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Cell Reproduction
Mitosis & Meiosis
Outcomes
• 1. Describe mitosis in detail (460-465)
– interphase, mitosis and cytokinesis (the cell cycle)
– explain the importance of maintaining
chromosome number through the processes of
cell and organism reproduction. (461-462)
• Observe, identify and describe the events of the cell
cycle.
– Include: growth, cytokinesis, chromosome
behaviour.
Cell Cycle
• G1 (Gap 1)- cells carry out
normal metabolic activities
for day to day functions.
• S (Synthesis)- DNA is
replicated (duplicated,
copied).
• G2 (Gap 2)- cell prepares
for cell division by doubling
cell organelles.
Mitosis (Asexual Reproduction)
• A stage of the cell cycle that involves the
division of the nucleus into two new nuclei that
contain identical copies of the genetic
information.
• Mitosis is divided into 4 stages based on the
cellular events:
– Prophase, Metaphase, Anaphase, Telophase. (PMAT)
– (PMAT - Please Make A Twin)
– If you include Interphase – The Full Cell Cycle
– (IPMAT – I’ll Produce Me A Twin)
Functions of Mitosis
1. Growth
2. Repair regenerate damaged tissues.
3. Regeneration of entire body parts simpler
organisms.
4. Maintenance of the body.
Mitosis and cytokinesis occur in our body cells
(aka somatic cells)
Stages of the Cell Cycle
Interphase - G1, S, G2
• G1 - Normal period of growth &
development for a cell.
• S- DNA/Chromatin becomes
Synthesized or copied
(replicated/duplicated)
• G2 – Cell prepared to divide by
increasing size and number of
organelles.
Key Features:
1. A sharp, round
nucleus.
2. Any dark staining
regions (nucleolus)
3. A more uniform
color background
for the nucleus
Cells that do not
divide (like neurons)
are always in
interphase.
Prophase
•DNA begins to shorten & thicken
–Now called chromatids/chromosomes
•Centromeres form
•Nuclear membrane breaks apart
•Spindle fibers begin to form
Key Features:
1. Nucleus is
breaking up, not
as circular.
2. The DNA
becomes
‘clumpy’.
“Clumpy & Bumpy”
Metaphase
•Sister Chromatids
(Chromosomes) line up in the
Middle
•Centromere attaches to
spindle fibers (visible)
Key Features:
1. Chromosomes are lined up at the Middle.
Anaphase
•Centromeres divide
•Spindle fibers shorten
•Sister chromatids separate to
opposite ends of the cell.
Key Features:
1. The sister chromatids are being pulled apart. They
look as though they are reaching for each other.
(Help me sister, I don’t want to go!!!)
Telophase
Spindle fibers begin to disappear
Chromosomes begin to uncoil
Nuclear membrane starts to re-form
For the Cell Membrane:
Cell plate (plant cell) or Cleavage furrow
(animal cell) becomes highly noticeable.
Cytokinesis (Cell Cutting)
• The division of the cytoplasm is know as cell
cutting. This signals the end of Mitosis.
• After the cells membrane or wall forms a
complete new barrier between the two sister
cells. We would say cytokinesis has just
occurred.
– Note: Cytokinesis is therefore not a technical stage
in Mitosis/Meiosis; it is a process ends telophase.
Interphase
Prophase
Metaphase
Anaphase
Telophase
Interphase
http://biology.nebrwesleyan.edu/benham/mitosis/
Mitosis Animation
http://www.johnkyrk.com/mitosis.html
Results of Mitosis
• Division of nucleus
– Nuclei are identical to each other
• Same number & type of chromosomes
Asexual Reproduction
• Offspring are produced from
one original organism
• The DNA between the parent
and offspring is identical
• Mitosis is one form of asexual
reproduction
• Link to Curriculum appendix
Cellular Sexual
Reproduction
Meiosis
Sexual Reproduction
A type of reproduction in which
two sex cells, usually an egg and
a sperm, join to form a zygote,
which will develop into a new
organism with a unique identity.
Sexual reproduction
results in a great
variety, or diversity,
of offspring.
Meiosis
• The nucleus divides twice
– Meiosis I
– Meiosis II
• Final Product will be 4 cells from one
parent that contains ½ the original
amount of chromosomes as the
parent.
Interphase
• Same as in Mitosis.
• Normal life cycle functions of the cell
• Divided into 3 stages:
– G1 (Gap 1)- Growth and development
– S (Synthesis Phase) - DNA is duplicated
– G2 (Gap 2)- Organelles double in
preparation for separation
Prophase I
•DNA begins to shorten & thicken
–Chromatin now called sister chromatids
or chromosomes
•Centromeres form
•Nuclear membrane breaks apart
•Spindle fibers begin to form
NEW from Mitosis
• Sister chromatids find their “match”
(similar chromosome from your other
parent) and CROSSING OVER may
occur.
Homologous Chromosomes
• Crossing over happens in Prophase 1
Metaphase I
• Homologous Chromosomes
(AKA Tetrads – two matching
pairs of sister chromatids) line up in middle of cell in
HOMOLOGOUS PAIRS
• Centromeres attach
to spindle fibers
Anaphase I
• Homologous chromosome pairs
separate and move to the poles.
–NOTE: Sister chromatids are
NOT pulled apart
–The sisters DO NOT separate
• A pair of sister chromatids
move to each ends of cell
http://biotech-adventure.okstate.edu/low/basics/meiosis/phases/anaphase-I/anaphase-i.gif
Telophase I
• May or May Not occur. BUT
Cytokinesis does!
• If it does:
–Normal telophase, X-somes
uncoil, nucleus appears, cell
divides.
• No INTERPHASE – i.e. there
is No further replication of
the DNA
http://biotech-adventure.okstate.edu/low/basics/meiosis/phases/telophase-I/telophase-I.gif
What we have at this Point
- Two new cells are created, each
carrying ½ the original # of
chromosomes.
- These cells are now HAPLOID (n)
- normal somatic cells are diploid (2n)
- This means each cells now carries
only one (duplicated) copy of a
homologous chromosome pair.
• The two new cells are not normally or
necessarily the same. They can carry
different combinations of genetic
information from the parent cell or the
other sister cell because of crossing over.
• Because the chromosome number has
decreased at this point meiosis 1 is known as
Reduction Division stage of meiosis.
Prophase II
• Similar to mitosis but:
–Starts with TWO cells instead
of one
• Spindle fibers appear
• Nucleus disappears (if it
reformed)
»(just like Mitosis)
http://biotech-adventure.okstate.edu/low/basics/meiosis/phases/prophase-II/prophase-II.gif
Metaphase II
• Now - Duplicated sister
chromatids/chromosomes line
up in middle of cell
• Spindle fibers attach to
centromeres
–(just like Mitosis)
http://biotech-adventure.okstate.edu/low/basics/meiosis/phases/metaphase-II/metaphase-II.gif
Anaphase II
• Centromere divides
–Sister chromatids separate &
move to ends of cell
• Chromatids are now individual
chromosomes
»(just like Mitosis)
http://biotech-adventure.okstate.edu/low/basics/meiosis/phases/anaphase-II/anaphase-II.gif
Telophase II
• Spindle fibers disappear
• Nuclear membranes reform
and cytokinesis takes place.
»(just like Mitosis)
• Results in 4 NEW cells
• Each with ½ the original number
of chromosomes and all are
unique (NOT like Mitosis)
http://biotech-adventure.okstate.edu/low/basics/meiosis/phases/telophase-II/telophase-II.gif
Meiosis Animation
http://www.johnkyrk.com/meiosis.html
In summary:
Meiosis
• Two cells form during meiosis I with
half the original number of
chromosomes, then in meiosis II
both of these cells divide again to
separate sister chromatids.
• These two divisions result in four sex
cells (gametes), each with one-half
the original number of chromosomes
as the parents cell.