BIOLOGY 201L Chapter Notes - Chapter 18: Spindle Apparatus, G1 Phase, Sister Chromatids
Chapter 18 - The Cell-Division Cycle
Monday, February 15, 2016
2:37 PM
Most basic function of cell: duplicate and segregate DNA accurately.
M phase - mitosis and cytokinesis; takes about an hour in mammal
Interphase - period between one M phase and the next; includes G1, S, and G2 phases
• G1 and G2 phase - gap phases where cell grows; genes are expressed
• S phase - synthesis; DNA replicates and sister chromatids form
• G1 --> S --> G2 --> M
Cell-cycle control system orders and coordinates events to ensure that they occur in the correct
sequence. Similar in all eukaryotes.
• Uses molecular brakes called "checkpoints" to pause cycle at certain transition points so doesn't
trigger the next step in cycle unless cell is prepared.
• Transition G1 --> S: confirm environment is favorable for replication. If unfavorable, delay progress
through G1 and even enter G0 resting state.
o Very important in animals. Cancer results if malfunction.
• Transition G2 --> M: checks if DNA is undamaged and fully replicated
• During M: checks that chromosomes are attached to mitotic spindle before being segregated.
Mitosis
Before mitosis, each chromosome has 2 chromatids held together by cohesion proteins. Mitosis cleaves
the proteins.
Centrosome is principal microtubule-organizing center in animal cells. Duplicates in centrosome cycle so
can help form poles of mitotic spindle so each daughter cell has own centrosome.
• Centrosome duplication starts with DNA replication. Triggered by Cdks-G1/S-Cdk.
• The two separate during mitosis and each nucleates a radial array of microtubules called an aster.
Asters move to opposite sides of nucleus to form mitotic spindle.
Prophase - mitotic spindle forms when microtubules growing from centrosomes interact with each
other and stabilize; sis chromatids condense; kinetochores on centromeres; centrosomes move apart
• Those centrosomes are called spindle poles.
• Centrioles and proteins in centrosomes matrix grow microtubules.
• Spindle assembly driven by motor proteins that help cross-link the microtubules.
Prometaphase - nuclear envelope disassembles, so now spindle microtubules can access chromosomes
at kinetochores (protein complexes on cent
romere)
• Each chromosome has 2 kinetochores, one on each chromatid. They face opposite directions.
• Chromosomes attach to opposite poles (bi-orientation) to generate tension on kinetochores.
• Each human kinetochore binds 20-40 microtubules. Yeast binds just one.
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