Cell Division (review) by steepslope9876

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									Cell Division                     (review)

Compare and contrast mitosis and meiosis.
How does a cell reproduce?

   Some internal or external signal initiates
    the process.
   The DNA is replicated and supplies of
    other cell components are built up.
   The DNA is distributed to each of the two
    new cells.
   Two unique cell membranes (and cell
    walls, if present) grow during cytokinesis.
The process in prokaryotes
is called (binary) fission.
 The cell responds to signals from the
  environment, such as food supplies.
 The single chromosome is replicated starting at
  the “ori” point and ending at the “ter” point.
 The two DNA strands stick to the cell membrane
  and as the membrane grows between them they
  are pulled apart.
 The plasma membrane pinches in due to the
  action of a tubulin-like protein and forms the two
  new cells.
Eukaryotes have a more complex
process called mitosis or meiosis.
 Once they are differentiated and mature some
  eukaryote cells divide rarely or not at all.
 Initiation signals are more complex and involve
  CDK (an enzyme) and cyclin proteins.
 Eukaryotes have multiple chromosomes and the
  replication process is more complex.
 The process of chromosome movement involves an
  intricate mechanism of microtubules.
 The presence of a distinct nuclear membrane also
  complicates things in eukaryotes.
 Cytokinesis is different in plants and animals.
Mitosis produces two
genetically identical cells.
 The cell typically grows in size.
 The DNA is replicated.
 Copies of all cellular organelles are made.
 The nuclear membrane dissolves.
 The DNA divides into two equal parts.
  (There is only one nuclear division.)
 Cytokinesis divides the cytoplasm and
 Meiosis produces four
 genetically unique cells.
 “Sex cells” undergo meiosis to make haploid
 The DNA is replicated only once but is divided twice,
  thus reducing the chromosome number in half.
 The homologous chromosomes (copies from each
  parent) are randomly assorted and segregated into
  the new cells at each division.
 Synapsis (crossing-over) often occurs.
 Genetically diverse gametes are produced.
 Division of the cytoplasm may be uneven.
The Cell Cycle
                  Most of the cell’s life is
                   spent in interphase.
                   Some cells never leave
                   the G1 stage. They live
                   and die without dividing.
                  The signal to divide is
                   during the G1 stage.
                  DNA replicates in the S.
                  G2 prepares for mitosis.
                  The M or mitotic phase
                   consists of four (or five in
                   some books) steps.
The Signal to Divide happens in G1.

 Cyclin-dependent kinases (enzymes) are
  activated by cyclin proteins to start the S
  stage using internal signals.
 Division only takes place if conditions in the
  cell are right to make both CDKs and cyclins.
 Some division is stimulated from outside the
  cell by growth factors (chemicals that bind on
  the cell surface to trigger cell division).
 Synthesis of DNA makes nearly
 exact copies of the chromosomes.

 Chromosomes are made of chromatin: DNA and
  proteins called histones.
 The replicated chromosomes consist of two
  chromatids attached at a centromere (an X shape
  once the chromatids condense during prophase).
 As the DNA is replicated a pair of centrosomes
  (and in some organisms, centrioles) are also
  formed from a single one near the nucleus.
 The centrosomes determine the cell “equator” and
  orient the plane of cell division.
G2 prepares the cytoplasm and
organelles for division.

 After the DNA is replicated the cell makes
  copies of some of its organelles and
  stockpiles the molecules it will need to
  produce new membranes and other parts.
 The microtubules that will form the cell
  spindle apparatus (the “ropes” that will
  move the chromosomes during division) are
  made at this time.
Mitosis Begins!
              Prophase: DNA supercoils
               around histones; centrosomes
               or centrioles move to opposite
               ends of cell; micotubules form.
              Late prophase(prometaphase):
               chromosomes attach to
               micotubules at kinetochores;
               nuclear membrane dissolves.
              Metaphase: chromosomes are
               all lined up at the cell equator.
              Anaphase: dyneins and
               shortening microtubules pull
               chromatids apart.
              Telophase: 2 new nuclei form
Cytokinesis – division of cytoplasm

 Once nuclear division (mitosis) is complete the
  cytoplasm and cell membranes divide.
 In animals a cleavage furrow pinches in the cell
  membrane to form two new cells. Actin and myosin
  microfilaments do this.
 In plants a cell plate forms between the two new
  nuclei and new cell wall and membrane grow there.
  Vesicles from the golgi apparatus start this.
 Each new daughter cell contains an exact copy of
  the DNA and copies of the organelles that were in
  the cytoplasm.
Meiosis is different in
several ways.
 There are two nuclear divisions (meiosis I and II)
 The homologous chromosomes inherited from
  each of the parents are separated to reduce the
  total number of chromosomes by half.
 Synapsis occurs during prophase of the first
  division (prophase I).
 The homologous chromosomes separate during
  anaphase I, BUT the chromatids do NOT separate
  at their centromeres.
 The chromatids separate during anaphase II.
DNA is replicated prior to prophase I
and sister chromatids are attached
at centromeres.
Homologous chromosomes pair up
during prophase I. Synapsis may
occur to recombine some genes.
Homologous chromosomes
segregate during anaphase I.
Meiosis II begins immediately after
cytokinesis of the first division.
Sister chromatids separate into
daughter chromatids during
anaphase II. Four 1n (haploid) cells
are made.
Unequal division of the cytoplasm
occurs in some animal eggs (ova).

                  During cytokinesis of
                   meiosis I and II, one of the
                   new cells gets nearly all of
                   the cytoplasm. The other
                   cells are called polar bodies.
                   They do not survive.

                  This ensures that the egg
                   cell will have a large supply
                   of stored materials in its
                   cytoplasm for the mitotic
                   divisions that will follow
                   fertilization and formation of
                   the zygote.
Errors of Meiosis

 Nondisjunction leads to aneuploidy.
      The homologous chromosomes do not segregate
       properly during meiosis I. One new cell gets both of
       that particular chromosome and the other gets none.
      If an egg cell with the extra chromosome is fertilized
       by a normal sperm a condition called trisomy will
       occur. That zygote will have three copies of that
       chromosome instead of two. Down Syndrome is
       caused by trisomy of chromosome 21 in humans.
      There are other examples of aneuploidy - trisomy
       and monosomy - that you can look up on the internet.

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