Chapter 10

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Chapter 10 Cell Division

• L2 Biology

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Size of most eukaryotic cells

• 10-100 micrometers (um)
• Diameter of high power field of view in our
  scopes is 500 um (0.5 mm)

How long is this paramecium cell? Is this cell
average, or smaller or larger than average?
500 um
Paramecium 400x
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A crocodile and an earthworm have cells the same
size.Why is that?
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Read pp 242-243 in your textbook.

• Surface area to volume ratio determines how long
  cells can grow before they either die or divide
• As cells grow larger, their surface area to
  volume ratio (increases or decreases?).
• If the volume is so big that stuff cant diffuse
  into or out of the entire cell in a reasonable
  amount of time, the cell will die from waste
  buildup or starvation.

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If these were cells, which would be most
efficient in taking in and getting rid of
materials in the same amount of time?
Which square would take longest to get stuff to
its middle?
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Why do cells divide?

• Growth

• Repair/regeneration

• Reproduction
• asexual

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Cells go through a life cycle
Cell division mitosis 1 hr
First growth phase 9 hr
Second growth phase 2 hr
interphase
Synthesis phase DNA is copied 10 hr
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Cell cycle phases These only occur if a cell is
going to divide

• G1 cell grows after being formed
• S DNA makes a copy of itself in preparation for
  cell division
• G2 short growth phase
• M mitosis nucleus is copied
• If a cell is not going to divide, it stays in G0
  phase doesnt prepare for cell division.

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Chromatin

• Invisible most of the time - Only visible during
  cell division (mitosis or meiosis)
• During S-phase the DNA replicates (makes an
  exact copy of itself)
• This means the cell has twice as much DNA in it
  after replication
• Once a chromosome has replicated, it shortens and
  thickens and can now be seen in our microscopes.

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One Chromosome

• Sister
• Chromatids
• Each strand is an identical
• copy of the other one
• Centromere
• Where the two chromatids
• Are attached to each other
• This is different for each
• chromosome

DNA
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Chromosome Number

• Each species has the same number of chromosomes
  in all their cells that are made by mitosis. This
  is the diploid number (2n). In humans this number
  is 46. So cells of your skin and muscle and liver
  each have 46 chromosomes in them. Look how many
  chromosomes are in the cells of these creatures

2n 42
2n 78
2n 38
2n 94
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• These 46 chromosomes are actually pairs, one came
  from your father, one from your mother. So each
  of your cells has 23 pairs of chromosomes. Each
  member of the pair is homologous to the other of
  that pair (same length, centromere in same place,
  genes for same traits in same location).

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• When cells divide by mitosis, each daughter cell
  receives the same number of chromosomes as its
  mother cell has 2n.
• In order to do this, the chromosomes must be
  copied first, then one of each copy is placed in
  the new cells.

46
46
46
46
46
92
46
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22 Homologous pairs of chromosomes one pair of
sex chromosomes
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Sex Chromosomes

• Homologous in females XX
• Not homologous in males XY

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Mitosis

• Nuclear division resulting in nuclei identical to
  parent cell asexual reproduction for some
  organisms.
• Begins after interphase
• Ends before cytokinesis
• Four phases Prophase
• Metaphase
• Anaphase
• Telophase

P M A T
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Prophase
Mother cell
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Prophase

• Chromosomes become visible
• Spindle forms from protein microtubules
• Nuclear envelope disintegrates
• Nucleolus disintegrates
• In animal cells, centrioles migrate to opposite
  ends of the cell (poles) and spindle fibers
  attach to them

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Metaphase

• Chromosomes line up single file at the equator of
  the cell

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Anaphase

• Sister chromatids are pulled apart toward
  opposite poles

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Telophase

• Nuclear membrane forms around each group of
  chromosomes
• Nucleolus reappears in each nucleus
• Spindle fibers disappear
• Chromosomes become invisible again as chromatin
• Cytokinesis begins in plant cell by formation of
  cell plate cleavage furrow in animal cell
  completely separates the two nuclei into two
  different cells.

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Cytokinesis

• Occurs after nucleus has
• been duplicated
• Begins in anaphase in
• animal cells by the
• formation of a cleavage furrow

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• Begins in telophase in plant cells
• by the formation of a cell plate.

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Zone of specialization
Zone of elongation
Zone of cell division
Zone of cell division
Root cap
Onion Root Tip where the actively dividing
cells are.
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Find the different stages of mitosis in these
onion cells
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Why is cell division important to understand?

• Cancer is uncontrolled cell division cells then
  spread to other parts of the body.

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This is how skin cancer looks a change in a
mole is the first symptom. If you know how your
moles usually look, you can identify any changes.

• Skin cancer cells

This is the worst kind of skin cancer Malignant
melanoma it has often metastesized by the time
it is diagnosed.
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Meiosis

• Cell division producing cells that have half the
  number of chromosomes of the mother cell
• Produces gametes eggs and sperm
• Occurs so that fertilization doesnt increase the
  number of chromosomes in each generation.

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Gametes sex cells

• Eggs or sperm
• Have half the normal number of chromosomes
  haploid (n) 23 in humans
• Sexual reproduction needs these to combine DNA
  from two different parents, producing offspring
  that is different from each parent

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Meiosis Reduction Division

• Two complete cell divisions
• First cell division separates homologous
  chromosomes (reduction of chromosome number)
• Second cell division separates sister
  chromatids (like mitosis) - Division
• Produces 4 haploid cells

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• M
• E
• I
• O
• S
• I
• S
• II

M E I O S I S I
Crossing over can occur Homologous pairs are
separated
Sister chromatids are separated
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Gametogenesis

• Oogenesis
• Production of an egg
• One mother cell produces one egg cell and three
  polar bodies that die

• Spermatogenesis
• Production of sperm
• One mother cell produces 4 equally sized sperm
  cells

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Fertilization
The sperm unites with the egg forming a zygote
(fertilized egg). The zygote then divides
by mitosis to produce the trillions of cells that
make up a multicellular body like yours.
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The End