Cells don't get bigger forever — they divide. Mitosis copies the genome; meiosis halves it. Both are exquisitely choreographed.
Every sub-topic below feeds at least one of these questions.
How can large numbers of genetically identical cells be produced?
How do eukaryotes produce genetically varied cells that can develop into gametes?
The required syllabus content for D2.1, in order. Each card is one lesson-sized checkpoint.
In all living organisms, a parent cell—often referred to as a mother cell—divides to produce two daughter cells.
Cytokinesis as splitting of cytoplasm in a parent cell between daughter cells
Equal and unequal cytokinesis
Roles of mitosis and meiosis in eukaryotes
DNA replication as a prerequisite for both mitosis and meiosis
Condensation and movement of chromosomes as shared features of mitosis and meiosis
Phases of mitosis
Identification of phases of mitosis
Meiosis as a reduction division
Down syndrome and non-disjunction
Meiosis as a source of variation
In every living organism, parent cells divide into daughter cells. Cytokinesis is the physical splitting of cytoplasm; nuclear division must come first.
A ring of actin and myosin forms around the cell equator. Myosin contracts; the ring tightens, pinching the cell in two.
Vesicles from the Golgi line up at the equator and fuse to form a cell plate. Cellulose is added to build a new cell wall between the daughter cells.
Usually division is equal — daughter cells of similar size. Unequal exceptions: oogenesis in humans (meiosis produces one large ovum and three small polar bodies); budding in yeast (a small daughter buds off a larger parent cell).
Both require DNA replication first. Mitosis produces two identical diploid cells. Meiosis produces four genetically varied haploid cells.
Used for growth, tissue repair, asexual reproduction. Maintains diploid chromosome number. One parent cell → two genetically identical daughter cells.
Used to produce gametes for sexual reproduction. Reduces diploid to haploid. One parent cell → four genetically varied daughter cells.
DNA replication first. Before either type of division, S phase replicates the entire genome. Each chromosome now consists of two identical sister chromatids joined at the centromere.
During interphase, DNA exists as diffuse chromatin. During prophase, it condenses into visible chromosomes — wrapped tightly around histones for safe movement.
DNA coils around histones into nucleosomes; nucleosomes coil into chromatin fibres; fibres coil further to form compact chromosomes. By prophase, the chromosomes are short and thick enough to be moved without tangling.
Spindle fibres (protein microtubules) attach to centromeres via kinetochores. During anaphase, microtubule motors at the kinetochores shorten the spindle fibres, pulling chromatids to opposite poles.
PMAT — Prophase, Metaphase, Anaphase, Telophase. You should be able to identify each from a micrograph.
Chromosomes condense and become visible. Nuclear envelope breaks down. Spindle forms.
Chromosomes line up at the cell equator. Spindle fibres attached to centromeres.
Sister chromatids pulled apart to opposite poles by shortening spindle fibres.
Chromosomes arrive at poles; decondense. Nuclear envelope reforms. Cytokinesis follows.
Meiosis I separates homologous chromosomes (reducing chromosome number); meiosis II separates sister chromatids (like mitosis).
Errors in chromosome separation during meiosis produce gametes with the wrong number of chromosomes. The most common consequence in humans is Down syndrome (trisomy 21).
Non-disjunction can occur during meiosis I (homologues fail to separate) or meiosis II (sister chromatids fail to separate). Either way, one resulting gamete has an extra chromosome (n+1) and one is missing one (n−1).
Down syndrome results when an n+1 gamete (extra chromosome 21) fertilises a normal gamete. The zygote has three copies of chromosome 21 — trisomy 21. Frequency rises with maternal age — eggs sitting in arrested meiosis for decades accumulate errors.
Meiosis is biology's variation-generator. Three independent mechanisms create genetically distinct gametes.
In prophase I, non-sister chromatids of homologous pairs exchange segments. Creates new combinations of alleles on each chromosome.
In metaphase I, the orientation of each bivalent is random. With 23 chromosome pairs in humans, that's 2²³ ≈ 8 million possible combinations from this step alone.
Any sperm can fertilise any egg. Combined with the variation already in each gamete, this multiplies variation enormously — >70 trillion possible offspring genotypes from any couple.
An extra 6 sub-topics for HL — same syllabus, deeper mechanism.
Cell proliferation for growth, cell replacement and tissue repair
Phases of the cell cycle
Cell growth during interphase
Control of the cell cycle using cyclins
Consequences of mutations in genes that control the cell cycle
Differences between tumours in rates of cell division and growth and in the capacity for metastasis and invasion of neighbouring tissue
If you can't define one of these in a sentence, that's where to revise next.
“What processes support the growth of organisms?”
“How does the variation produced by sexual reproduction contribute to evolution?”