Centromere

After a chromosome copies itself, it has two matching halves called chromatids. They stay joined at one narrow point. That point is the centromere. During cell division, spindle fibres grip the chromosome here. Later they pull the two chromatids apart.

Three words sound alike. Keep them apart:

• Centromere: the point that joins two chromatids.
• Centrosome: a small body near the nucleus of an animal cell. It holds the centrioles and forms the spindle.
• Chromatid: one of the two copies of a chromosome.

A small example

Picture the letter X as two curved halves, like ) and (, touching in the middle. Each half is a chromatid. The point where they touch is the centromere. In a drawing of metaphase, the centromeres line up across the middle of the cell.

Where marks go

The 2024 ICSE Biology paper had a one-mark question on where copied chromosomes are joined. The answer was centromere. CISCE's report on the 2024 ICSE Biology paper names the error: answers "referred to the 'centrosome' as the 'centromere'".

The 2025 paper asked for the same point again. CISCE's report on it lists the difference between centrosome and centromere among the topics candidates found difficult. The ICSE 2027 specimen paper gives the statement "duplicated chromosomes are joined at centrosome". You must spot it as false.

About the author

Deepti Sardana

Founder Member & Academic Lead

An IIT alumna (M.Sc., M.Phil.) with over 25 years in teaching, child psychology and learning assessment. As Academic Lead at EduBrahma, she leads the method TRUtesting uses to find where and why a child loses marks.

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