Transcript Cell Growth and Division

Chapter 10
Limits to cell size
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Ability of DNA to control cell functions
◦ Think of only having one set of directions and tools
to build an entire city. Would all the workers be able
to get directions and enough tools to do their job?
◦ When a cell gets too big, its DNA cannot meet the
needs of the entire cell
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Surface Area to Volume ratio
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When a cell gets bigger, the volume and
surface area increase….
BUT ~ the volume increases much more
quickly
This means that there is relatively LESS cell
membrane for a big cell than a little one
Why do you think this might be a problem?
Look at these two “cells.” The one on the left is 5X5
and the one on the right is 8X8. Which one has the
higher surface area to volume ratio?
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Organisms keep growing, even though cells
generally stay about the same size….How?
Cells divide!
A big organism’s cells are about the
same size as a little organism’s
cells, there are just more cells.
The art of making more cells
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Cells constantly go through a cycle of growth
and division
This cycle is divided into two parts:
Interphase and Cell Division
Cell Division consists of mitosis and
cytokinesis, but the cell must prepare for
division during interphase
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G1 (gap 1)
◦ Cells increase in size and produce new proteins and
organelles
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S phase (synthesis)
◦ DNA is synthesized and chromosomes are
duplicated in preparation for mitosis
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G2 (gap 2)
◦ Organelles and molecules needed for mitosis are
produced
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After G2, mitosis occurs. But first we need to
know some terms!
◦ Mitosis
◦ Cytokinesis
◦ Chromatid
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•Centromere
•Centriole
•Spindle
It is important to realize the human cell has
46 chromosomes normally (23 pairs)
When it gets ready for mitosis, it doubles this
and makes 46 pairs of homologous sister
chromatids (During S phase)
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Prophase
Metaphase
Anaphase
Telophase
Cell division is not
complete after
mitosis….
Cytokinesis
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Chromatin condenses to form chromosomes
Centrioles separate and move to opposite
ends of the cell
Spindle begins to form
Nuclear envelope breaks down and nucleolus
disappears
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Chromosomes line up at the center of the cell
Spindle fibers connect to each of the
chromosomes at the centromere
Think of the boxers all meeting
in the middle of the ring to
touch gloves
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The sister chromatids separate into single
chromosomes
Chromosomes are pulled to opposite ends of
the cell by the spindle
Think of Ana, the ring girl,
pushing the boxers apart
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Chromosomes have moved to opposite ends
of the cell and start to unravel into loose DNA
Two separate nuclear envelopes start to form
Spindle begins to break down
Nucleolus reappears in both nuclear
envelopes
Cytokinesis begins here….
See the cell membrane start to
pinch together?
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Even though mitosis is complete, cell division
is not!
The cytoplasm must now divide
between the two new cells
In animal cells, the cell membrane
simply “pinches off” until 2 new cells
are created
In plant cells, a cell plate forms between the 2
new cells, creating a cell membrane. Then a
cell wall forms.
Why is it important?
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Normal cells have ways to tell when they
should stop growing
What do you think will happen to the cells
below when they touch each other and have
no more room on the plate?
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This is an example of EXTERNAL REGULATORS
of the cell cycle
Cells respond to proteins on other cells and
stop growing
◦ This kind of regulator is helpful in wound healing
and embryo growth
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This is when the cell itself regulates the cell
cycle
For example, the cell will produce certain
proteins when it has replicated its
chromosomes.
This tells the cell that it is time to
proceed with mitosis.
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Cancer.
Cells no longer respond to the proteins
(cyclins) and will reproduce continuously
This causes uncontrolled cell growth, or
tumors.
Tumors that invade the rest of the body and
cause direct damage are known as cancer