BIOB11H3 Lecture Notes - Lecture 17: Sister Chromatids, Coiled Coil, Condensin
Lec 17- Cell Cycle II: Chromosomes Condensation & Movement
Lecture Notes
• Chromosome Condensation
• Overview of Critical Steps in Mitosis →
o (prophase) chromosomes condense; chromatids visible
o cytoskeleton disassembles; spindle apparatus forms; nuclear membrane
breakdown
o (prometaphase) chromosomes captured by MTs at centromeres/kinetochores;
begin movement to equator
o (metaphase) chromosomes aligned on metaphase plate
o (anaphase) sister chromatids separate; begin poleward movement; poles also
move apart
o (telophase) chromosomes cluster at opposite poles, decondense; nuclear
membrane reforms
• Cyclin-CDK complexes promote cell cycle progression
• START: G1 cyclin/CDK triggers G1/S transition
• MPF: mitotic cyclin/CDK triggers G2/M transition
• During S phase:
• 1. DNA replication must be initiated
• 2. Centrioles must be replicated
• For M phase to occur →
• 1. Nuclear membrane breaks down (lamins)
• MPF activity promotes lamina disassembly
o Lamina- plays structural role; proteins Lamin A, B & C interact & some
chromatin attaches to these lamin proteins
• i) MPF phosphorylation of lamins A & C
• ii) Induces disassembly of lamina & release of chromatin
• iii) Chromatin now free to condense
• 2. Chromosomes condense (condensin/cohesion)
• MPF also targets cohesion & condensin
• Fig 14-14: Model for roles of condensin & cohesion in formation of mitotic
chromosomes → After replication, DNA helices of a pair of sister chromatids held in
associated by cohesion molecules that encircle the sister DNA helices. As cell enters
mitosis, the compaction process begins as condensin forms a ring around supercoiled
loops of DNA within chromatin. Cooperative interactions btwn condensin molecules
organize the supercoiled loops into larger coils which are then folded into a mitotic
chromosome fiber.
• Cohesin: 2 large SMC coiled coil proteins + several regulatory subunits (target of MPF
phosphorylation); maintains sister chromatid cohesion until metaphase checkpoint has
passed
o Loss of cohesion along axis from interphase to prophase
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Document Summary
Lec 17- cell cycle ii: chromosomes condensation & movement. Centrioles must be replicated: for m phase to occur , 1. As cell enters mitosis, the compaction process begins as condensin forms a ring around supercoiled loops of dna within chromatin. Cytoskeleton undergoes dramatic reorganization (mt associated proteins: centrosome: microtubule organizing center (mtoc, centrioles: (2 per centrosome) perpendicular pairs of structures consist of 9 fibrils of microtubules duplicated at s-phase, at mitosis, spindle apparatus forms. During s phase, daughter procentrioles form adjacent to maternal centrioles so that two pairs of centrioles become visible within the centrosome. The daughter procentrioles continue to elongate during g2 phase & at the beginning of mitosis, the centrosome splits, with each pair of centrioles becoming part of its own centrosome. Molecular motors: drives chromosomes along microtubule tracks: dyenin: minus-end directed motor, kinesin: plus-end directed motor, both use atp hydrolysis energy to walk cargo, 2.
