Cell Division

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Cell Division

• L3 Biology

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Why do cells divide?

• Growth
• Repair/regeneration
• Reproduction
• asexual

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The Cell Cycle life cycle of a cell that will
divide
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Cells go through a life cycle
Cell division mitosis 1 hr followed by
division of the cytoplasm
First growth phase 9 hr
Second growth phase 2 hr
interphase
Synthesis phase DNA is copied 10 hr
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Chromosomes

• 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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• When cells divide by mitosis, each daughter cell
  receives the same number of chromosomes as its
  mother cell has.
• 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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Humans have 22 pairs of similar chromosomes one
pair of sex chromosomes
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Sex Chromosomes

• Similar in females XX
• Not similar in males XY

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Mitosis

• Cell division resulting in nuclei identical to
  parent cell asexual reproduction for some
  organisms.
• Four phases Prophase
• Metaphase
• Anaphase
• Telophase

P M A T
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Prophase
Mother cell
Chromosomes become visible, nucleus
disintegrates, spindle fibers form, centrioles
move to poles
Chromosomes line up in the middle
Chromatids separate and move to opposite poles
Chromosomes become invisible, nuclear membranes
form around new nuclei, spindle fibers disappear.
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Prophase

• Chromosomes become visible
• Spindle fibers form
• Nuclear membrane disintegrates
• Nucleolus disintegrates
• In animal cells, centrioles move 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 middle of
  the cell

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Anaphase

• Sister chromatids are pulled apart toward
  opposite poles

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Telophase
Cell plate

• 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 in an animal cell

• Occurs after nucleus has
• been duplicated
• Begins in anaphaseby 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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Find the different stages of mitosis in these
onion root cells
B
C
D
A
E
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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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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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Gametes sex cells

• Eggs or sperm
• Have half the normal number of chromosomes 23
  in humans
• Combine DNA from two different parents, producing
  offspring that is different from each parent

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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 Gametogenesis the formation of eggs
and sperm

• 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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Egg formation
Sperm formation
4 sperm
One egg
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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