CELL CYCLE AND CELL DIVISION - QUICK REVISION.
CELL CYCLE: The sequence of events by which a cell duplicates its genome, synthesises other constituents and divides into two daughter cells. A human cell in culture divides about once every 24 hours; yeast in about 90 minutes. The cycle has two basic phases: INTERPHASE and M PHASE (mitosis). Cell division proper lasts only about an hour, so interphase is more than 95% of the cycle.
INTERPHASE has three sub-phases: G1 (Gap 1) - metabolically active, grows, no DNA replication; S (Synthesis) - DNA replication, DNA doubles from 2C to 4C but chromosome number stays the same (2n stays 2n); in animal cells the centriole also duplicates; G2 (Gap 2) - proteins synthesised for mitosis while growth continues. Cells that stop dividing exit G1 into the quiescent stage (G0), e.g. heart cells. In animals mitosis occurs in diploid somatic cells; plants can show mitosis in both haploid and diploid cells.
M PHASE (MITOSIS) = equational division (parent and progeny have the same chromosome number). Karyokinesis (nuclear division) has four stages, followed by cytokinesis. PROPHASE: chromosomal material condenses into compact chromosomes, each of two chromatids joined at the centromere; centrosomes (duplicated in S) move to opposite poles, radiating asters; spindle forms; by its end golgi, ER, nucleolus and nuclear envelope are no longer seen. METAPHASE: nuclear envelope completely disintegrates; chromosomes most condensed (best for studying morphology); kinetochores (disc-shaped, on centromeres) attach to spindle fibres; chromosomes align at the equator on the metaphase plate. ANAPHASE: centromeres split, sister chromatids (now daughter chromosomes) separate and move to opposite poles, centromere leading and arms trailing. TELOPHASE: chromosomes decondense and lose identity; nuclear envelope forms around each cluster making two daughter nuclei; nucleolus, golgi and ER reform. CYTOKINESIS: division of cytoplasm. Animal cells - a furrow in the plasma membrane deepens and joins. Plant cells - a cell-plate forms in the centre and grows outward (becomes the middle lamella). If karyokinesis is not followed by cytokinesis a multinucleate syncytium forms (e.g. coconut liquid endosperm).
SIGNIFICANCE OF MITOSIS: produces diploid daughter cells with identical genetic complement; growth of multicellular organisms; restores the nucleo-cytoplasmic ratio; cell repair (epidermis, gut lining, blood cells); continuous plant growth via meristematic tissues (apical and lateral cambium).
MEIOSIS: reduction division forming haploid gametes from diploid cells. Two sequential divisions (meiosis I and II) but only ONE round of DNA replication. Pairs homologous chromosomes and allows recombination; four haploid cells result. MEIOSIS I - PROPHASE I (long, five sub-stages): Leptotene (chromosomes become visible), Zygotene (synapsis - pairing of homologous chromosomes; synaptonemal complex; pair = bivalent or tetrad), Pachytene (recombination nodules; crossing over between non-sister chromatids by enzyme recombinase), Diplotene (synaptonemal complex dissolves; homologues separate except at chiasmata, the X-shaped crossover sites), Diakinesis (terminalisation of chiasmata; full condensation; nucleolus disappears, nuclear envelope breaks down). Metaphase I: bivalents align on the equatorial plate. Anaphase I: homologous chromosomes separate, sister chromatids stay together. Telophase I: nuclear membrane and nucleolus reappear, cytokinesis gives a dyad of cells. INTERKINESIS - short stage between the two divisions, no DNA replication. MEIOSIS II (resembles mitosis): Prophase II (nuclear membrane disappears), Metaphase II (chromosomes at equator, spindle attaches to kinetochores of sister chromatids), Anaphase II (centromeres split, sister chromatids move to opposite poles), Telophase II (nuclear envelopes reform, cytokinesis gives a tetrad of four haploid cells).
SIGNIFICANCE OF MEIOSIS: conserves the specific chromosome number of a species across generations (halved in gametes, restored at fertilisation) and increases genetic variability, which is important for evolution.