Why Cell Division Matters

Evaluate requires a judgement of importance, not a description of four processes. Since all four are named, the judgement cannot be "they all matter" — it has to say how each matters, and on what basis they can be ranked or not.

7 marks. State the criterion for importance, judge each of the four against it, then deliver a verdict that answers whether one can be ranked above the others.

The importance of each process is best judged by what would be lost if it failed — whether the consequence is a defective cell, a defective organism, or the loss of a capacity on which the species depends. This distinguishes processes that are important to the individual cell from those important to the continuity of life itself.

DNA replication is the most fundamental of the four, because it is the only one that produces the genetic information the others distribute. Without it a cell could still divide, but each daughter would receive roughly half a genome, lacking the instructions for most of its proteins, and would be non-viable. Its semi-conservative, proofread mechanism also determines the accuracy of everything downstream: replication errors that escape correction become permanent mutations inherited by every descendant cell. Replication is therefore important both for the existence of the copy and for its fidelity.

Mitosis is important because it distributes that copy accurately. Its elaborate ordered sequence exists to guarantee that each daughter receives exactly one complete set of chromosomes rather than a random selection, which underpins growth from a single zygote to a multicellular organism, the repair of wounds, the continual replacement of cells such as skin and gut lining, and asexual reproduction in many species. Its importance is also demonstrated by the consequences of its failure: uncontrolled mitosis is cancer, while a failure of mitosis prevents growth and healing.

Meiosis is important on a different level: it is not required for an individual to grow, repair itself or survive, and many organisms live entirely without it. Its importance is to the species rather than the individual, since it halves the chromosome number so that fertilisation restores rather than doubles it, and since crossing over, independent assortment and random segregation generate the genetic variation on which natural selection acts. A population reproducing only by mitosis would be genetically uniform and far more vulnerable to a change in its environment or to a new disease.

Cytokinesis is the least conceptually complex of the four but is not dispensable: it is the physical division of the cytoplasm that completes the process, and without it a cell would accumulate multiple nuclei rather than producing separate daughter cells. It also distributes the cytoplasm, mitochondria and other organelles, so each daughter can function independently from the moment it exists. Its importance is one of completion rather than of information, which is a genuinely different kind of importance.

All four are essential, but not equally and not in the same way. Replication and mitosis are indispensable to the individual organism, since neither growth, repair nor survival is possible without them, and replication has the strongest claim of the four because it produces the information the others handle. Cytokinesis is necessary to complete division but contributes nothing to the genetic outcome. Meiosis is dispensable to the individual and indispensable to the species. Importance therefore depends on the level considered: at the level of the cell, replication is paramount; at the level of the population across evolutionary time, meiosis is, because it is the only one of the four that generates anything new.

Discuss means identify issues and provide points for and against, or from more than one perspective. Two failures are named, and they fail at different levels, so treat each in turn and then say what the pair of them reveals.

5 marks. Roughly two on uncontrolled mitosis, two on the meiotic failure, and one on what the contrast shows about why division is so tightly regulated.

The cell cycle is regulated by checkpoints at the G1, G2 and metaphase transitions, which verify cell size, the completeness of replication and the correct attachment of the spindle before allowing the cycle to proceed. If mutations disable these controls, cells divide without regard to those conditions, continuing past damage that should have halted them and dividing when no growth or repair is required.

The immediate result is a tumour: a mass of cells accumulating where the tissue has no need of them, consuming nutrients and oxygen and compressing surrounding structures. Because the damaged cells continue dividing without repairing DNA errors, further mutations accumulate, and cells may become malignant, invading neighbouring tissue and spreading through the blood or lymph to establish secondary tumours elsewhere. The damage is to the individual organism, and can be fatal to it, but it is not inherited, since the affected cells are somatic.

If the homologous pairs failed to separate at meiosis I, or the sister chromatids at meiosis II — a failure called non-disjunction — a gamete would receive an extra chromosome or lack one, and in a complete failure of the reduction division a gamete would be diploid rather than haploid.

Fertilisation involving such a gamete produces a zygote with an abnormal chromosome number. Because the quantity of DNA sets the balance of gene products, an extra or missing chromosome disrupts development throughout the organism: in humans most such zygotes fail to develop, and those that survive show conditions arising from a whole-chromosome imbalance. A complete failure of halving would give a triploid zygote, which in animals is almost always non-viable. Unlike a somatic mutation, this error is present in every cell of the resulting organism and is potentially passed to the next generation.

The two failures show why cell division is regulated so tightly and at so many points. An error in mitosis damages the individual in which it occurs, through a lineage of cells that grows within an otherwise normal body; an error in meiosis damages an entire new organism from its first cell and may be transmitted onward. The checkpoints of the cell cycle and the pairing and separation mechanisms of meiosis therefore protect different things — the integrity of a body, and the integrity of a genome passed between generations — which is why both are conserved across virtually all eukaryotic life.