Mitosis

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Mitosis
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• The nucleus of eukaryotic cells contains
  chromatin, a mixture of protein, DNA, and RNA.
• Chromatin makes up the chromosomes.
• Each chromosome may contain 100s to 1000s of
  genes, the basic units of heredity.

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• Most organisms consist of two main cell types
  somatic cells and gametes.
• Gametes are sex cells (ex. sperm and eggs/ova)
  all of the other cells are somatic cells (ex.
  epithelial cells, muscle cells, nerve cells, bone
  cells, blood cells, fat cells, etc.)
• Each individual of a given species contains a
  characteristic number of chromosomes in the
  nuclei of their cells in humans, this number is
  46

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Diploid vs. Haploid

• Human somatic cells normally contain 46
  chromosomes, but human gametes normally contain
  23
• Cells containing the full set of chromosomes
  characteristic of the species are diploid
  (represented by 2n) these cells have pairs of
  chromosomes, consisting of one from the mother
  (maternal chromosome) and one from the father
  (paternal chromosome)

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Diploid vs. Haploid (continued)

• Gametes contain only half of the characteristic
  chromosome number, due to the fact that they may
  eventually fuse (during fertilization) to form a
  zygote
• Sex cells contain no pairs of chromosomes, so
  they are haploid
• In humans, the diploid chromosome number is 46
  the haploid chromosome number is 23

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• Once cells reach a certain size, they must stop
  growing, die, or divide
• Nerve cells, skeletal muscle cells, and red blood
  cells do not normally divide, though, once they
  are mature
• Cell division that results in the production of
  new somatic cells consists of mitosis
  cytokinesis

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Cell Cycle

• The life cycle of a cell is called the cell cycle
• Different types of cells have different life
  spans
• Bacteria have very short livesmost divide about
  every 20 minutes
• Neurons in the brain and spinal cord do not
  usually divide after birth

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Cell Cycle (continued)

• 5 stages to the Cell Cycle
• Interphase
• Prophase
• Metaphase
• Anaphase
• Telophase

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Cell Cycle (continued)

• The 5 stages are continuous, with no breaks
  between stages
• Interphase is not usually considered to be a
  stage of cell division it occurs before division
  starts
• Mitosis is the division of the nucleus of a cell
  it consists of prophase, metaphase, anaphase, and
  telophase

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Cell Cycle (continued)

• Cytokinesis is the division of the cytoplasm and
  the rest of the cell it occurs during telophase
• In animal cells, cytokinesis is accomplished by
  the formation of a cleavage furrow the plasma
  membrane of the animal cell pinches in near the
  center and separates the cell into 2 daughter
  cells
• In plant cells, cytokinesis occurs as a cell
  plate forms near the center and splits the cell
  in two

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Interphase

• Most of the life of a cell is spent in this stage
  (the longest stage of the cell cycle)
• The cell is growing and carrying out its normal
  activities necessary for daily life
• This stage is subdivided into
• G1 (first gap phase)
• S (synthesis phase)
• G2 (second gap phase)

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Interphase (continued)

• During the G phases, the cell grows and prepares
  for division
• During the S phase, DNA replication occurs DNA
  is duplicated so that each new daughter cell will
  receive a complete copy of it

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Prophase

• First stage of mitosis
• Longest stage of mitosis
• Chromatin material coils, shortens (condenses),
  and separates to form chromosomes
• Since DNA replication has occurred, each
  chromosome consists of 2 chromatids held together
  by a centromere

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Prophase (continued)

• The nuclear membrane breaks down so that the
  hereditary material can be evenly distributed to
  the new cells
• The nucleolus breaks down, releasing RNA for
  distribution
• The spindle forms from microtubules within the
  cell it spreads from one end of the cell (pole)
  to the other the centromeres attach the
  chromosomes to the spindle

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Prophase (continued)

• In animal cells, paired centrioles separate and
  move to opposite poles
• Near the poles and surrounding the centrioles,
  microtubules form a radiating cluster called an
  aster asters are not found in plant cells

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Fig. 5.5aa
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Fig. 5.5ab
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Metaphase

• X-shaped, double-stranded chromosomes line up
  (single file and unpaired) at the metaphase
  plate/ equator of the cell
• Chromosomes can be seen best during metaphase
  they can be photographed during this stage and
  studied to see if abnormalities exist
• A karyotype is a photograph of chromosomes used
  for genetic study the chromosomes are arranged
  by pairs and numbered, based upon size and other
  distinguishing characteristics

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Fig. 5.5ba
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Anaphase

• Shortest stage of mitosis shortest stage of the
  cell cycle
• Each chromosome (consisting of 2 chromatids)
  separates at the centromere each chromatid is
  now considered a chromosome
• The microtubules of the spindle move the new
  single-stranded chromosomes toward opposite poles
  of the cell

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Fig. 5.5bb
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Telophase

• Final stage of mitosis
• Opposite of prophase
• Chromosomes return to their previous chromatin
  state
• Nucleolus and nuclear membrane re-form
• Spindle disappears
• Cytokinesis occurs

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Telophase (continued)

• Cytokinesis is the division of the cytoplasm to
  yield 2 daughter cells
• In animal cells, cytokinesis is accomplished by
  formation of a cleavage furrow that pinches in at
  the equator to separate the cell into two
• In plant cells, cytokinesis occurs by formation
  of a cell plate that splits the cell into two
• Organelles (ex. mitochondria, chloroplasts, etc.)
  are evenly distributed between the 2 daughter
  cells as cytokinesis occurs

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Fig. 5.5bc
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Twins

• Identical twinsform when the cells of an embryo
  separate from one another, and each goes on to
  form a separate individual these twins have the
  same DNA, the same sex, and are genetically
  identical only environmental factors can make
  them different
• Fraternal twinsform from separate eggs
  fertilized by separate sperm these twins do not
  have the same DNA, are not genetically identical,
  and may not even be of the same sex they may not
  even look alike (ex. hair color, eye color, etc.)

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Results of Mitosis

• Two identical daughter cells are produced, each
  having the same number of chromosomes as the
  parent cell in humans, a parent cell (diploid)
  with 46 chromosomes divides by mitosis and
  cytokinesis to produce two daughter cells
  (diploid), each having 46 chromosomes
• The purpose of mitosis is for growth and repair
  cells of the body are replaced by new cells
  formed by mitosis a zygote develops into an
  embryo by repeated mitotic divisions (and
  differentiation, the process by which the cells
  of an embryo become specialized)