Cell division has two stages: karyokinesis (nuclear division) and cytokinesis (cytoplasmic division). (p.100)
Somatic cells: body cells other than gametes; diploid (2n = 46 in humans); produced by mitosis. (p.100)
Gametes: reproductive cells; haploid (n = 23 in humans); produced by meiosis. (p.100)
Homologous chromosomes: a pair of paternal and maternal chromosomes with the same structural characteristics. Chromatin is a chromosome that looks like a long thread. (p.100)
6.2 Cell Cycle and Mitosis
The cell cycle is DNA multiplication followed by cell division to form two daughter cells; it has interphase (G1, S, G2) and M phase (mitosis and cytokinesis). (p.101)
G1 phase: cell grows, organelles and proteins are produced, chromosomes are chromatin. (p.101)
S phase: DNA is replicated; each chromosome becomes two identical sister chromatids joined at the centromere. (p.101)
G2 phase: cell continues to grow and prepares for division. (p.101)
Mitosis: division of the nucleus into two nuclei with the same chromosome number and genetic content as the parent cell. (p.102)
Prophase: chromatin shortens and thickens into visible chromosomes (two sister chromatids); nuclear membrane and nucleolus disappear; centrioles move to opposite poles; spindle fibres form. (p.102)
Metaphase: chromosomes line up on the equatorial plane held by spindle fibres; ends when centromeres begin to divide. (p.103)
Anaphase: centromeres divide; spindle fibres shorten and pull sister chromatids to opposite poles. (p.103)
Telophase: chromosomes become chromatin, nucleoli and nuclear membranes re-form, spindle fibres disappear; followed by cytokinesis. (p.103)
Plant cells have no centrioles but still form spindle fibres. (p.104)
Animal cytokinesis: plasma membrane constricts in the middle (cleavage furrow) via microfilaments. (p.104)
Plant cytokinesis: vesicles form a cell plate at the centre that expands to form a new cell wall, strengthened by cellulose. (p.104)
Necessity of mitosis: embryo development and growth, replacing dead or damaged cells (wound healing), regeneration (lizard tail), asexual reproduction (hydra budding). (p.104)
Applications: plant tissue culture in agriculture; stem cell therapy (bone marrow stem cells for damaged cartilage); cultured animal stem cells for meat. (p.105)
6.3 Meiosis
Meiosis occurs in the testis and ovary and produces haploid gametes from diploid cells. (p.105)
Needed to form gametes, maintain the diploid chromosome number across generations and produce genetic variation. (p.105)
Prophase I: homologous chromosomes pair (synapsis) to form bivalents (tetrads); crossing over at chiasma exchanges genetic material between non-identical chromatids, giving new gene combinations. (p.106)
Metaphase I: homologous pairs line up on the equatorial plane; sister chromatids remain joined. (p.106)
Anaphase I: homologous chromosomes separate and move to opposite poles; each is still two sister chromatids. (p.106)
Telophase I and cytokinesis: two haploid daughter cells. DNA does not replicate between meiosis I and II. (p.106)
Prophase II: nuclear membrane and nucleoli disappear; spindle fibres form in both daughter cells. (p.107)
Metaphase II: chromosomes arranged randomly on the equatorial plane. (p.107)
Anaphase II: centromeres separate; sister chromatids move to opposite poles and are now chromosomes. (p.107)
Telophase II: four haploid daughter cells with genetic content different from the parent. (p.107)
6.4 Issues of Cell Division on Human Health
Uncontrolled cell division forms tumours: benign (not dangerous, removed surgically) or malignant (cancer). (p.108)
Causes of cancer: radiation (X-ray, gamma, UV), tar in tobacco, carcinogens (formaldehyde, benzene), genetic factors, bacteria and viruses. (p.108)
Cancer cells spread through lymph and blood vessels and destroy normal cells. (p.108)
Down syndrome is caused by nondisjunction (chromosomes fail to separate in anaphase I or II), giving gametes with 22 or 24 chromosomes. (p.109)
Fertilisation of a normal gamete (23) with a gamete of 24 gives a zygote with 47 chromosomes: trisomy 21. (p.109)
Down syndrome features include mental retardation, stunted growth, slanted eyes and slightly protruding tongue. (p.109)