The Cell Cycle & Mitosis

Account for means give reasons — here, reasons why each phase exists. Describing what happens in each phase is only half the task; the question is why a cell needs that phase at all, and why in that order.

7 marks, four phases. For each: what occurs, and why the cycle would fail without it. Finish on why the order is fixed.

A dividing cell must produce two daughter cells each having a complete and accurate copy of the genome and enough cytoplasm, organelles and protein to function independently. Division itself takes only a small part of the cycle; the majority, interphase, exists because a cell that divided without preparing would produce daughters that were too small, under-equipped, or carrying incomplete genetic information.

In gap 1 the cell grows in size, synthesises proteins and produces additional organelles, including mitochondria and ribosomes, while continuing its normal functions. This phase exists because division halves the cytoplasm and organelles between two cells: without a prior period of growth, cells would become progressively smaller with each division and would soon lack the machinery to operate. G1 also contains a checkpoint at which the cell assesses its size, its nutrient supply and any DNA damage, and either proceeds or halts.

In the synthesis phase the entire genome is replicated, so that each chromosome comes to consist of two identical sister chromatids joined at the centromere. This phase exists because mitosis distributes existing chromosomes rather than creating them: unless a complete second copy is made beforehand, the two daughter cells could receive only half the genetic information each. Replication is confined to a single dedicated phase so that it is complete and can be checked before separation begins.

In gap 2 the cell grows further and synthesises the proteins specifically required for division, including the tubulin from which spindle fibres are built, and it proofreads the newly replicated DNA, repairing errors. This phase exists as a final preparation and quality check: a second checkpoint verifies that replication is complete and undamaged, and division is delayed if it is not, because an error carried into mitosis would be copied into both daughter cells permanently.

Mitosis is the ordered separation of the sister chromatids so that each new nucleus receives one complete identical set, and it is followed by cytokinesis, in which the cytoplasm divides. Mitosis exists because accurate separation cannot be left to chance: the spindle apparatus attaches to every centromere and pulls the chromatids apart simultaneously, which is the only way to guarantee that each daughter receives exactly one copy of every chromosome rather than a random selection.

The sequence is fixed by dependency: the cell must be large enough and undamaged before replicating, so G1 precedes S; the genome must be fully copied before it can be separated, so S precedes G2 and mitosis; and the copy must be checked before separation makes any error permanent, so G2 lies between them. The checkpoints at each transition enforce the order, and their loss is what allows the uncontrolled division seen in cancer.

Describe the named phases means all four by name, in order, each with what happens to the chromosomes. Cytokinesis is named separately in the question because it is not a phase of mitosis, and treating it as one loses a mark.

5 marks. Prophase, metaphase, anaphase, telophase, then cytokinesis. One sentence each, every sentence saying what the chromosomes are doing.

In prophase the chromosomes, already replicated during the synthesis phase, condense by coiling tightly and become visible under a light microscope as two sister chromatids joined at a centromere. The nuclear membrane breaks down, the nucleolus disappears, and spindle fibres begin to form from the centrosomes as they move to opposite poles of the cell.

In metaphase the spindle fibres attach to the centromere of each chromosome, and the chromosomes are moved to line up singly along the equator of the cell, midway between the two poles, with the sister chromatids of each facing opposite poles.

In anaphase the centromere of each chromosome divides and the spindle fibres shorten, pulling the sister chromatids apart and drawing them towards opposite poles. Each chromatid is now an individual chromosome, so each pole receives one complete identical set.

In telophase the chromosomes reach the poles and begin to uncoil, the spindle fibres break down, and a new nuclear membrane forms around each set, producing two nuclei within the one cell. Mitosis, the division of the nucleus, is complete at this point.

Cytokinesis is the division of the cytoplasm, which follows mitosis and physically separates the two nuclei into two cells. In an animal cell the membrane pinches inward as a cleavage furrow that deepens until the cell splits; in a plant cell, whose rigid wall cannot pinch, vesicles fuse along the midline to build a new cell plate that becomes a dividing cell wall. The result is two genetically identical daughter cells.