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BIOB11H3 (13)
Dan Riggs (8)
Chapter 14

chapter 14 notes

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Department
Biological Sciences
Course Code
BIOB11H3
Professor
Dan Riggs

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Chapter 14
Two major phases in cell cycle: M phase (mitosis + cytokinesis) and interphase
(growth + preparation for dividing + other).
Time between start of labeling period to appearance of the first labelled mitotic
chromosomes = approximate length of G2.
DNA replication + synthesis of additional histones occurs at the S phase.
Most cells in the body are present in a state that precedes DNA synthesis, called
G0. They only proceed to S phase to complete a cell
cycle if they receive an internal signal.
Cell fusion experiments show that the cytoplasm of a replicating cell contains
diffusible factors that stimulate initiation of DNA
synthesis in G1-phase nuclei, whereas G2-phase nuclei cannot respond to initiation
factors present in S-phase cell cytoplasm.
Prereplication complex can only occur during early G1 phase.
G1 + M = premature chromosomal compaction to form a set of elongated compacted
chromosomes
G2 + M = premature chromosome compaction but doubled
S + M = chromatin became compacted -> "pulverized" chromosomal fragments
rather than intact.
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So, transition G1 -> S and G2 -> M are under positive control --> induced by a
stimulatory agent.
The entry of cell into M phase is initiated by maturation-promoting factor (MPF)
which consists of 2 subunits; one with kinase activity
and a regulatory subunit called cyclin. Cyclin levels regulate the activity of kinase,
which phosphorylates other proteins and allows
the cell to enter M phase.
At late G1 stage, G1 cyclins levels rise and bind to cdc2 kinase. This period is called
START. After START, the cell is irrevocably
committed to replicating DNA at sites where prereplication complex had previously
assembled.
Passage from G2 to mitosis requires activation of cdc2 by mitotic cyclins. However,
mitotic cyclins are not enough. Other regulatory
enzymes: CAK (Cdk-activating kinase), phosphorylates a critical threonine residue -
Wee1 phosphorylates a key tyrosine residue in the
ATP-binding pocket of the enzyme, important for keeping Cdk inactive until the end
of G2 where the inhibitory phosphate Tyr 15 is
removed by the phosphatase Cdc25.
In contrast to yeast, mammal cell mitosis is controlled my more than one type of
Cdk.
Study stages of mitosis from slides
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Description
Chapter 14 Two major phases in cell cycle: M phase (mitosis + cytokinesis) and interphase (growth + preparation for dividing + other). Time between start of labeling period to appearance of the first labelled mitotic chromosomes = approximate length of G2. DNA replication + synthesis of additional histones occurs at the S phase. Most cells in the body are present in a state that precedes DNA synthesis, called G0. They only proceed to S phase to complete a cell cycle if they receive an internal signal. Cell fusion experiments show that the cytoplasm of a replicating cell contains diffusible factors that stimulate initiation of DNA synthesis in G1-phase nuclei, whereas G2-phase nuclei cannot respond to initiation factors present in S-phase cell cytoplasm. Prereplication complex can only occur during early G1 phase. G1 + M = premature chromosomal compaction to form a set of elongated compacted chromosomes G2 + M = premature chromosome compaction but doubled S + M = chromatin became compacted -> pulverized chromosomal fragments rather than intact. www.notesolution.com
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