Chapter at a glance
- All cells reproduce by dividing; cycles of growth and division allow a single cell to produce millions of cells.
- The cell cycle is the sequence of events by which a cell duplicates its genome, synthesises other constituents and divides into two daughter cells.
- A typical eukaryotic cell cycle (human cells) lasts ~24 h; interphase occupies >95 % and M phase ~1 h.
- Interphase comprises G1 (cell growth, no DNA replication), S (DNA synthesis, DNA content doubles from 2C to 4C while chromosome number remains 2n) and G2 (protein synthesis and growth).
- Non-dividing cells exit G1 into the quiescent G0 stage; they remain metabolically active but do not proliferate.
- M phase (equational division) includes karyokinesis (prophase–metaphase–anaphase–telophase) followed by cytokinesis.
- Mitosis produces two diploid daughter cells with identical genetic complement; it maintains nucleo-cytoplasmic ratio and enables growth and repair.
- Meiosis involves one round of DNA replication followed by two sequential divisions (meiosis I and II), yielding four haploid cells; it conserves species chromosome number and generates genetic variability.
Definitions and laws
- Cell cycle: “The sequence of events by which a cell duplicates its genome, synthesises the other constituents of the cell and eventually divides into two daughter cells is termed cell cycle.”
- Interphase: “The phase between two successive M phases … the time during which the cell is preparing for division by undergoing both cell growth and DNA replication in an orderly manner.”
- M phase: “The phase when the actual cell division or mitosis occurs.”
- Quiescent stage (G0): “Cells that do not divide further exit G1 phase to enter an inactive stage called quiescent stage (G0) of the cell cycle.”
- Mitosis / equational division: Division in which “the number of chromosomes in the parent and progeny cells is the same.”
- Meiosis: Specialised division that “reduces the chromosome number by half” and produces haploid daughter cells.
- Crossing over: “The exchange of genetic material between two homologous chromosomes … enzyme-mediated process and the enzyme involved is called recombinase.”
- Synapsis: Pairing of homologous chromosomes during zygotene.
- Bivalent / tetrad: “The complex formed by a pair of synapsed homologous chromosomes.”
- Chiasmata: X-shaped structures formed at sites of crossing over where homologous chromosomes remain linked after dissolution of the synaptonemal complex.
(No formulas or SI units appear in the chapter.)
Important diagrams and activities
- Figure 10.1 – Diagrammatic view of cell cycle indicating formation of two cells from one cell (shows relative durations of interphase and M phase).
- Figure 10.2 (a–e) – Stages of mitosis (prophase, metaphase, anaphase, telophase, cytokinesis) – must be drawn to show chromosome behaviour and spindle formation.
- Figure 10.3 – Stages of meiosis I (bivalents, chiasmata, separation of homologues).
- Figure 10.4 – Stages of meiosis II (resembles mitosis; separation of sister chromatids to produce tetrad of haploid cells).
Common misconceptions and exam pitfalls
- Assuming chromosome number changes in S phase (it remains 2n; only DNA content doubles from 2C to 4C).
- Confusing chromatids with chromosomes after S phase; sister chromatids are still one chromosome until centromere splits in anaphase.
- Stating mitosis occurs only in diploid cells (plants show mitosis in haploid cells too; male honey bees are an animal exception).
- Believing cytokinesis is identical in plants and animals (furrow in animals; cell-plate in plants).
- Mixing anaphase of mitosis (sister chromatids separate) with anaphase I of meiosis (homologues separate, sister chromatids remain together).
- Forgetting that meiosis produces four haploid cells while mitosis produces two diploid cells with identical genomes.
- Overlooking that G0 cells are metabolically active and can re-enter the cycle when required.
Formula sheet
No quantitative formulas or equations are present in the chapter.