Classification of chromosomes based on centromere position



The position of the centromere is the chromosome (which is constant to a given chromosome) varies i.e., it may occupy different positions. Based on this, four morphological shapes have been identified in chromosomes. These are:

1. Metacentric:

The Centromere occupies a middle position with reference to the length of the chromosoem. The two arms thus resulted are almost equal in length. During anaphasic movement in cell division, metacentric chromosomes ap­pear 'V' shaped. Eg: Trillium. Tradescantia etc.,

2. Sub metacentric:

When the centromere is located some distance away from the middle region of the chromosome, the position is said to be median and the chromosome will be shorter than the other. During anaphasic movement, sub metacentric chromosomes appear 'L' shaped eg: Human beings.

3. Acrocentric:

In this case, the centromere is situated almost near one end of the chromo­some. As a result, one arm of the chromosome will be extremely short and the other very long. The centromere is said to occupy a subterminal postion. eg: Grass hoppers.

4. Telocentric:

When the centromere is situated exactly at one end, the chromosome will be having only one long arm. Telocentric chromosomes are very rare. Truly telocentric chromosomes have been identified by Marks (1957) in certain plants, protozoa and certain mammals.

Cleveland (1949) has also reported in certain protozoa inhabiting the digestive tract of wood termites, the occurrence of telocentric chromosomes. The earlier report regarding chro­mosome IV of Drosophila melangoaster, being telocentric has been disproved as a second arm has been seen.