Colchicine, Mitotic Index and Root Growth
A Class 10 student germinates onion bulbs and collects four root tips of similar length. Two tips are kept in distilled water for 4 hours; two are kept in 0.05% colchicine solution for the same time. She stains squashed tips with acetocarmine and counts 400 cells from each tip. In the water-treated tips she records 36 dividing cells: 20 in prophase, 8 in metaphase, 4 in anaphase and 4 in telophase. In the colchicine-treated tips she records 92 dividing cells: 10 in prophase, 76 in metaphase, 4 in anaphase and 2 in telophase. The roots treated with colchicine look slightly swollen and do not elongate as much. She repeats the count on a second root tip and gets similar values. Her partner says colchicine 'makes cells divide faster', so the higher number of dividing cells proves it is a growth promoter. The student is not convinced.
1. Calculate the mitotic index for each treatment and decide whether the partner's claim is supported by the data.
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- Mitotic index = (dividing cells ÷ total cells) × 100.
- Water-treated tips: 36 ÷ 400 × 100 = 9.0%.
- Colchicine-treated tips: 92 ÷ 400 × 100 = 23.0%.
- The index is higher after colchicine, but that alone does not prove faster division. A higher index can also result from cells being delayed or arrested in mitosis.
- Look at the phase distribution. In water, metaphase cells are 8 out of 36 dividing cells, about 22%. In colchicine, metaphase cells are 76 out of 92, about 83%. This is a strong accumulation at metaphase, not a general speed-up.
- The partner's claim is therefore not supported. To test division rate properly, the student would need counts over several time intervals, evidence that new cells are produced, and root length or cell-number data.
2. Explain the cellular reason for the swollen roots and reduced elongation. Predict what may happen if the colchicine is washed out after 4 hours.
Read the worked answer
- Root elongation depends mainly on division in the root apical meristem and on cell expansion.
- Colchicine interferes with spindle formation. Chromosomes can condense, but sister chromatids cannot be pulled to opposite poles, so many cells stop at metaphase.
- Fewer completed divisions means fewer new cells for growth, so the root elongates less. The root may look swollen because cells enlarge and remain blocked rather than because more tissue is being produced.
- If colchicine is washed out, the spindle may reform and some blocked cells may resume division. However, chromosomes may have already separated without cell division, so some daughter cells can end up with altered chromosome number. Recovery may therefore be partial and some cells may die; the root is unlikely to return completely to normal.