BIOLOGY 201L Chapter Notes - Chapter 18: Spindle Apparatus, G1 Phase, Sister Chromatids

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14 Dec 2016
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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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