Cell Cycle

1
Cell CycleMitosis
2
Cell Cycle

• Cell cycle repeating cycle of events in the
  life of a cell.
• Three major phases of the cell cycle
• 1. Interphase
• 2. Mitosis (includes several subphases)
  dividing the nucleus
• 3. Cytokinesis dividing the cytoplasm

3
Interphase

• Interphase period of time between cellular
  divisions. The cell carries on its normal
  functions, and grows and matures during this
  time. It will replicate a full set of the
  chromosomes towards the end of interphase to
  prepare for cell division.
• Interphase has three phases
• 1. G1 (gap) time between cell division and DNA
  replication. The time when the cell grows
  matures.
• 2. S (synthesis) DNA is replicated
• 3. G2 cell produces proteins and molecules
  needed for mitosis.

4
Uses of Mitosis

• 1. Development and growth in multicellular
  organisms.
• 2. Repair by replacing damaged cells. Injured
  parts of the body are repaired by the production
  of new cells.
• 3. Asexual reproduction producing a new
  organism involving only mitotic cell division.
  Used in plant and many unicellular organisms like
  bacteria.

5
Asexual Reproduction

• A. Fragmentation when a colonial organism is
  broken in two, and the missing cells are replaced
  by mitosis.
• B. Budding producing a new small organism on
  the side of the parent.
• C. Spore a cell or cells with a hard
  protective covering, which will open when
  conditions are favorable the cells will form a
  new organism by mitosis

6
Mitosis

• Mother cell cell that has replicated its DNA
  and is ready to go through mitosis.
• Mitosis process in which the copies of the
  genetic material are divided into two sets so
  that each daughter cell is identical to the
  original cell.

7
Mitosis

• Four phases of mitosis
• I. Prophase
• A. Chromatin condenses into chromosomes
  consisting of sister chromatids joined together
  at the centromere.
• B. Special proteins attach to the centromere to
  form the kinetochore (point of attachment for
  spindle fibers).
• C. Nuclear membrane disintegrates.
• D. Centrosomes (organelle that organizes the
  microtubules) migrate to opposite poles of the
  nucleus by motor proteins on microtubules in the
  cytoplasm. They then form mitotic spindle that
  will direct movements of the chromosomes.
• E. Mitotic spindle begins moving sister
  chromatids toward the center of the cell.
• Two types of spindle fibers
• 1. kinetochore fibers extend from
  centrosomes attach to the kinetochore of the
  centromere
• 2. polar fibers extend out from one
  centrosome to the middle of the cell to join
  other polar fibers from the opposite centrosome.

8
Mitosis

• II. Metaphase
• A. Sister chromatids line up at the center of
  the cell, the equatorial plane.
• III. Anaphase
• A. Enzymes break down the proteins in the
  centromeres, allowing the chromatids to separate.
• B. Kinetochore fibers pull the chromosomes
  toward opposite ends of cell.
• C. Polar fibers push the centrosomes apart
  which elongates the cell.
• IV. Telophase
• A. Chromatids reach the end of the spindle
• B. New nuclear membranes form around the
  chromosomes on separate poles of the cell.
  (opposite of prophase)
• C. Chromosomes begin to uncoil returning to
  chromatin form (opposite of prophase)
• D. Mitotic spindle is deconstructed (opposite
  of prophase)

9
Cytokinesis

• Cytokinesis the final phase, in which the
  division of the cytoplasmic contents must be
  divided. Occurs differently in plants cells than
  animal cells.
• Animal cells-
• Actin filaments constrict the membrane in the
  center forming a cleavage furrow which deepens
  until the cell is pinched into two cells.
• Plant cells
• 1. Vesicles formed by the Golgi bodies align
  fuse to form a membrane-bound cell plate.
• 2. Cellulose and other components that form the
  cell wall are produced and form a mature cell
  wall separating the two daughter cells.

10
Variations in Mitosis

• In some unicellular organisms, all the phases
  of mitosis occur in the nuclear membrane. The
  nuclear membrane will be pinched to form 2
  nuclei.
• The length of time for mitosis differs for
  various types of cells.
• May take from 10 minutes to 3 hours.
• Embryos have very short interphase, because they
  must produce cells rapidly.
• Most types of cells can be replaced when damaged
  or die.
• Most nerve cells in the human brain may grow in
  size and replace worn-out or damaged cells , but
  will not divide.
• Most brain cells cant be replaced when they
  die.