Cell Division

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Cell Division

• Lab 10

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Cell Division

• According to cell theory cells only come from
  other living cells.
• Information encoded within DNA must be
  replicated so each new cell has a copy.

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Eukaryote Cell Cycle

• DNA is contained in multiple chromosomes (a
  single long DNA molecule packaged with proteins
  into a compact shape).

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Cell Division in Eukaryotes

• Mitosis cell division in somatic
  (nonreproductive) cells.
• Meiosis cell division in reproductive cells.
• Results in eggs or sperm.

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The Cell Cycle

• Interphase G1, S, and G2 together.
• G1 phase first growth phase
• S phase synthesis phase, replication of DNA.
• G2 phase second growth phase, replication of
  mitochondria, chromosomes condense.
• M phase mitosis
• C phase - cytokinesis

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Chromosomes

• Homologous chromosomes or homologues two nearly
  identical versions of a chromosome.
• Diploid cells have two of each type of chromosome
  ( 23 pairs or 46 in humans).

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Chromosome Structure

• During S phase, each of the 2 homologues
  replicates, resulting in identical copies called
  sister chromatids.
• Chromatids remain connected at a linkage site
  called the centromere.

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Cell Division

• Interphase Cells grow, DNA replicates and winds
  up tightly (condensation).

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Cell Division

• Mitosis nuclear cell division in somatic cells.
• Prophase
• Metaphase
• Anaphase
• Telophase
• Continuous process where stages flow one into the
  next.

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Cell Division

• Prophase Chromosomes condense enough to be seen
  with a light microscope.

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Cell Division

• Metaphase Alignment of the chromosomes along
  center of cell.

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Cell Division

• Anaphase Separation of the chromatids.
• Sister chromatids split apart one moving to
  each pole.

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Cell Division

• Telophase re-formation of the nuclei.

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Cell Division

• Cytokinesis division of the cytoplasm.
• Two complete, diploid cells.

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Cytokinesis

• In plants, a cell plate forms dividing the cell
  in two.
• Cellulose is laid down to form new cell wall.

• In animal cells, the cell pinches in two.
• This produces a cleavage furrow which deepens
  until the cell splits.

http//www.youtube.com/watch?vVGV3fv-uZYI
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Reproduction

• When organisms reproduce sexually, a cell from
  each parent fuses together.
• In order to avoid a doubling of chromosome
  number, there must be a way to reduce the number
  of chromosomes in the gametes (eggs sperm).

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Meiosis

• Meiosis is the special type of cell division that
  produces eggs and sperm.
• In meiosis, a diploid cell with two sets of
  homologous chromosomes will divide so that the
  daughter cells are haploid and have one set of
  chromosomes.

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Fertilization

• One haploid egg and one haploid sperm can then
  fuse in the process called fertilization forming
  a diploid zygote.

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Meiosis

• Chromosomes have replicated during interphase
  just as in mitosis.
• Meiosis actually consists of two separate
  divisions.
• Meiosis I serves to separate the two versions
  of the chromosome (homologues).
• Meiosis II serves to separate the two replicas
  of each version (sister chromatids).

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Meiosis

• Because there is only one replication of DNA but
  two cell divisions, each of the four daughter
  cells is haploid has only one set of
  chromosomes.

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Meiosis I

• Prophase I Chromosomes become visible.
• The 2 versions of each chromosome pair up and
  exchange segments. This is called crossing over.

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Meiosis I

• Metaphase I Chromosomes line up in the middle.
• Which chromosome faces which pole is random.
  This is called independent assortment.

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Meiosis I

• Anaphase I
• Homologues are pulled apart and move toward
  opposite poles.
• Sister chromatids NOT separated yet.
• Each pole has half as many chromosomes (one set
  rather than two) as the original cell.
• Telophase I the chromosomes gather at the two
  poles and wait for the onset of meiosis II.

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Meiosis II

• After a brief interphase in which NO DNA
  synthesis occurs, meiosis II begins.
• Meiosis II is just like mitosis except that the
  sister chromatids are no longer identical due to
  crossing over.

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Meiosis II

• Prophase II nuclear envelopes break down.
• Metaphase II chromosomes line up in the middle
  of the cell.

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Meiosis II

• Anaphase II sister chromatids move to opposite
  poles.
• Telophase II The nuclear envelope reforms
  around the four sets of daughter chromosomes.

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Meiosis II

• The resulting 4 daughter cells are haploid.
• No 2 cells are alike due to crossing over.
• In animals, these cells develop directly into
  gametes (eggs sperm).

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Unique Features of Meiosis

• Synapsis occurs when homologous chromosomes
  line up along their entire length.
• Crossing over - occurs when segments of the
  chromosomes are then exchanged.

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Unique Features of Meiosis

• Another unique feature is that the homologous
  chromosomes separate in meiosis I rather than
  sister chromatids.
• Sister chromatids separate in meiosis II.
• Meiosis is a reduction division.

http//www.youtube.com/watch?vD1_-mQS_FZ0NR1