Transcript 1. Cell division functions in reproduction, growth, and repair

The Cell Cycle
Cellular and nuclear division
Boehm 2014
The Cell Cycle
Boehm 2013
Why do cells divide?
• To reproduce- unicellular organisms.
• To repair dead or damaged cells
• To grow and get bigger- multicellular organisms
• Cells need to remain small!
– Think agar block lab
Fig. 12.1
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DNA and Cell Division
• A dividing cell replicates its DNA, separates
the two copies to opposite ends of the cell,
and then splits into two daughter cells.
Boehm 2013
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• DNA molecules are packaged into
chromosomes for cell division.
• Human somatic cells (body cells) have 46
chromosomes.
• Human gametes
(sperm or eggs)
have 23 chromosomes,
half the number in
a somatic cell.
• When the cell is not
Fig. 12.2
undergoing division, the DNA is in the form of
chromatin (unwound DNA).
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• Each duplicated chromosome consists of two
sister chromatids which contain identical copies
of the chromosome’s DNA
.
• The region where the
strands connect is the
centromere.
Fig. 12.3
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Division of the cell
• Mitosis, is followed by division of the cytoplasm,
cytokinesis.
– These processes take one cell and produce two cells that
are the genetic equivalent of the parent.
• Each of us inherited 23 chromosomes from each
parent: one set in an egg and one set in sperm.
• The fertilized egg or zygote underwent trillions of
cycles of mitosis and cytokinesis to produce a fully
developed multicellular human.
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Review and reflect
• What are 2 reasons why cells must divide?
• Do you think all cells continually divide?
• Where is DNA located in eukaryotic cells?
• How does the original cell compare to the 2
daughter cells?
• Any other questions/points unclear?
Cell Cycle
• The mitotic (M) phase of the cell cycle
alternates with the much longer interphase.
– The M phase includes mitosis and cytokinesis.
– Interphase accounts
for 90% of the cell
cycle.
Fig. 12.4
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Interphase
• Interphase has three subphases:
– the G1 phase
• The cell grows and produces organelles
– the S phase
• the chromosomes are copied
– G2 phase
• The cell finishes growing and makes sure all
organelles are copied.
• DNA is in the form of chromatin during
interphase- why?
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Mitosis
– For description, mitosis is usually broken
into 4 subphases:
•
•
•
•
Prophase,
metaphase,
anaphase, and
Telophase.
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Prophase
• Chromatin condenses into chromosomes.
• The nuclear envelope
disappears.
• Spindle fibers begin to form
from the centrioles.
• Spindle fibers attach to the
centromere of each
Fig. 12.5b
chromosome.
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Metaphase
• The spindle fibers push the sister
chromatids until they are all arranged at
the metaphase plate
– An imaginary line between the poles
• The centrioles are now at
the poles.
Fig. 12.5d
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Anaphase
• The centromeres divide, separating the
sister chromatids.
• Each is now pulled toward the pole to
which it is attached by spindle fibers.
Fig. 12.5e
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Telophase
• Two nuclei begin to form
• DNA (chromatin) becomes
less tightly coiled.
• Cytokinesis, division
of the cytoplasm,
begins.
Fig. 12.5f
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Fig. 12.5 left
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Fig. 12.5 right
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Fig. 12.9
Boehm 2013
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Cytokinesis
• Cytokinesis, division of
the cytoplasm, typically
follows mitosis.
• In animals, the first sign
of cytokinesis (cleavage)
is the appearance of a
cleavage furrow in the
cell surface near the old
metaphase plate.
Fig. 12.8a
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Review and reflect
• In what stage of mitosis do chromosomes line
up in the middle of the cell?
• What do you think happens when cells divide
too quickly and out of control?
• Any other questions/points unclear?