Cell cycle

• Mitosis consists of five stages: Prophase, Prometaphase, Metaphase, Anaphase, and Telophase.
• The mitotic spindle is essential for all stages of mitosis.
• Centrosomes operate as microtubule organising centres (MTOC) during mitosis.
• Organelles cannot be produced de novo therefore they must be divided equally between daughter cells.
• ER is divided during cytokinesis and used to manufacture all other organelles.
• Mitochondria number is high enough to ensure adequate inheritance.
• Cytokinesis ensures inheritance of a full set of organelles.
• Mitosis ensures inheritance of a full set of chromosomes.
• The MTOC produces and organises three classes of microtubule spindle fibre: Astral microtubules anchor the MTOC to the cell membrane, Polar microtubules from opposite poles overlap and allow separation, Kinetochore microtubules attach to chromosomes.
• Centrosomes duplicate and mature in S-phase.
• In M-phase, centrosomes move to opposite poles.
• Centrosomes must only be copied once during mitosis.
• During Prophase, chromosomes condense to become thick and visible, the nuclear envelope breaks down, centrosomes separate and spindle fibres form.
• Separation of centrosomes during Prophase is caused by overlapping plus ends of polar microtubules.
• At the end of Prophase, chromosomes are free in the cytoplasm.
• Chromosome condensation in Prophase disentangles chromatin, facilitates accurate chromosome segregation, and prevents DNA breakage in mitosis.
• Facilitated by condensins which use ATP to remove cohesins, by the end of Prophase, centromeres are the main attachment point.
• In Prometaphase, the nuclear envelope fragments completely, each chromatid has a kinetochore, and kinetochore microtubules search for chromosomes.
• Dynamic instability in Prometaphase increases, plus ends of polar microtubules overlap, and motor proteins assist microtubule binding.
• In Metaphase, centrosomes at opposite poles release microtubules that connect to kinetochores, tension aligns chromosomes at the metaphase plate, and astral microtubules attach to the cell membrane.
• Contraction causes the cytoplasm to divide as the furrow deepens during cytokinesis.
• Telophase is the complete depolymerisation of the spindle, with tubulin entering the cytoskeleton.
• The breakdown of the nuclear envelope occurs in late prophase and is triggered by the phosphorylation of the chromosomes.
• A contractile ring forms at the old metaphase plate and creates a cleavage furrow during cytokinesis.
• The contractile force can bend a glass needle.
• Chromosomes become less condensed and enter the interphase state during telophase.
• Cell surface area is increased as the membrane incorporates vesicles during cleavage.
• The contractile ring is made of two filaments: Actin and Myosin II, plus many regulatory and structural proteins.
• Nuclear pore complexes pump in nuclear proteins and the nucleus expands during telophase.
• Astral microtubules connect to the kinetochores during mitosis.
• During anaphase, two mechanisms move chromosomes: Motor proteins 'walk' chromosomes along microtubules from the plus end, and the microtubule depolymerises once the protein has moved past.
• The nuclear envelope reforms from fragments left after prophase and prometaphase during telophase.
• The plane of division in cytokinesis is determined by signals generated by the spindle during anaphase.
• Anaphase A is the separation of chromosomes, driven by depolymerisation of microtubules at the plus end.
• Cytokinesis starts in anaphase and finishes immediately after telophase.
• Anaphase B is the separation of spindles, partly driven by depolymerisation of microtubules at the minus end and partly by activity of motor proteins at the plus end.
• Motor proteins at the MTOC 'reel in' microtubules from the minus end, and the microtube depolymerises once past the MTOC.
• Sister chromatids are bound together by cohesion rings before they are allowed to separate during anaphase.
• Seperase cleaves cohesins at the centromere in Anaphase, chromatids are now called chromosomes, and depolymerisation at both plus and minus ends of kinetochore microtubules moves chromosomes polewards.
• In Anaphase, polar microtubules lengthen causing cells to expand, the extent of overlap is less in Anaphase than Metaphase, and at the end of Anaphase poles contain an identical set of chromosomes.