The eukaryotic nucleus during interphase contains thin thread like chromatin fibers which condence longitudinally during cell divisions to appear like independent small rod-like structure known as chromosomes. Their gross structure is studied during metaphase as these become most distinct at this stage because of maximum condensation. The metaphase chromosome consists of two chromatids which run parallel to each other and held together by a single centromere. During anaphase, the centromere splits and the chromatids become independent chromosomes, each with two arms with a centromere. Basing on the position of centromere, the chromosomes of four types:

1. Metacentric:

Centromere remains at mid point and both arms are more or less equal in length.

2. Submetacentric:

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Arms are unequal in length.

3. Acrocenteric:

One arm is very short compared to other.

4. Telocentric:

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Telimore occupies the terminal position chromosome with one arm only. Besides the centromere or primary constriction, some chromosomes may possess secondary constrictios towards distal part of one or both arms, separating a small part form the main body called satellite. The previous idea on structure of chromosome was that the two arms are bounded by a limiting membrane called pellicle, inside which the colourless matrix is present.

Chromonemata are embedded in the matrix. However, electron microscopic studies reveal that an enormously long chain of DNA is associated with proteins to form nucleosome which coil in definite pattern to form as structure of this type. Chromosomes are bearer of hereditary materials or genes and thus responsible for inheritance of characters. They take major parts during cell division.