colorless fluid called matrix, which is achromatic material. The chromonemata are suspended in matrix.
It is believed that nucleolar material and matrix is interchangeable i.e. when nucleolus appears matrix disappears and vice versa.
Matrix is bounded by its outer limit or membrane called pellicle. The chromosomes are having genetical material (DNA) in chromatins which take light stain and occupies major portion called euchromatin.
The remaining “part take dark stain, but genetically inert, having ample RNA called heterochromatin.