Revise Cell and nuclear division for IB Biology (D2.1)

A dividing cell does not simply split in two on demand; it moves through an ordered sequence called the cell cycle. The cycle has two main parts. Interphase is the long preparatory part and is itself divided into three stages: G1 (Gap 1), in which the cell grows and makes proteins and organelles; S (Synthesis), in which the DNA is replicated so that every chromosome becomes two identical sister chromatids joined at a centromere; and G2 (Gap 2), in which the cell completes its growth and assembles what it needs to divide. The M phase (Mitotic phase) then follows and is much shorter: it consists of mitosis, the division of the nucleus, and cytokinesis, the division of the cytoplasm. The essential point is that DNA is copied once, during S phase, before either kind of nuclear division starts.

Mitosis is the division of a nucleus into two daughter nuclei that are genetically identical to each other and to the original, and that keep the full (diploid) chromosome number. It is a continuous process, but for description it is divided into four stages. In prophase the chromosomes condense and become visible as pairs of sister chromatids, and a spindle of microtubules begins to form. In metaphase the chromosomes are moved to the equator (the middle) of the cell and held there by the spindle. In anaphase the sister chromatids of each chromosome are pulled apart to opposite poles, so each pole receives one identical copy of every chromosome. In telophase the chromosomes arrive at the poles and a nuclear membrane re-forms around each set, giving two nuclei. Because DNA was copied exactly in S phase and the identical chromatids were shared out one-to-each, the two nuclei carry identical genetic information. This is precisely what growth, tissue repair and asexual reproduction require: new cells that are exact copies.

Mitosis divides the nucleus, but the cell is not yet two cells. Cytokinesis is the division of the cytoplasm that follows, physically pinching or partitioning one cell into two. In animal cells the plasma membrane is drawn inwards by a ring of protein filaments, forming a cleavage furrow that deepens until the cell splits. In plant cells, whose stiff cell wall cannot be pinched, vesicles gather along the middle and fuse to build a new cell plate that grows outwards into a dividing wall. The important distinction for the exam is that mitosis and cytokinesis are separate events: mitosis is nuclear division, cytokinesis is cytoplasmic division, and together they complete the M phase to yield two distinct daughter cells.

Meiosis is the reduction division used to make gametes. Starting from one diploid cell it carries out TWO successive divisions after only a single round of DNA replication, producing four haploid cells — cells with half the original chromosome number. In humans, for example, a diploid cell of 2n = 46 gives rise to gametes of n = 23. The two divisions do different jobs. The first division (meiosis I) separates the homologous chromosomes, one of each pair going to each new cell, and this is the step that halves the chromosome number. The second division (meiosis II) then separates the sister chromatids of each chromosome, much like a mitosis but in the now-haploid cells, giving four cells in total. Crucially, the four products are not identical: meiosis is built to create variation.

Two events inside meiosis make the four gametes genetically different. The first is crossing over, which happens in prophase I: homologous chromosomes pair up closely, and corresponding segments are exchanged between them. Because the maternal and paternal chromosomes of a pair carry different alleles of the same genes, this swap produces chromosomes with NEW combinations of alleles that neither parent chromosome had. The second is independent assortment, which happens in metaphase I: each homologous pair lines up on the equator and then separates independently of every other pair, so whether a given cell inherits the maternal or the paternal member of each pair is decided at random. With n pairs of chromosomes, independent assortment alone can produce 2^n different combinations of whole chromosomes. On top of these two internal mechanisms, sexual reproduction adds a third source of variation after meiosis is over: random fertilisation, since it is a matter of chance which sperm fertilises which egg. Together these mean each new individual is a genetically unique reshuffling of its parents' alleles.

It is worth holding the two divisions directly against each other, because most exam errors come from blurring them. Both are preceded by one round of DNA replication in S phase and both use a spindle to move chromosomes. But mitosis is a single division that keeps homologous chromosomes separate throughout and produces two diploid cells identical to the parent, whereas meiosis is two divisions in which homologous chromosomes first pair and then separate, producing four haploid cells that are all different. Only meiosis has crossing over and independent assortment of homologous pairs, and only meiosis halves the chromosome number. In short: mitosis is for making more of the same cell (growth, repair, asexual reproduction); meiosis is for making varied gametes (sexual reproduction).

The cell cycle is normally held under tight control, pausing at checkpoints until it is safe to proceed. When the genes that regulate the cycle mutate, this control can be lost, and a cell may divide again and again without restraint. The result is a tumour, an abnormal mass of cells produced by uncontrolled mitosis. A benign tumour stays where it is, but a malignant (cancerous) tumour can invade surrounding tissue and shed cells that travel in the blood or lymph to start secondary tumours elsewhere, a process called metastasis. Mutations of this kind can be triggered by mutagens such as certain chemicals, some viruses and ionising or UV radiation, and cancer usually requires several such mutations to accumulate. The key idea for D2.1 is simply that cancer is what happens when the normal control of cell division breaks down — division itself is the same process, but its regulation has failed.

D2.1 explanation questions are marked analytically: each distinct valid biological point is worth one mark, up to the total available. Method-style points (M) credit correct mechanism, and where a question builds on an earlier value, error-carried-forward (ECF) means a wrong figure early on does not cost you the marks that follow, provided your reasoning is shown. Study how each mark below is pinned to a specific named idea — a stage and a mechanism — rather than to loose wording, because that is exactly how the Practice engine will read your answer.

These two divisions underpin much of the rest of the course. Mitosis is the engine of growth and repair and the basis of asexual reproduction and cloning; understanding its loss of control is the foundation for the biology of cancer. Meiosis, with its crossing over, independent assortment and the random fertilisation that follows, is the ultimate source of the genetic variation that natural selection acts on — so it connects directly to inheritance, gene linkage and evolution. Get the products straight (two identical diploid cells versus four different haploid cells) and you have a template that anchors every genetics and continuity topic to come.