While examining the mitotic stage in a tissue, one finds some cells with 16 chromosomes and some with 32 chrom

NCERT Exemplar · Q28

Q. While examining the mitotic stage in a tissue, one finds some cells with 16 chromosomes and some with 32 chromosomes. What possible reasons could you assign to this difference in chromosome number. Do you think cells with 16 chromosomes could have arisen from cells with 32 chromosomes or vice versa?

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Concept understanding — Anaphase Chromatid Separation

Anaphase Chromatid Separation

Imagine you are holding a long, stretched-out spring between your two hands. If you cut it exactly at the middle, each hand now holds one half. That is the core picture of anaphase — but with a crucial twist: the "cut" happens only when the cell is ready, and the two halves are actively pulled apart, not just released.

The Intuition: Why Separate at All?

A cell has spent hours duplicating every chromosome into two identical copies (sister chromatids) held together at a central region called the centromere. These copies are like a pair of identical twins holding hands. For the cell to divide into two daughter cells, each new cell must get one complete set of genetic instructions. The twins must let go and walk to opposite ends of the cell. That letting-go and walking is anaphase.

The separation is not passive. The cell does not simply dissolve the glue and let the chromatids drift. It actively reels them in, like a fisherman pulling a line, ensuring each daughter cell gets exactly one copy of every chromosome.

The Precise Statement

Anaphase chromatid separation is the simultaneous splitting of the centromere that holds sister chromatids together, followed by the poleward movement of each daughter chromatid along shortening spindle fibres attached to its kinetochore.

Important

The key event is the sudden, synchronous cleavage of the cohesin protein complex at the centromere. This is triggered by the enzyme separase, which is activated only after the cell has verified that all chromosomes are properly attached to the spindle (the spindle assembly checkpoint). Until that checkpoint is satisfied, separation is blocked.

The Two Movements in Detail

Anaphase is often split into two concurrent processes:

Anaphase A — Chromatids move to the poles. The spindle fibres (microtubules) attached to the kinetochore at the centromere shorten by losing tubulin subunits at the kinetochore end. The chromatid is literally reeled in, like a rope being pulled from the pole. The kinetochore "walks" along the shortening microtubule.

Anaphase B — The poles themselves move apart. The spindle fibres between the two poles (interpolar microtubules) elongate, pushing the poles farther from each other. This stretches the cell and increases the separation between the two sets of chromatids.

Both movements happen simultaneously in most cells. The result is that by the end of anaphase, each pole has a complete set of chromosomes, and the cell is ready to divide its cytoplasm (cytokinesis).

Watch out

A common mistake is to think the centromere splits before the spindle fibres attach. In reality, the spindle fibres attach during prometaphase, but the centromere remains intact until anaphase onset. The attachment happens first; the separation happens only when the cell is certain everything is aligned.

The Molecular Trigger …

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Seeing both 16- and 32-chromosome cells in the same mitotic tissue most plausibly reflects the same cell caught at different moments — 32 being the transient anaphase count of a cell whose stable number is 16, not a genuinely different lineage.

Mitosis is specifically described as an equational division: the chromosome number in the parent cell and in the progeny cells stays exactly the same, neither halved nor increased. This has a direct bearing on how to interpret two different chromosome counts observed within what is described as a single mitotic tissue.

If a cell's stable, resting chromosome number is 16, then a completed, normal mitotic division of that cell should produce two daughter cells that also each have 16 chromosomes — not 32 and not 8. Mitosis conserving the number rules out one true lineage of cells simply becoming another through ordinary division.
The doubling that does legitimately occur within a single cell happens transiently, at anaphase. Anaphase begins when every chromosome on the equatorial plate splits simultaneously, its centromere dividing and its two sister chromatids separating. The moment this happens, each separated chromatid is reclassified as a full daughter chromosome in its own right. So a cell with a true chromosome number of 16, caught at exactly this moment before cytokinesis has physically split it in two, would appear under the microscope to contain 32 separate chromosomes. …

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