Cell Size and the Cell Cycle
Chapter 5, Section 1
In your textbook
Cell Size and Cell Division
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Most cells are between 2-200 um in
diameter (1 micrometer = 1 x 10-6
m)
Some bacterial cells are only
several nanometers in diameter (1
nm = 1 x 10-9 m)
Largest known cell = yolk of an
ostrich egg, at 8 cm in diameter
Why do Cells Divide?
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to replace old / damaged cells
to help the organism grow
to prevent individual cells from
getting too large
Why Can’t a Cell Become Really
Large?
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Substances need to get in and out
of the cell at a reasonable rate to
keep the cell supplied with nutrients
and prevent wastes from building
up
These substances typically move
through the cytoplasm by diffusion;
as the distance they must travel
increases, diffusion becomes less
efficient (it takes longer)
…Too Large (cont’d)

The surface-area-to-volume ratio
must be high enough to allow
diffusion to occur fast enough; as a
cell grows, its volume increases
faster than its surface area
…Too Large (cont’d)
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There must be enough DNA present
to program the cell’s activities and
keep them running; in some larger
cells (like Pelomyxa – a giant
amoeba), multiple nuclei are
present
The Cell Cycle
Before cell division can take
place…
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a cell must replicate its genetic
material so that the daughter cells
created during division have the
same amount of genetic material as
the parent cell.
What happens during the cell
cycle?
The cell cycle is the continuous series of
events that occur during a cell’s
“lifetime”. It consists of four phases:
(1) G1 – First “Gap” Phase
During this phase, the cell grows and
prepares for division. Mainly, its
cytoplasm is increasing in volume.
Cell Cycle (cont’d)
(2) S – “Synthesis” Phase
During this phase, the cell’s DNA is
replicated.
(3) G2 – Second “Gap” Phase
During this phase, the cell continues to
grow and prepare for division. Mainly,
its organelles are being replicated.
What happens during the cell cycle?
(cont’d)
These first three phases (G1, S, and G2) are often
grouped together and called interphase.
What happens during the cell cycle?
(cont’d)
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M – Mitosis
During this phase, the nucleus and
the nuclear material divides
Cytokinesis
During this phase, the cytoplasm
divides.