Mitosis — Phases (Karyokinesis)

NCERT grounding

NCERT Class 11 Biology, Chapter 10 (Cell Cycle and Cell Division), §10.2, calls the M phase "the most dramatic period of the cell cycle, involving a major reorganisation of virtually all components of the cell." Because the chromosome number of parent and progeny cells is the same, it is named the equational division. The text divides nuclear division — karyokinesis — into four stages: prophase, metaphase, anaphase and telophase, while cautioning that division is a continuous, progressive process with no clear-cut lines between stages.

Karyokinesis in four phases

Karyokinesis is the division of the nucleus. It follows the S and G2 phases of interphase, during which the DNA has already been replicated so that each chromosome now consists of two identical sister chromatids joined at the centromere. Karyokinesis does not make new DNA; its entire job is to organise these duplicated chromosomes, split them at the centromere, and deliver one chromatid of each chromosome to each of the two daughter nuclei. Because the parent's chromosome number is preserved unchanged, mitosis is the equational division. The four phases below describe how this is achieved in a strict order — and that order itself is a recurring NEET item.

Prophase — condensation and spindle assembly

Prophase is the first stage of karyokinesis and follows the S and G2 phases. In those phases the newly formed DNA molecules are not distinct but intertwined; prophase is marked by the initiation of condensation of the chromosomal material. As chromatin condenses, the tangled material becomes untangled and resolves into compact mitotic chromosomes. Each chromosome is now seen to be composed of two chromatids attached together at the centromere — the visible product of S-phase replication.

Simultaneously the centrosome, which had already duplicated during the S phase of interphase, begins to move towards the opposite poles of the cell. Each centrosome radiates microtubules called asters; the two asters together with the spindle fibres constitute the mitotic apparatus. By the close of prophase the cell is structurally stripped down for division.

Metaphase — alignment on the metaphase plate

The complete disintegration of the nuclear envelope marks the start of metaphase, so the chromosomes are now spread through the cytoplasm. Condensation is complete and the chromosomes can be observed most clearly — this is the stage at which the morphology of chromosomes is most easily studied. A metaphase chromosome is made of two sister chromatids held together by the centromere.

Small disc-shaped structures on the surface of the centromeres, called kinetochores, serve as the sites of attachment of spindle fibres to the chromosomes. One chromatid of each chromosome is connected by its kinetochore to spindle fibres from one pole, and its sister chromatid is connected by its kinetochore to spindle fibres from the opposite pole. The chromosomes are drawn to the centre of the cell, and the plane along which they align is the metaphase plate (equatorial plate).

Anaphase — the centromere splits

At the onset of anaphase, each chromosome that was arranged on the metaphase plate is split simultaneously at the centromere. The two sister chromatids, now referred to as daughter chromosomes of the future daughter nuclei, begin migrating towards the two opposite poles. The defining geometric detail is the leading edge: as each chromosome moves away from the equatorial plate, its centromere remains directed towards the pole — so the centromere leads while the chromosome arms trail behind. The two key events of anaphase are simply that centromeres split and chromatids separate, then move to opposite poles.

Telophase — reformation of two nuclei

At the beginning of the final stage, the chromosomes that have reached their respective poles decondense and lose their individuality; the individual chromosomes can no longer be seen as discrete elements, and each set of chromatin material collects at one of the two poles. A nuclear envelope develops around each chromosome cluster, forming the two daughter nuclei. The structures lost in prophase now return in reverse: the nucleolus, Golgi complex and ER reform. Karyokinesis is complete — nuclear division has produced two genetically identical nuclei from one. Cytoplasmic division (cytokinesis) typically follows to finish the M phase, but that is a separate event from the four karyokinetic stages.

One subtlety underlies the whole sequence: chromosome number and chromatid count diverge until anaphase. From prophase through metaphase, each chromosome carries two chromatids, so a 2n cell has 2n chromosomes but 4n chromatids. The instant the centromere splits at anaphase, each chromatid is recounted as a full chromosome — momentarily doubling the chromosome number to 4n at the poles before cytokinesis halves it back to two 2n cells. NEET routinely probes this counting at a named phase.

Worked examples

Common confusion & NEET traps

The single largest source of error in this subtopic is conflating the phase at which a named event occurs. NEET deliberately pairs "alignment" with "splitting" and tests whether you can keep metaphase and anaphase apart, and whether you know that spindle fibres attach to the kinetochore — not the centromere itself.