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Lecture 16

BIOB11H3 Lecture Notes - Lecture 16: Sister Chromatids, Coiled Coil, Condensin


Department
Biological Sciences
Course Code
BIOB11H3
Professor
Dan Riggs
Lecture
16

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BIOB11-L16: Cell Cycle II (Chromosome condensation & Movement)
Events of Mitosis: Overview of Critical Steps
Prophase:
- Chromosomes condense; Sister chromatids become visible
- Cytoskeleton disassembles; Spindle apparatus forms; Nuclear membrane breakdown
Prometaphase:
- Chromosomes ‘captured’ by MTs at centromeres/kinetochores; begin movement to equator
Metaphase:
- Chromosomes aligned on metaphase plate (equator)
Anaphase:
- Sister chromatids separate; Begin poleward movement, Poles also move apart
Telophase:
- Chromosomes cluster at opposite poles, Decondense. Nuclear membrane reforms
Chromosome Terminology
- After Replication 2 identical sister chromatids
- Each sister chromatid assembles a functional kinetochore at the centromere region
Cyclin-CDK complexes promote cell cycle progression
- START: G1/S cyclin + CDK Triggers G1/S transition
- MPF: Mitotic cyclin + CDK Triggers G2/M transition
What are the targets of these protein kinases?
During S phase:
1. DNA replication must be initiated (Replication licensing events)
2. Centrioles must be replicated
Targets for MPF For M phase to occur:
1. Nuclear membrane breaks down (lamins)
2. Chromosomes condense (condensin / cohesin)
3. Cytoskeleton undergoes dramatic reorganization (MT associated proteins)
MPF target 1) Promotes lamina disassembly
- MPF phosphorylates Lamins A and C
- Induces disassembly of lamina Release of chromatin
- Chromatin now free to condense (Able to form mitotic chromosome)
MPF target 2) Cohesin & Condensin
Cohesin: 2 large SMC coiled coil proteins (Smc1,3) + several regulatory subunits (Scc1,3)
- Wrap along axis after Replication occurs & first part of Prophase
- Metaphase: Cohesin remains only at centromere regions
- Maintains sister chromatid cohesion until metaphase checkpoint has passed
Condensin: 2 SMCs (Smc2,4) + regulatory proteins
- Organizes chromatin into supercoiled loops Forming mitotic chromosome
- Act as a hinge, when hinge closes, bring chromatin into a more condense shape
Cohesin & Condensin function in chromosome architecture
Cohesin: Added as replication occurs
Held sister chromatids together during most of S, G2, M phase
Condensin: Exerts its function at Prophase to condense chromatin
Cohesin lost from chromosome arms but remains at centromere
Cohesin lost after passing Metaphase checkpoint Able to be separated in Anaphase
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