Download Chapter 10

Chapter 10
Cell Growth &
Division
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10 –1 Cell Growth
• Which has larger cells: an
adult elephant or a baby
elephant?
• Neither! They are the same
size. The adult just has more
cells.
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Two main reasons why cells divide
rather than continuing to grow
indefinitely:
• The larger a cell becomes, the more demands
the cell places on its DNA. DNA "Overload"
– A huge cell needs a lot of DNA, but cells only have
one set of DNA.
• The cell has more trouble moving enough
nutrients and wastes across the cell membrane.
– it would be more difficult to get sufficient amounts of
oxygen and nutrients in and waste products out
• Cells don’t get bigger… Organisms do!!
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Division of the Cell
• Cell divides into 2 new daughter
cells.
• DNA duplication occurs before
division (own genetic set)
• Each daughter cell has increased
surface area to volume, which
allows exchange of materials with the
environment.
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10–2 Cell Division
• Every cell must first copy its genetic
information before cell division begins.
• Each daughter cell then gets a complete
copy of that information.
• Eukaryotes are more complex than
Prokaryotes. In Eukaryotes:
– The first stage, division of the cell nucleus, is
called mitosis. The second stage, division of
the cytoplasm, is called cytokinesis.
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Chromosomes
• genetic information is passed on from one
generation of cells to the next
• Chromosomes – made up of DNA and
proteins
• every organism has a specific number of
chromosomes
– fruit flies = 8 chromosomes
– humans = 46 chromosomes
– carrot = 18 chromosomes
– Chicken = 78 chromosomes
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Chromosomes (cont.)
• chromosomes aren’t visible except during
cell division…otherwise the DNA and
proteins are spread throughout the
nucleus.
• at the beginning of cell division,
chromosomes condense and become
visible.
• before cell division, each chromosome is
duplicated.
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Parts of the Chromosome
• chromatids – sister chromatids: each
chromosome consists of 2 identical sister
chromatids. (separated during cell division)
• centromere – where each pair of
chromatids is attached near center (protein
disk)
• entering cell division in humans = 46
chromosomes each with sister chromatids.
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The Cell Cycle
• recurring events in the life of the cell.
• the series of events that cells go
through as they grow and divide
• During the cell cycle, a cell grows,
prepares for division, and divides to
form two daughter cells, each of
which then begins the cycle again.
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The Cell Cycle
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4 Phases of the Cell Cycle
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G1  S  G2  M
G1 = intense growth and activity
S = DNA replication
G2 = intense growth and activity
[Interphase = G1, S, G2 ]
M = Mitosis (nucleus division) and
cytokinesis (cytoplasm division)
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The Cell Cycle Animation
• http://www.cellsalive.com/cell_cycle.htm
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Events of the Cell Cycle
• G1 = cells do most of their growing.
• During this phase, cells increase in size and
synthesize new proteins and organelles.
• S = DNA is replicated
• synthesis of DNA molecules
• Key proteins associated with the chromosomes are
also synthesized
• G2 = shortest phase of interphase
• organelles and proteins required for cell division
are produced
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Mitosis
• 4 phases: [P M A T]
– Prophase
– Metaphase
– Anaphase
– Telophase
• Lasts from a few minutes to several days
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Prophase
• longest phase of mitosis (50 to 60 percent of
the total time of Mitosis)
• chromosomes become visible
• centrioles separate – go to opposite ends
of the cell
– centrioles lie in a region called the centrosome
that helps to organize the spindle
• condensed chromosomes become attached
to fibers in the spindle at a point near the
centromere of each chromatid
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Prophase (cont.)
• nucleolus disappears (breaks up)
• nuclear envelope breaks down
• chromosomes coil more tightly
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Early prophase
Mid prophase
Late prophase
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Metaphase
• lasts only a few minutes
• the chromosomes line up across the
center of the cell
• Microtubules connect the centromere of
each chromosome to the two poles of the
spindle
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Anaphase
• the centromeres split
• sister chromatids separate  become
individual chromosomes
• chromosomes continue until they have
separated into two groups near the poles
of the spindle
• Anaphase ends when the chromosomes
stop moving
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Telophase
• Chromosomes begin to uncoil into a
tangle of dense material (chromatin)
• A nuclear envelope re-forms around
each cluster of chromosomes
• The spindle begins to break apart
• A nucleolus becomes visible in each
daughter nucleus
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Mitosis Animation
• http://www.cellsalive.com/mitosis.htm
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Cytokinesis
• two nuclei — each with a duplicate
set of chromosomes — are formed,
within the cytoplasm of a single cell
• the division of the cytoplasm itself
• occurs at the same time as telophase
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• animal cells = cell membrane is drawn
inward until the cytoplasm is pinched into
two nearly equal parts
• plants = the cell plate forms midway
between the divided nuclei
– The cell plate gradually develops into a
separating membrane.
– A cell wall then begins to appear in the cell
plate.
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10-3 Regulating the Cell Cycle
• Controls on Cell Division
–When cells come into contact with
each other, cells respond by
stopping growth
–when space is put between cells,
cells begin growing once again.
–controls on cell growth can be
turned on and off
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Contact Inhibition
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Cell Cycle Regulators
• Cyclins (proteins) regulate the timing
of the cell cycle in eukaryotic cells.
• two types of regulatory proteins:
those that occur inside the cell and
those that occur outside the cell.
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Internal Regulators
• Proteins that respond to events inside the
cell
• allow the cell cycle to proceed only when
certain processes have happened inside
the cell
– For example, several regulatory proteins make sure
that a cell does not enter mitosis until all its
chromosomes have been replicated.
– Another regulatory protein prevents a cell from
entering anaphase until all its chromosomes are
attached to the mitotic spindle.
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External Regulators
• Proteins that respond to events outside
the cell
• direct cells to speed up or slow down the
cell cycle.
– Growth factors are among the most important
external regulators. They stimulate the growth
and division of cells. Growth regulators are
especially important during embryonic
development and wound healing.
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Uncontrolled Cell Growth
• cells that divide uncontrollably and form
masses are called tumors
• Cancer cells do not respond to the
signals that regulate the growth of most
cells and may break loose from tumors
and spread throughout the body (cancer)
• Metastasis = is the spread of cancer from
its primary site to other places in the body
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Causes of cancer
• smoking tobacco
• radiation exposure
• even viral infection
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Why does cancer occur?
• Many (but not all) cancers have a defect in
a gene called p53
• p53 normally halts the cell cycle until all
chromosomes have been properly
replicated
• Damaged or defective p53 genes cause
the cells to lose the information needed to
respond to signals that would normally
control their growth
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Mitosis Animation
• http://www.cellsalive.com/mitosis.htm
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