Chapter 10: Cell Cycle and Cell Division (Cell: Structure and Function)

About 24 hours (human cells in culture divide once in about 24 hours), of which the M phase takes only about an hour.

Karyokinesis is the division of the nucleus: separation of daughter chromosomes into two nuclei (prophase to telophase). Cytokinesis is the division of the cytoplasm that follows, forming two daughter cells: by a furrow in animal cells and by a cell plate in plant cells. Karyokinesis may occur without cytokinesis (multinucleate coconut endosperm).

Interphase (over 95% of the cycle) is the preparation for division, with growth and DNA replication in order. G₁: the cell is metabolically active and grows but does not replicate DNA. S: DNA replicates, DNA content doubles from 2C to 4C (chromosome number unchanged, 2n); in animal cells the centriole duplicates. G₂: proteins needed for mitosis are made and growth continues.

G₀ is an inactive stage that cells enter when they exit G₁ and stop dividing. Such cells (e.g. heart cells) remain metabolically active but no longer proliferate, unless called on to divide according to the organism's needs (e.g. to replace lost cells).

Because the number of chromosomes in the parent cell and in each daughter cell is the same (2n → 2n), and the daughter cells receive identical genetic material.

(i) Metaphase (ii) Anaphase (of mitosis; also anaphase II of meiosis) (iii) Zygotene of prophase I (iv) Pachytene of prophase I.

(Diagram in the concept notes.) (a) Synapsis: the pairing of homologous chromosomes during zygotene, accompanied by the formation of the synaptonemal complex. (b) Bivalent (tetrad): the complex of a pair of synapsed homologous chromosomes, with four chromatids, clearly seen in pachytene. (c) Chiasmata: the X-shaped points where non-sister chromatids of homologues remain attached after crossing over, seen in diplotene when the homologues begin to separate.

In animal cells, a furrow appears in the plasma membrane, deepens and meets in the centre, dividing the cytoplasm (from the periphery inwards). Plant cells have an inextensible wall, so a new wall forms from the centre outwards: a cell plate (precursor of the middle lamella) forms in the centre and grows to meet the existing lateral walls.

Equal: spermatogenesis (four equal sperms from a primary spermatocyte) and microsporogenesis (four pollen grains in a tetrad). Unequal: oogenesis in animals, where unequal cytokinesis gives one large ovum and small polar bodies; in plants, megasporogenesis gives a linear tetrad of which one megaspore is functional and three degenerate.

(i) Mitosis in somatic (and some haploid) cells; meiosis in diploid cells forming gametes. (ii) One division vs. two divisions with one replication. (iii) Equational (2n → 2n) vs. reductional (2n → n). (iv) Two identical daughter cells vs. four genetically different haploid cells. (v) Simple prophase vs. long prophase I with synapsis, crossing over and chiasmata. (vi) Single chromosomes vs. bivalents at metaphase (I). (vii) Chromatids vs. homologues separate at anaphase (I). (viii) Growth and repair vs. gamete formation and variation (see the table in the concept notes).

(i) It conserves the specific chromosome number of each species across generations: gametes are haploid and fertilisation restores diploidy. (ii) It increases genetic variability by crossing over and the random separation of homologues; variation is the raw material for evolution.

(i) Male honey bees (drones), and males of wasps and ants, are haploid; their cells divide by mitosis (gametes form by mitosis). In lower plants the haploid gametophytes of bryophytes (moss, Marchantia) and pteridophytes (prothallus), haploid algae (Spirogyra, Chlamydomonas) and fungi grow by mitosis. (ii) In higher plants, the haploid synergids and antipodal cells of the embryo sac (and the egg cell, until fertilised) do not divide.

No. Mitosis separates the two sister chromatids of each chromosome to the two daughter nuclei. These chromatids are made by DNA replication in S phase; without it there would be only one chromatid per chromosome and the daughter cells would receive half the DNA. Cell-cycle controls prevent mitosis until replication is complete.

Yes. DNA may replicate repeatedly without nuclear or cell division (endoreduplication): e.g. polytene chromosomes of Drosophila salivary glands, and polyploid cells such as some liver cells. Also, cells that replicate DNA and then arrest (G₂) do not divide.

In meiosis: after meiosis I each cell has n chromosomes and 2C; after meiosis II, n and C.