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Lecture 7

Biology 2382B Lecture Notes - Lecture 7: G2 Phase, Schizosaccharomyces Pombe, Chromosome Segregation


Department
Biology
Course Code
BIOL 2382B
Professor
Jessica Kelly
Lecture
7

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Lecture 7/8 - Mitosis & Cell Cycle Control
(Chp 18: p. 781-783)
(Chp 20: p. 847-853, p. 858-863, p. 887)
Function of the Cell Cycle
Essential mechanism by which all living things reproduce and pass on genetic
information to next generation of cells
Ensures that DNA in each chromosome is faithfully replicated to produce 2 copies
Replicated chromosomes must be accurately distributed (segregated) to 2
genetically identical daughter cells
Coordination of growth (increase in cell mass) with division
Checkpoints exist during the cycle to ensure that each daughter cell has the same full
set of chromosomes segregated properly and has grown sufficiently that the daughter
cells will be viable upon division…research has shown these mechanisms are highly
conserved among all eukaryotic cells…
The Cell Cycle & Its Phases
Cell cycle - the ordered sequence of events
in which a cell duplicates its chromosomes
and divides into two genetically identical
cells
4 Phases:
G1-S-G2 = Interphase
Cell increases in size
In M phase (mitosis) most dramatic
events can be observed microscopically
All Cells are generated from pre-existing
cells…
Difficult to determine where a cell is in
interphase but easy when in Mitosis…

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Phases of The Cell Cycle
G1 (Gap 1) Phase:
Generalized growth and metabolism of the cell
Where most cells arrest when not dividing (i.e. enter Go)
Variable length (~11 hours in mammalian cells)
S (Synthesis) Phase:
DNA replication occurs (~6-8 hours)
Cell stops actively metabolizing during this pahase
G2 (Gap 2) Phase:
Preparation for chromosome segregation and cell division (~4 hours)
M (Mitosis) Phase:
Chromatin condensation
Nuclear envelope breakdown (retracts into ER)
Sister chromatids attach to mitotic spindle (which duplicates)
Segregation of chromatids
Decondense and reformation of intact nuclei
Cytokinesis (~1 hour)
G1 being the longer phase of the cell cycle means that more cells are likely to be in this
phase of the cell cycle…
Some Eukaryotic Cell-Cycle Times
FACS can be used to quantify proportion of
cells are in each cell cycle phase
This is achieved by measuring the
intensity of fluorescence from the
Hoescht stain bound to DNA at the
end of S phase cell has double the
DNA and therefore fluorescence
intensity is also doubled
G1 being the longer phase of the cell cycle
means that more cells are likely to be in this
phase of the cell cycle…

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Human nerve cells are an example of cells that are permanently in G0
M phase (mitosis)
High accuracy and fidelity are required to assure that the chromosomes will be
segregated properly…
checkpoint system to ensure that previous phases are
complete before advancing to the next…
Chohesins - link the sister
chromatids together
APC/C - breaks these interactions
and allows the chromatids to separate
to opposite poles of the cell
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