Chapter 5 – Cell Division by steepslope9876


									Cell Division
   L3 Biology
Why do cells divide?
   Growth

   Repair/regeneration

   Reproduction
     asexual
The Cell Cycle – life cycle of a cell
         that will divide:
    Cells go through a life cycle
               Cell division – mitosis – 1 hr
               – followed by division of the

Second                                          First growth phase
growth                  interphase
                                                9 hr
phase – 2 hr

         Synthesis phase – DNA is copied – 10 hr
 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.
                   One Chromosome

Each strand is an identical
copy of the other one

Centromere                          DNA
Where the two chromatids
Are attached to each other –
This is different for each
               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
   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            92
     Humans have 22 pairs of similar
chromosomes; one pair of sex chromosomes
Sex Chromosomes
   Similar in females: XX

   Not similar in males: XY
 Cell division resulting in nuclei identical
  to parent cell – asexual reproduction for
  some organisms.
 Four phases: Prophase
                   Mother cell

                                        Chromosomes become visible,
                                        nucleus disintegrates, spindle
                                        fibers form, centrioles move to

                                                   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.

 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.
   Chromosomes line up single file at the
    middle of the cell
   Sister chromatids are pulled apart toward
    opposite poles
         Telophase            Cell plate

   Nuclear membrane forms around
    each group of chromosomes
   Nucleolus reappears in each nucleus
   Spindle fibers disappear
   Chromosomes become invisible again as
   Cytokinesis begins in plant cell by formation of
    cell plate; cleavage furrow in animal cell
    completely separates the two nuclei into two
    different cells.
                Cytokinesis in an
                  animal cell
Occurs after nucleus has
been duplicated
Begins in anaphaseby the
formation of a cleavage furrow
Begins   in telophase in plant cells
  by the formation of a cell plate.
    Find the different stages of mitosis in these onion root cells:



 Cell division producing cells that have half the
  number of chromosomes of the mother cell
 Produces gametes – eggs and sperm
 Occurs so that fertilization doesn’t increase the
  number of chromosomes in each generation.
         Why is cell division important to
   Cancer is uncontrolled cell division – cells then
    spread to other parts of the body.
      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
This is how skin cancer looks – a change in a mole is
the first symptom. If you know how your moles           Skin cancer cells
usually look, you can identify any changes.
                       This is the worst kind of skin cancer –
                       Malignant melanoma – it has often
                       metastesized by the time it is
Meiosis = Gametogenesis – the
formation of eggs and sperm
 Oogenesis                Spermatogenesis
Production of an egg      Production of sperm

One mother cell           One mother cell
 produces one egg cell     produces 4 equally
 and three polar bodies    sized sperm cells
 that die
 Sperm       Egg
 formation   formation

4 sperm
             One egg
The sperm unites with the egg forming a zygote (fertilized

The zygote then divides by mitosis to produce the trillions of
cells that make up a multicellular body like yours.
The End

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