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| Chromosomes and Centromere |
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| Centromere is the primary constriction in the chromatid of a chromosome where the spindle tubules become attached during cell division. A chromosome generally has only one centromere and such a chromosome is described as monocentric. Sometimes two centromeres may occur in the chromosome as in the case of maize plant. Such a chromosome is described as dicentric. In such a chromosome each chromatid will have three arms. In the round worm Parascaris equorum the chromosomes have more than two centromeres and are hence described as polycentric. Very rarely, a centromere may be absent and such a chromosome may be described as acentric or holocentric. In such chromosomes the entire surface of the chromatid functions as the centromere. |
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| In monocentric chromosomes the centromere does not always occur in the centre. Based on the position of the centromere, the monocentric chromosome can be distinguished into four types, namely |
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 Metacentric, where the centromere is exactly in the centre and the two arms are of the same length. Such a chromosome assumes the shape of 'V' during anaphase. |
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 Sub-metacentric, where the centromere is slightly away from the centre and hence the two arms are unequal. Such a chromosome attains the shape of 'L' during anaphase. |
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 Acrocentric, where the centromere is situated far away from the centre so that the two arms are highly unequal. Such a chromosome attains the shape of 'J' during anaphase. |
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 Telocentric, where the centromere is situated at the tip so that the chromatid has only one arm. Such a chromosome attains the shape of 'I' during anaphase. |
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| All the chromosomes in a given cell need not be of the same type with reference to the position of centromere. However, the two chromosomes in any homologous pair will be of the same type. In some cases only one particular type of chromosomes may occur. For e.g. In locusts, all the chromosomes are acrocentric, while in amphibians all the chromosomes are metacentric. |
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