Glossary: Cell Division

This chapter's terms in context

Chapter 6 rests on five terms that distinguish the two forms of cell division tested at SPM level: mitosis, meiosis, chromosome, diploid and haploid. A chromosome is a thread-like structure made of tightly coiled DNA and protein, found in the nucleus, carrying the genes that determine an organism's characteristics; each species has a characteristic chromosome number, and human body cells normally contain 46 chromosomes arranged as 23 homologous pairs. Diploid describes a cell containing two complete sets of chromosomes, one set inherited from each parent, written as 2n, and every human body cell except the gametes is diploid. Haploid describes a cell containing only one complete set of chromosomes, written as n, and in humans only the gametes, the sperm and the egg cell, are haploid.

Mitosis is the type of cell division that produces two daughter cells, each genetically identical to the parent cell and to each other, both diploid if the parent cell was diploid. Mitosis is responsible for growth, the repair of damaged tissue and asexual reproduction, and because the daughter cells are identical copies, an organism relies on mitosis whenever it needs more of the same kind of cell rather than genetic variety. Meiosis, in contrast, is the type of cell division that produces four daughter cells, each genetically different from one another and from the parent cell, and each haploid, containing exactly half the chromosome number of the parent cell. Meiosis occurs specifically during the formation of gametes, and its purpose is twofold: to halve the chromosome number so that fertilisation restores the full diploid number in the offspring, and to generate genetic variation through the independent assortment and crossing over of chromosomes.

The most common structural error in this chapter is describing meiosis simply as 'like mitosis but producing four cells', without stating the two features that actually distinguish it: the halving of chromosome number and the genetic difference between the resulting cells. A second frequent confusion is between 'chromosome' and 'chromatid', a chromosome that has replicated consists of two identical sister chromatids joined at a centromere, and these chromatids only become separate chromosomes once they are pulled to opposite poles of the cell during division, a stage-by-stage detail that Paper 2 questions on the cell cycle frequently probe.

Because fertilisation combines one haploid gamete from each parent to restore the diploid number, mitosis and meiosis work as a matched pair across the whole life cycle: meiosis halves the chromosome number to make variable haploid gametes, and fertilisation immediately restores it, while mitosis then multiplies that new diploid cell into every other cell of the growing organism. Losing sight of this cycle, for instance, forgetting that a zygote formed by fertilisation is diploid even though it grows entirely by mitosis from that point onward, is a frequent source of confusion when meiosis, mitosis and gamete formation are combined in one exam question.

Exam questions on this chapter frequently supply a diagram of a cell at an unlabelled stage of mitosis or meiosis and ask a candidate to identify the stage and state the chromosome number present, testing whether n and 2n notation has been genuinely understood rather than memorised as a symbol. A common scoring gap is correctly identifying a stage of division from its diagram but stating the wrong chromosome number, particularly at stages where chromosomes have already replicated into sister chromatids but have not yet separated, a point at which the chromosome number has not changed even though the amount of DNA has doubled. Practising with diagrams showing several consecutive stages of mitosis or meiosis, and stating both the stage name and the chromosome number for each one in a single sentence, closes this specific and recurring gap.