Transcript 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, 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
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Chromosomes (cont.)
• chromosomes not visible except during
cell division…otherwise the DNA and
proteins are spread throughout the
nucleus.
• at beginning of cell division, chromosomes
condense and are 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
• 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 = Chromosome 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 = chromosomes are replicated and the
• synthesis of DNA molecules
• Key proteins associated with the chromosomes are
also synthesized
• G2 = shortest phase of interphase
• organelles and molecules 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
• 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 (spindle fibers) 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
• 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 (petri dish)
– when put with space 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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In the body
• Cut the skin or break a bone
• Cells at the edges of the injury are
stimulated to divide rapidly.
• Produce new cells
• When the healing process nears
completion, the rate of cell division slows
down
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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
• Cancer cells do not respond to the
signals that regulate the growth of most
cells
• divide uncontrollably
• form masses of cells called tumors
• may break loose from tumors and spread
throughout 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 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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http://msjensen.cehd.umn.edu/webanatomy/
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