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BIOC 212 Lecture Notes - G1 Phase, Spindle Apparatus, G2 Phase

Course Code
BIOC 212
Thomas Duchaine

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Lecture 26: Cell Cycle (March 14th, 2012)
Cell Cycle (Lecture 24-26, Slide 1)
- Very fundamental for all cells: bacteria, mammalian, …
- They all need to duplicate:
o Cell content
o Genome
- Two identical cells = the result of cell cycle
- 2 separate processes involved with the cell cycle
o Cell growth
Cell duplicates contents
Driven by protein synthesis
o Cell division
DNA replication
Chromosome segregation
- It has been studied for a long time but it’s only a few years that they could study it at a molecular level
What is seen under the microscope? Figure below. (Lecture 24-26, Slide 2)
- You see two phases (for mammalian cells for example)
o M phase
Cells look completely round
The cells have to:
Make the spindle poles
Break down the nucleus
Break down the nuclear membrane
Attach the spindle poles to the different chromosomes
DNA has to condense into chromosomes
Chromosomes have to line up in the middle
Chromosomes are then pulled apart
o Cell is divided in 2
Nuclear membrane is recreated
Spindle poles are degraded
Figure 17–2 The events of eucaryotic cell division as seen under a microscope.
The easily visible processes of nuclear division (mitosis) and cell division
(cytokinesis), collectively called M phase, typically occupy only a small fraction of
the cell cycle. The other, much longer, part of the cycle is known as interphase.
The five stages of mitosis are shown: an abrupt change in the biochemical state
of the cell occurs at the transition from metaphase to anaphase. A cell can pause
in metaphase before this transition point, but once the point has been passed, the
cell carries on to the end of mitosis and through cytokinesis into interphase. Note
that DNA replication occurs in interphase. The part of interphase where DNA is
replicated is called S phase (not shown).

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o Interphase
Cells look flat
Cells are attached to the cell culture dish (where cells are grown)
Unless you’re examining blood cells
o They are liquid
o They float
o Difficult to tell which phase they are in
o They are not attached to the cell culture dish
- People still didn’t know how this worked
o They would stain the DNA to see it all happen
o They were curious to see how it all worked & how it was regulated
Further conclusions were drawn about the cell cycle (Lecture 24-26, Slide 3)
- M phase
o Nuclear division
o Cytoplasmic division
o Maybe 1 hour
- Gap phases
o Not in all cells
o Only present in adult cells
Used as checkpoints
To assure readiness of commitment to M or S phase
Don't want to go into S phase if DNA is damaged by carcinogens or radioactivity
which lead to mutations or cancer
o Repair phases
o Used to assure genome integrity
o Not in embryonic cells
Due to rapid production of the embryo before it gets degraded by someone else
o G1 & G2
o In cancer, these checkpoints are gone
Cell although mutated, or not
attached to a spindle, it still
enters M phase
In normal cells there are
sensors, in cancer cells these
sensors disappear
Accumulation of damages occur
- S phase
o DNA division
o Maybe 12 hours
Because one 1m long DNA
- You can study this cycle in mammalian cells
o Grow them in culture
o Study the cycle this was more difficult
because the genomes were not sequenced back in the day
o Mammalian cells are diploid therefore every gene is doubled so when you want to remove a
gene, it’s double the work
o Still to this day it takes a long time to remove the genes in diploid cells, hence the use of haploid
o Want to use less complex organisms to study yeast is a model organism for this reason
Figure 17–3 The phases of the cell cycle.
The cell grows continuously in interphase,
which consists of three phases: DNA
replication is confined to S phase; G1 is
the gap between M phase and S phase,
while G2 is the gap between S phase and
M phase. In M phase, the nucleus and then
the cytoplasm divide.
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