Cell division or mitosis

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Cell division or mitosis

• All Organisms Grow and Reproduce
• All Species Pass Their Hereditary Information on
  to Their Offspring

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CELL DIVISION IN BACTERIA Prokaryote (you are
an eukaryote)

• Binary Fission Is Bacterial Cell Division
• Copying of the DNA circle occurs at the
  replication origin
• Requires a battery of enzymes
• End result two side-by-side circles of DNA on
  membrane

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• Composition of the bacterial genome Exists as one
  double-stranded circle of DNA
• Attached to one point on the interior of the cell
  membrane
• Division Initiated by Growth of the Cell to a
  Certain Size
• New plasma membrane and cell wall materials laid
  down
• Growing membrane pinches inward, cell constricted
  in two
• Each cell contains a copy of the genome

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• Lets take a look at the cell cycle and how it is
  controlled.
• Cell Cycle An Interactive Animation
• Introduction

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So, the cell cycle consists of

• G1 growth and preparation of the chromosomes
  for replication, there is a restriction point
  which once passed commits the cell to divide.
• S synthesis of DNA (and centrosomes) see DNA
  Replication
• G2 preparation for division with replication of
  organelles and increased cytosol
• M mitosis
• G0 cells which do not divide (liver nerves)
• When a cell is in any phase of the cell cycle
  other than mitosis, it is often said to be in
  interphase.

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Regulation of the cell cycle

• How cell division (and thus tissue growth) is
  controlled is very complex. The following terms
  are some of the features that are important in
  regulation, and places where errors can lead to
  cancer. Cancer is a disease where regulation of
  the cell cycle goes awry and normal cell growth
  and behavior is lost.

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p53

• is a protein that functions to block the cell
  cycle if the DNA is damaged. If the damage is
  severe this protein can cause apoptosis (cell
  death).

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• 1. p53 levels are increased in damaged cells.
  This allows time to repair DNA by blocking the
  cell cycle.
• 2. A p53 mutation is the most frequent mutation
  leading to cancer. An extreme case of this is Li
  Fraumeni syndrome, where a genetic a defect in
  p53 leads to a high frequency of cancer in
  affected individuals.

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Control of the Cell Cycle

• Controlled by proteins in the cytoplasm. Among
  the main players in animal cells are
• 1. Cyclins
• a G1 cyclin (cyclin D)
• S-phase cyclins (cyclins E and A)
• mitotic cyclins (cyclins B and A)
• Their levels in the cell rise and fall with the
  stages of the cell cycle.

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Steps in the cycle (you are not expected to know
each step)
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A game for you to play. Read the directions

• The Cell Cycle Mitosis Tutorial
• Here is an explanation of cell proliferation Anima
  tions

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Duration of Cell Cycle Is Variable

• Embryos exhibit shortest cycles
• Divide as quickly as DNA can be replicated
• Half of cycle is S, half is M, virtually no G1 or
  G2
• Mature cells have longer cycles
• Mammalian cell cycle averages 24 hours Growth
  occurs during G1 and G2 G phases may be referred
  to as gap phases They separate the S phase from
  the M phase M phase takes only small portion of
  cycle

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• Length of cycle variability is in G1 Many cells
  pause in a G0 resting stage May remain there for
  days to years, some remain permanently Most body
  cells are in G0 at any one time Injury may
  stimulate some cells to enter G1 from G0
• Rediscovering Biology - Unit 8 Cell Biology and
  Cancer Animations and Images

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MITOSIS

• Mitosis subdivided into four continuous stages
• Prophase
• Metaphase
• Anaphase
• Telophase

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Preparing for Mitosis Interphase

• G1 phase cells undergo major portion of growth
• S phase chromosome replicates to produce sister
  chromatids Remain attached at the centromere

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• Specific DNA sequence bound to a protein
  kinetochore
• Location specific to each chromosome

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• G2 phase chromosomes begin process of
  condensation
• Motor proteins involved in rapid, final
  condensation

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• Animation of coiling. Note histones
• Educational science animation DNA Coiling

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Mitosis (an overview)

• Cell Division BioClip
• Animations
• http//www.bio.unc.edu/faculty/salmon/lab/mitosis/
  Bloodlily.mov

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The details

• When a eukaryotic cell divides into two, each
  daughter or progeny cell must receive the
  following.
• a complete set of genes (for diploid cells, this
  means 2 complete genomes, 2n)
• a pair of centrioles (in animal cells)

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• some mitochondria and, in plant cells,
  chloroplasts as well
• some ribosomes, a portion of the endoplasmic
  reticulum, and perhaps other organelles

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• this is 3 of the DNA of only one of the 46
  chromosomes in the human diploid cell. So the
  problem faced is how to coil and double all of
  the material and not have a mess !

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The answer

• 1. Duplicate each chromosome during the S phase
  of the cell cycle.
• 2, This produces dyads, each made up of
  2 identical sister chromatids. These
  are held together by a ring of proteins called
  cohesins.

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• 3. Condense the chromosomes into a compact form.
  This requires ATP and a protein complex called
  condensin

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• 4. Separate the sister chromatids and
• 5. distribute these equally between the two
  daughter cells.
• Steps 3 - 5 are accomplished by mitosis.

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Interphase

• The cell is engaged in metabolic activity and
  performing its prepare for mitosis (the next four
  phases that lead up to and include nuclear
  division). Chromosomes are not clearly discerned
  in the nucleus, although a dark spot called the
  nucleolus may be visible. The cell may contain a
  pair of centrioles (or microtubule organizing
  centers in plants) both of which are
  organizational sites for microtubules.

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Interphase plant and animal
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Prophase

• Chromatin in the nucleus begins to condense and
  becomes visible in the light microscope as
  chromosomes. The nucleolus disappears. Centrioles
  begin moving to opposite ends of the cell and
  fibers extend from the centromeres. Some fibers
  cross the cell to form the mitotic spindle.

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Prophase Animal / Plant
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Prometaphase

• The nuclear membrane dissolves, marking the
  beginning of prometaphase.
• Proteins attach to the centromeres creating the
  kinetochores.
• Microtubules attach at the kinetochores and the
  chromosomes begin moving.

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Prometaphase
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Prometaphase
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Metaphase

• Spindle fibers align the chromosomes along the
  middle of the cell nucleus. This line is referred
  to as the metaphase plate. This organization
  helps to ensure that in the next phase, when the
  chromosomes are separated, each new nucleus will
  receive one copy of each chromosome.

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metaphase
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Anaphase

• The paired chromosomes separate at the
  kinetochores and move to opposite sides of the
  cell. This is the only time there is a double set
  of chromosomes in the cell Motion results from a
  combination of kinetochore movement along the
  spindle microtubules and through the physical
  interaction of polar microtubules.

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Telophase

• Chromatids arrive at opposite poles of cell, and
  new membranes form around the daughter nuclei.
  The chromosomes disperse and are no longer
  visible under the light microscope. The spindle
  fibers disperse, and cytokinesis or the
  partitioning of the cell may also begin during
  this stage.

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Cytokinesis animal

• In animal cells, a belt of actin filaments forms
  around the perimeter of the cell, midway between
  the poles. The interaction of actin and a myosin
  (not the one found in skeletal muscle) tightens
  the belt, and the cell is pinched into two
  daughter cells.

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In plants

• In plant cells, a membrane-bounded cell plate
  forms where the metaphase plate had been. The
  cell plate, which is synthesized by the Golgi
  apparatus, supplies the plasma membrane that will
  separate the two daughter cells. Synthesis of a
  new cell wall between the daughter cells also
  occurs at the cell plate.

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review

• Mitosis
• Animations MITOSIS
• Animation Quizzes
• The Cell Cycle Mitosis Tutorial