CH 10 - HOW CELLS DIVIDE

            I. PROKARYOTIC

A.   BINARY FISSION (p.187) - ASEXUAL

B.   SINGLE, CIRCULAR CHROMOSOME

                        DNA (Genome) replicates, then divides

            II. EUKARYOTIC = CELL CYCLE  - Regular sequence of growth & division        

                        2 major parts, 5 phases (p. 192) - ASEXUAL

                 A. INTERPHASE: Growth (80-95% of time)

                        1.  G₁  = Growth in cell size and # of organelles

                        2.  S = Synthesis - DNA (genome) is replicated.

                        3.  G₂ = 2^nd Growth - Cell prepares for division.

                 B. DIVISION: (5-20% of time)

                        4.  Mitosis - Division of the chromosomes

                        5.  Cytokinesis - Division of the cytoplasm & organelles

            III. MITOSIS  - Gk. "mitos" = thread (p. 194-195)

                 -2 genetically identical cells, NO VARIETY, ASEXUAL

                        -Bone marrow, skin and spleen divide often.

                        -Liver and kidney cells divide infrequently è G₀

                        -Nerve and muscle cells rarely if ever divide in an adult è G₀

                        -Purpose = Growth, repair and replacement.

                        - 25,000,000/second in humans!

                 A. PROPHASE

                   Chromatin = DNA (40%) + histone proteins (60%).  Looks like thin strands. (p. 190)

                        1. Chromatin condenses & becomes visible (becomes chromosomes)  

                        2. Spindle begins to form

                        3. Nuclear envelope disappears.

                        4. Nucleolus disappears.

                        5. Chromosome structure (p. 191)

                                    a. 2 sister chromatids

                                    b. joined @ centromere

                                    c. 46 chromosomes, 92 chromatids, 23 homologous pairs

                  B. METAPHASE

                        1. Spindle attaches to chromatids at centromere (p. 193)

                        2. Region of attachment – Kinetochore

                        3. Chromosomes line up @ equatorial plane (p. 195)

                 C. ANAPHASE (p. 195)

                        1. Separation of sister chromatids at centromere, pulled to poles.

                        2. Each chromatid becomes a chromosome.

                 D. TELOPHASE – OPPOSITE of Prophase!

                        1. Nuclear envelope & nucleolus reform/reappear.

                        2. Chromosomes decondense, uncoil into chromatin.

                        3. Spindle disappears.

                        4. Plasma membrane begins to pinch in the middle.

            IV. CYTOKINESIS: (p. 195 & 197)

                 A. ANIMALS - CELL/CLEAVAGE FURROW - outside è in

                        1. Cell membrane furrows & pinches off forming 2 daughter cells.

                        2. Invagination leads to separation.        

                 B. PLANTS - CELL PLATE - inside è out (p. 198)

                        1. Vesicles coalesce to form cell wall between 2 cells.

                        2. A wall is formed from within.

            V. CONTROL OF DIVISION

                 A. CHECKPOINTS - (p. 200 - 201)

                        1. G1/S

                        2. G2/M

                        3. Spindle

                 B. CONTACT INHIBITION - stops growth after repair

                 C. EXTERNAL - MITOGENS = Mitosis generators, growth factors

                 D. CANCER - loss of control of growth & division (p. 204)