4.1 Cell Division: Mitosis & Meiosis

Why This Matters

Unit C is the heaviest unit on the Biology 30 diploma, accounting for about 40% of the exam. Cell division underpins everything that follows: genetics only makes sense once you understand how chromosomes are copied, sorted, and passed on. Expect both multiple-choice and numerical-response questions asking you to count chromosomes or chromatids at a given stage.

The central question is simple: how does a single fertilized egg become trillions of cells, and how do parents pass exactly half their genetic information to each offspring? Mitosis answers the first; meiosis answers the second. Confusing the two is one of the most common errors students make on this unit.

Key Vocabulary First

Get these terms straight before reading further:

• A chromosome is a single molecule of coiled DNA carrying many genes.
• After replication, a chromosome has two identical halves called sister chromatids, joined at a centromere.
• Homologous chromosomes are a matched pair — one inherited from each parent — that carry the same genes in the same order.
• Diploid (2n) cells have chromosomes in homologous pairs (humans: 46); haploid (n) cells have one of each (humans: 23).

Watch the trap: "chromosome number" counts centromeres, not chromatids. A replicated chromosome with two chromatids is still one chromosome.

The Cell Cycle and Interphase

Before any division, a cell spends most of its life in interphase, divided into three stages:

• G1 (gap 1) — the cell grows and carries out normal functions.
• S (synthesis) — DNA replication occurs; each chromosome is copied so it now consists of two identical sister chromatids joined at a centromere.
• G2 (gap 2) — the cell grows more and prepares proteins for division.

A key trap: after S phase, the chromosome number does not change, but the DNA amount doubles because each chromosome now has two chromatids. A human cell entering mitosis still has 46 chromosomes, but 92 chromatids.

Mitosis: The PMAT Phases

Mitosis is a single nuclear division that produces two genetically identical diploid (2n) cells. Its purpose is growth, tissue repair, and asexual reproduction. The phases, in order, are remembered as PMAT:

1. Prophase — chromatin condenses into visible chromosomes; the nuclear membrane breaks down; spindle fibres form.
2. Metaphase — chromosomes line up single-file along the cell's equator (metaphase plate).
3. Anaphase — sister chromatids separate and are pulled to opposite poles.
4. Telophase — nuclear membranes re-form around each set; chromosomes uncoil.

Cytokinesis then divides the cytoplasm, yielding two daughter cells. Each is 2n and genetically identical to the parent. Because no shuffling occurs, mitosis maintains the genome exactly — essential for replacing skin, blood, and gut cells throughout life. Uncontrolled mitosis, by contrast, is the basis of cancer, a common diploma application.

Meiosis I and Meiosis II

Meiosis is two consecutive divisions (meiosis I and meiosis II) that produce four genetically unique haploid (n) gametes from one diploid cell.

Meiosis I — the reduction division separates homologous pairs:

• Prophase I — homologous chromosomes pair up (synapsis) and crossing over exchanges segments between non-sister chromatids, shuffling alleles.
• Metaphase I — homologous pairs line up; their random orientation is independent assortment.
• Anaphase I — whole homologues (still two chromatids each) separate. Chromosome number is halved here: 2n → n.

Meiosis II resembles mitosis: there is no further DNA replication, and the sister chromatids of each chromosome finally separate, producing four haploid cells. The net result of both divisions is one diploid cell → four haploid gametes, each with a unique combination of alleles. In males this yields four sperm; in females, one functional egg and polar bodies (covered in Unit B).

Mitosis vs Meiosis

Sources of variation in meiosis are: (1) crossing over in prophase I, (2) independent assortment in metaphase I, and (3) random fertilization when any sperm meets any egg. Together these generate enormous diversity.

Nondisjunction and Down Syndrome

Nondisjunction is the failure of chromosomes (or sister chromatids) to separate properly during meiosis. The result is a gamete with too many or too few chromosomes. If such a gamete is fertilized, the zygote has an abnormal chromosome number — an aneuploidy.

The classic example is trisomy 21: a gamete carries an extra chromosome 21, so the zygote has three copies of chromosome 21 (47 chromosomes total). This causes Down syndrome. Nondisjunction of sex chromosomes can produce conditions such as Turner syndrome (XO) or Klinefelter syndrome (XXY). Nondisjunction risk rises with maternal age — a frequent context for diploma questions.