Mitosis and Meiosis Cell Division

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Mitosis and MeiosisCell Division
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The Cell Cycle

• The lifetime of a cell
• It grows
• And reproduces
• See an animation of the cell cycle at Cells
  Alive
• Cell Cycle at Cell's Alive

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Lets play the Cell Cycle Game

• Control of the Cell Cycle

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Cell division

• Cells arise from other cells from cell division
• Body cells are called somatic cells
• Sex cells, gametes, or egg and sperm are called
  autosomal cells.
• Mitosis produces identical cells and is used with
  body cells for replacement and growth. These are
  clones. They have the same amount of chromosomes.
• Meiosis is for sex cells and produces gametes
  that are not identical and have half of the
  chromosome numbers information as the parent
  cell. The 2 gametes will then fuse at
  fertilization to get the required number of
  chromosomes.

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Daughter cells and parent cells

• Parent cells are the original cell
• Daughter cells are the products of the division
  of the parent cell.

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Mitosis

• Mitosis is the process that our body uses to grow
  and replace cells -- in that case, you want to
  make sure that when the cell divides, each of the
  daughter cells (the cells resulting from the
  division) have the identical (and complete)
  genetic makeup of the parent cell (they cell they
  started as, before dividing). For us humans, that
  means they each have all 46 chromosomes.

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Mitosis Continued

• Normal somatic (body cell) division of the cell
  is known as mitosis. The entire process can take
  between 30 minutes and 2 hours. The product of
  this process is two identical daughter cells. The
  nuclear division is usually followed by cell
  division.

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The process takes place in five stages

• Interphase
• Prophase
• Metaphase
• Anaphase
• Telophase
• These stages differ in terms of duration. The
  second and fifth stages are fairly long, whilst
  the third and fourth stages are faster. When a
  cell is not undergoing cell division, it is in
  the interphase stage.

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Interphase

• Interphase is the "holding" stage or the stage
  between two successive cell divisions. Some 90
  percent of a cell's time in the normal cellular
  cycle may be spent in interphase. While the name
  might sound "passive" there are a number of
  processes that occur in interphase. If viewed
  under a microscope, the cell may appear to be
  dormant but in actuality biochemical activity is
  high during interphase.

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Prophase

• Prophase is a beautiful menagerie of changes that
  occur in both the cytoplasm and nucleus of the
  dividing cell. Many consider prophase (versus
  interphase) to be the first true step of the
  mitotic process. In prophase, the chromatin
  condenses into discrete chromosomes. The nuclear
  envelope breaks down and spindles form at
  opposite "poles" of the cell.

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There are many distinct changes that occur in a
cell during prophase

• The nucleoli disappear.
• Chromatin fibers become coiled into chromosomes
  with each chromosome having two chromatids joined
  at a centromere.
• The mitotic spindle, composed of microtubules and
  proteins, forms in the cytoplasm.
• In animal cells, the mitotic spindle initially
  appears as structures called asters which
  surround each centriole pair. The two pair of
  centrioles (formed from the replication of one
  pair in Interphase) move away from one another
  toward opposite ends of the cell due to the
  lengthening of the microtubules that form between
  them.

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In late prophase

• The nuclear envelope breaks up.
• Polar fibers, which are microtubules that make up
  the spindle fibers, reach from each cell pole to
  the cell's equator.
• Kinetochores, which are specialized regions in
  the centromeres of chromosomes, attach to a type
  of microtubule called kinetochore fibers.
• The kinetochore fibers "interact" with the
  spindle polar fibers connecting the kinetochores
  to the polar fibers.
• The chromosomes begin to migrate toward the cell
  center.

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Metaphase

• In metaphase, the spindle fully develops and the
  chromosomes align at the metaphase plate (a plane
  that is equally distant from the two spindle
  poles).

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• Changes that occur in a cell during metaphase
• The nuclear membrane disappears completely.
• In animal cells, the two pair of centrioles align
  at opposite poles of the cell.
• Polar fibers (microtubules that make up the
  spindle fibers) continue to extend from the poles
  to the center of the cell.
• Chromosomes move randomly until they attach (at
  their kinetochores) to polar fibers from both
  sides of their centromeres.
• Chromosomes align at the metaphase plate at right
  angles to the spindle poles.
• Chromosomes are held at the metaphase plate by
  the equal forces of the polar fibers pushing on
  the centromeres of the chromosomes

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Anaphase

• In anaphase, the paired chromosomes (sister
  chromatids) separate and begin moving to opposite
  ends (poles) of the cell. Spindle fibers not
  connected to chromatids lengthen and elongate the
  cell. At the end of anaphase, each pole contains
  a complete compilation of chromosomes.

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• Changes that occur in a cell during anaphase
• The paired centromeres in each distinct
  chromosome begin to move apart.
• Once the paired sister chromatids separate from
  one another, each is considered a "full"
  chromosome. They are referred to as daughter
  chromosomes.
• Through the spindle apparatus, the daughter
  chromosomes move to the poles at opposite ends of
  the cell.
• The daughter chromosomes migrate centromere first
  and the kinetochore fibers become shorter as the
  chromosomes near a pole.
• In preparation for telophase, the two cell poles
  also move further apart during the course of
  anaphase. At the end of anaphase, each pole
  contains a complete compilation of chromosomes.

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Telophase

• In telophase, the chromosomes are cordoned off in
  distinct new nucleuses in the emerging daughter
  cells.

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Telophase Continued

• Changes that occur in a cell during telophase
• The polar fibers continue to lengthen.
• Nuclei begin to form at opposite poles.
• The nuclear envelopes of these nuclei are formed
  from remnant pieces of the parent cell's nuclear
  envelope and from pieces of the endomembrane
  system.
• Nucleoli also reappear.
• Chromatin fibers of chromosomes uncoil.
• After these changes, telophase/mitosis is largely
  complete and the genetic "contents" of one cell
  have been divided equally into two.

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Cytokinesis

• Division of cytoplasm
• In plants there are cell plates that are formed
• Animal cells just pinch down the center and
  divide.

Cytokinesis Similarly, the division of the
original cell's cytoplasm is called cytokinesis.
It begins prior to the end of mitosis and
completes shortly after telophase/mitosis. At the
end of cytokinesis, there will be two distinct
daughter cells.
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Watch an animation on Mitosis

• Mitosis on cellsalive.com

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Great Animation of Mitosis to Music

• Cell Division BioClip

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Sexual reproduction

• Involves the fusion of nuclear material from two
  cells.
• The resulting cell is called a zygote.
• Meiosis is the process by which an individual
  makes the gametes or reproductive cells.
• The starting cells are diploid and the resulting
  cells are haploid

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Haploid vs. Diploid

• Diploid (2n) are cells that have 2 sets of
  chromosomes in them. Ex. Human skin cells
  (somatic cells) have 46 chromosomes or 23 pairs.
• Haploid (n) have 1 set in them. Ex. Human sperm
  or eggs ( gametes or autosomes)
• Have 23 chromosomes total.

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Meiosis

• Meiosis is used to produce the cells used in
  reproduction egg cells, pollen, sperm, and the
  like. In this case, those cells are going to be
  combining with other cells and thus only need
  HALF of the original genetic material (that means
  23 for us humans). Also, the daughter cells of
  meiosis (which are called "gametes") are
  dissimilar from one another, in addition to being
  unlike the parent.

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Watch a really cool video on youtube

• YouTube - meiosis

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Prophase I

• doubled chromosomes begin to condense
• homologous chromosomes "pair" through a protein
  mediated process of synapsis
• non-sister chromatids exchange parts of
  chromosomes (crossing over)
• in females, division stops here until receipt of
  hormonal signals to continue - this cessation
  will last between 12 and 50 years (!)
• near the end of Prophase I, the nuclear membrane
  disappears
• the spindle forms

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Crossing Over

• When chromosomes pair in the early prophase of
  the first division of meiosis (Meiosis I), a
  crossover occurs between two non-sister
  chromatids. This results in an exchange of
  genetic material between the maternal and
  paternal chromosomes. If there are genetic
  markers (alleles) on the chromosomes, it is
  possible to recover new combinations of alleles
  at different genes as a result of these
  crossovers.

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Crossing Over Continued

• The crossover occurs between the two genes and
  results in four, different arrangements of the
  alleles - AB, Ab, aB, and ab. Two of the
  arrangements are like those of the original
  paternal and maternal chromosomes (AB and ab) and
  two of the arrangements are new combinations or
  recombinations (Ab and aB).

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Anaphase I

• homologous chromosomes separate from each to go
  to opposite poles of the cell
• centromeres do not divide so each chromosome
  remains in the doubled state
• at this point the number of chromosomes has been
  halved
• since the chromosomes are doubled, they appear to
  have "four" arms as they are pulled to the
  opposite poles of the cell

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Telophase I

• doubled chromosomes arrive at the poles of the
  cell
• spindle disappears and nuclear membrane reappears
  
• cell division occurs
• the two cells do not go through a G1, S, and G2
  cycle but proceed into Meiosis II
• in females, cell division is grossly asymmetric
  producing a small polar body and a large

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Prophase II

• the chromosomes, which had doubled prior to
  Meiosis I, re-condense but do not pair
• the spindle appears and the nuclear membrane
  disappears
• in females, this is triggered by fertilization

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Metaphase II

• Doubled chromosomes align on the metaphase plate
  through attachment of spindle fibers to the
  centromeres

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Anaphase II

• finally, the centromeres divide resulting in the
  division of the doubled chromosomes by separation
  of chromatids
• chromosomes are pulled to opposite poles of the
  cell
• this resembles a mitotic anaphase except that the
  number of chromosomes has been halved

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Telophase II

• chromosomes de-condense
• nuclear membrane reappears
• cell division occurs - cytokinesis
• in females this division is asymmetric again such
  that only one egg cell is produced
• in males, sperm maturation of all four cells
  follows

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See some cool animations

• Animations

Meiosis
meiosis double cell division
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Mitosis vs. Meiosis

• Mitosis
• In body cells
• Produces 2 identical cells
• cells are 2n or diploid
• 1 set of divisions IPMAT
• Meiosis
• Gametes or sex cells
• 2 divisions IPMAT MAT
• 4 Unique cells produced
• Haploid cells (n) with half chromosome content

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Practice learning about Mitosis and Meiosis

• Mitosis and Meiosis