Page 49 - Demo 1

membrane, and the cell eventually divides in two, distribung organelles and
other cell contents to the two daughter cells.
In plants, the dividing cell forms a similar structure called the cell plate.
Cytokinesis in plant cells involves the building of new plasma membranes and
cell walls between the daughter cells.

CONTROL OF THE CELL CYCLE

In order for the cell to reproduce successfully, the cell cycle must be
controlled. The cell cycle has key checkpoints where feedback signals from the
cell about its size and the condion of its chromosomes can either trigger
subsequent phases of the cycle or delay them to allow more me for the current
phase to be completed. Three principal checkpoints control the cell cycle in
eukaryotes, one in each of the G1, G2, and mitoc phases.

G1 Checkpoint is when cell growth is assessed. Just before entry into
the S phase and near the end of G1, the G1 checkpoint makes a crical decision
for the cell to divide, delay division, or enter a resng stage. If the cell does not
pass this checkpoint, it can enter G0, during which it performs its normal
funcons but does not divide. Proper growth signals must be present for a cell
to pass this checkpoint. Also, the integrity of the cell’s DNA is checked such
that a damaged DNA can stop the cycle at this checkpoint and iniate DNA
repair. If that is not possible, it can cause the cell to undergo programmed cell
death, or apoptosis.

G2 Checkpoint is when DNA replicaon is assessed. The cell cycle stops
briefly at the G2 checkpoint unl the cell verifies that DNA has replicated.
Addionally, if DNA is damaged due to solar radiaon or x-ray exposure, stopping
the cell cycle at this checkpoint allows me for its repair, ensuring that the
damage is not passed on to daughter cells. If this checkpoint is passed, the
many molecular processes that signal the start of mitosis are set in moon.

M Checkpoint takes place at metaphase and iniates the exit from
mitosis and cytokinesis and the beginning of G1. This checkpoint ensures that
the chromosomes are properly aached to the spindle and will be distributed

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