The Cell Cycle

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The Cell Cycle
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Cell Cycle in Eukaryotes

• The splitting of a parent cell into two
  genetically identical daughter cells is called
  mitosis.
• Unicellular organisms use cell division for
  reproducing whereas multicellular organisms use
  cell division for growth and repair.

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Why must cells divide? why cant they just keep
getting bigger and bigger?

• Cells have upper and lower size limits
• If cells were too small, they could not contain
  all of the necessary organelles and molecules
  needed for survival
• If cells get too large (even if surrounded by
  plenty of nutrients) the cell is unable to
  adequately exchange materials with its
  surroundings
• The upper limit on cell size is due to the ratio
  of cell surface area to volume
• As a cell grows, its volume increases more
  rapidly than its surface area. When the surface
  area-to-volume ratio is too small, the cell
  cannot move materials into and out of the cell at
  a sufficient rate or in sufficient quantities.
• A tennis ball has a larger surface areavolume
  ration than a soccer ball. It is easier for
  material to get into and out of a tennis ball
  than a soccer ball simply because of size.
  Material must travel farther to get to the center
  of the soccer ball than the tennis ball.
• Another way to think of this is that a cell with
  a large surface areavolume ratio has more units
  of cell membrane (surface area) taking care of
  the cytoplasm (volume) than a cell with a small
  surface areavolume ratio
• Cells need a large surface areavolume ratio

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Calculate SAV ratio

• Surface area of a cube (length x width x of
  sides)
• Volume of a cube (length x width x height)
• Practice
• SA
• V
• SA V Ratio
• The cell with the largest SAV ratio is the cell
  that is the smallest or largest?

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Cell Cycle in Eukaryotes

• The cell cycle is the repeating set of events
  that make up the life of a cell.
• The cell cycle consists of interphase (the time
  between cell divisions) and cell division.
• Interphase consists of three stages G1, S, and
  G2
• During interphase, cells carry out their normal
  functions and undergo critical growth and
  preparation for cell division
• Cell division (mitotic phase) consists of two
  stages mitosis and cytokinesis
• Mitosis is the division of the cell nucleus and
  its contents
• Cytokinesis is the process that divides the cell
  cytoplasm
• The result of this phase is two daughter cells
  that are genetically identical to the original
  cell

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Cell Cycle in Eukaryotes

• 3 stages of Interphase (this is the phase of the
  cell cycle that a cell spends most of its life)
• G1 a cell carries out its normal functions and
  grows
• Checkpoint during G1, the cell must pass a
  critical checkpoint before it can proceed to the
  S stage
• S The cell makes a copy of its DNA
• G2 The cell continues to carry out its normal
  functions and continues to grow
• Checkpoint during G2, the cell must pass another
  critical checkpoint before the cell goes through
  mitosis and division
• Cells need enough nutrition, adequate size, and
  relatively undamaged DNA to divide successfully

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Factors that Affect Rate of Cell Division

• Type of Cell
• Because prokaryotes do not have the
  membrane-bound organelles and cytoskeleton found
  in eukaryotes, prokaryotic cells typically divide
  much faster than do eukaryotic cells
• Bodys need for those cells
• A typical eukaryotic cell spends 12 hours in
  interphase, but
• Cell division is greater in embryos and children
  than it is in adults because many of their organs
  are still developing
• Cells that receive a lot of wear and tear divide
  and are replaced more frequently (ex skin cell
  has a 2 week life span whereas a liver cell
  divides every 1-2 years)

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Cell Cycle in Eukaryotes

• Some cells (such as fully developed cells in the
  nervous system) normally never divide once they
  are mature.
• In this case, the cell is said to have entered
  the G0 phase (they have exited the normal cell
  cycle but continue their normal functions)
• During the G0 phase, cells do not copy their DNA
  and do not prepare for cell division.

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Cell Cycle in Eukaryotes

• After the G2 phase, cells enter the M phase (M
  Mitosis)
• There are four steps in mitosis prophase,
  metaphase, anaphase, and telophase.
• The purpose of these stages of mitosis is to sort
  two sets of DNA and divide them between two
  nuclei

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Terms you must know to understand what is
happening during the stages of mitosis

• Chromosome? one long continuous thread of DNA
  that consists of numerous genes along with
  regulatory information
• (Your body cells all cells except egg and sperm
  - each have 46 chromosomes)
• Histones? proteins around which DNA wraps to help
  organize and condense itself
• Chromatin? uncoiled or stretched out DNA
  (chromatin will coil up or condense to form
  chromosomes)
• Chromatid?one half of the duplicated chromosome
  (half of the X)
• the two identical chromatids are called sister
  chromatids
• Centromere? the place where sister chromatids are
  held together
• Telomere? the ends of the DNA molecules
  (teloend)
• Telomeres are made of repeating nucleotides that
  do not form genes. A short section of nucleotides
  is lost from a new DNA molecule each time it is
  copied. It is important that these nucleotides
  are lost from telomeres, not from the genes
  themselves. Are the shortening of telomeres a
  biological cause of aging?

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Mitosis - Prophase

• Chromatin (uncoiled DNA) coils up to form
  chromosomes
• When the chromosomes coil up, then can now be
  seen with a compound light microscope
• The nuclear envelope and the nucleus break down
  and disappear
• In animal cells, centrioles and spindle fibers
  appear and move toward opposite ends of the cell
• As the centrioles move, spindle fibers (made of
  microtubules) radiate from the centrioles in
  preparation for cell division
• This array of fibers is called the mitotic
  spindle and serves to equally divide the
  chromatids between the two new cells during cell
  division.

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Mitosis - Prophase
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Mitosis - Metaphase

• Spindle fibers attach to the centromere of each
  chromosome and align the chromosomes along the
  cell equator (the middle of the cell)

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Mitosis - Anaphase

• The chromatids of each chromosome separate at the
  centromere
• The spindle fibers begin to shorten, which pulls
  the sister chromatids away from each other and
  toward opposite sides of the cell
• When the chromatids separate, they are considered
  to be individual chromosomes.

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Mitosis - Telophase

• A complete set of identical chromosomes is
  positioned at each pole (or end) of the cell
• The mitotic spindle fibers disassemble
• The chromosomes uncoil and return to being
  chromatin (uncoiled DNA)
• A nuclear envelope reforms around each new set of
  chromosomes

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Summary of the Four Stages of MitosisLink to
classzone animation of mitosis
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Last phase of cell division - Cytokinesis

• Cytokinesis is not considered part of mitosis.
• Cytokinesis is the process of the cell actually
  dividing into two new cells
• Each daughter cell is approximately equal in
  size.
• Each daughter cell receives
• 1. An identical copy of the original cells
  chromosomes
• 2. Approximately one half of the original cells
  cytoplasm and organelles.

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Cytokinesis Animal versus Plant

• Animal cells
• The cell membrane pinches inward forming a
  cleavage furrow
• Plant cells
• Instead of the cell membrane pinching in, a new
  cell wall (called a cell plate) forms between the
  two daughter cells

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Cytokinesis Animal versus Plant
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Mitosis Animations
http//www.cellsalive.com/mitosis.htm http//www.b
iology.arizona.edu/cell_bio/tutorials/cell_cycle/c
ells3.html http//www.johnkyrk.com/mitosis.html ht
tp//www.csuchico.edu/jbell/Biol207/animations/mi
tosis.html http//www.lewport.wnyric.org/jwanamake
r/animations/mitosis.html http//highered.mcgraw-h
ill.com/sites/0073031216/student_view0/exercise13/
mitosis_movie.html http//biotech-adventure.okstat
e.edu/low/basics/mitosis/mitosis-animation/index.h
tm http//www.phschool.com/science/biology_place/b
iocoach/mitosisisg/intro.html
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Regulation of the Cell Cycle
Cancer results from uncontrolled cell
growth Carcinogens are mutagens (substances that
mutate DNA) When DNA is mutated, the proteins
that help control the cell cycle may not be made
correctly, and therefore may stop
functioning Without proteins to control the cell
cycle, the cell starts dividing uncontrollably,
and cancer may result External factors that
affect the cell cycle Internal factors that
affect the cell cycle
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Regulation of the Cell Cycle
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Regulation of the Cell Cycle
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Prokaryotes versus Eukaryotes

• Prokaryotes are simpler cells that do not have a
  nucleus and typically have only one chromosome
  that is circular rather than linear. This one
  chromosome is attached to the plasma membrane
  unlike in eukaryotic cells where the chromosomes
  are contained in the nucleus of the cell.
• The process by which one prokaryotic cell splits
  into two is called binary fission (this is a type
  of asexual reproduction b/c only one parent is
  needed).
• Prokaryotic cells typically divide much faster
  than do eukaryotic cells because prokaryotes do
  not have the membrane-bound organelles and
  cytoskeleton found in eukaryotes

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Prokaryotes versus Eukaryotes

• 3 steps for prokaryotes
• 1. The one chromosome makes a copy of itself.
• 2. The parent cell continues to grow until it
  reaches twice its original size
• 3. A cell wall forms between the two chromosomes
  and the cell splits into two new cells.

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Binary Fission
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Asexual vs. Sexual Reproduction
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Multicellular Organization
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Multicellular Organization
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Multicellular Organization