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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.
Replication of Chromosomes
Replication is the process of duplicating a chromosome Occurs prior to division Replicated copies are called sister chromatids Held together at centromere
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Occurs in Interphase
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
• Spindle fibers attach to the Centromere (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/
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.
Why Do we Need Meiosis?
It is the fundamental basis of sexual reproduction
Two haploid (n) gametes are brought together through fertilization to form a diploid (2n) zygote
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Fertilization – “Putting it all together”
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1n =3
2n = 6
Facts About Meiosis
Follows normal Interphase (G1, S, G2) Two stages–
Meiosis I (Reduction Division)
Meiosis II
Know as:
Original cell is diploid (2n) (full number of chromosomes)
Four daughter cells produced that are haploid (n) (half the number of X-somes
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More Meiosis Facts
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Start with 46 duplicated chromosomes (2n) After 1 division - 23 duplicated chromosomes (n) After 2nd division - 23 single chromosomes (n) Occurs in our germ cells that produce gametes
Produces gametes through gametogenesis (eggs & sperm) Occurs in the testes in males to make sperm (Spermatogenesis)
Occurs in the ovaries in females ova/eggs (Oogenesis)
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Meiosis Forms Haploid Gametes Meiosis must reduce the chromosome number
by half Fertilization then restores the 2n number
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from mom from dad child
meiosis reduces
genetic content
too
much!
The right
number!
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.
Tetrads Form in Prophase I
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Homologous chromosomes
(each with sister
chromatids)
Join to form a TETRAD
Called Synapsis
Homologous Chromosomes
• Crossing over happens in Prophase 1
Crossing-Over
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Crossing-over multiplies the already huge number of different gamete types
produced by independent assortment
A Replicated Chromosome - Tetrad
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Homologs (same genes, different alleles)
Sister Chromatids (same genes, same alleles)
Gene X
Homologs separate in meiosis I and therefore different alleles separate.
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)
Prophase II
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Nuclear envelope fragments. Spindle forms.
Metaphase II
• Now - Duplicated sister chromatids/chromosomes line up in middle of cell
• Spindle fibers attach to centromeres
–(just like Mitosis)
Metaphase II
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Chromosomes align
along equator of cell.
Anaphase II
• Centromere divides
–Sister chromatids separate & move to ends of cell
• Chromatids are now individual chromosomes
»(just like Mitosis)
Anaphase II
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Sister chromatids separate and move to opposite poles.
Equator
Pole
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)
Telophase II
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Nuclear envelope assembles. Chromosomes decondense. Spindle disappears. Cytokinesis divides cell into two.
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.
Gametogenesis
Oogenesis or
Spermatogenesis
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Spermatogenesis Occurs in the testes
Two divisions produce 4 spermatids
Spermatids mature into sperm
Men produce about 250,000,000 sperm per day
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Spermatogenesis
• Begins as a diploid (2n) germ cell called a spermatogonium.
• This cell enlarges and undergoes Meiosis 1 & 2
• Final product is 4 haploid (n) spermatid cells.
– Each cell has equal amount of cytoplasm and the same number of chromosomes
• Spermatids then move off to the epididymis for maturation.
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Spermatogenesis in the Testes
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Spermatid
Spermatogenesis
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Oogenesis Occurs in the ovaries
Immature Begins as a diploid (2n) germ cell called an oogonium and then through the two meiotic divisions will create 3 polar bodies that die and 1 healthy egg. Polar bodies die because of unequal division of cytoplasm
Immature eggs at the time of birth are called locked in Prophase 1 and called Primary oocytes.
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Starting at puberty, one primary oocyte is selected to mature and be ovulated as a secondary oocyte.
Only if fertilization occurs will the final round of meiosis be completed.
NOTE: The stages of Meiosis 1 & 2 are not continuous.
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• Why the unequal distribution??
• To provide the ovum with sufficient nutrients to support the developing zygote in the first few days following fertilization.
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Oogenesis in the Ovaries
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Comparing Mitosis and Meiosis
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See Page 478 in Text
• Make Sure you are able to describe and compare the structure differences between sperm and egg.
– Sizes, energy reserves, mitochondria, numbers produced, motility, and additional structures (acrosomes)
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Comparison of Divisions
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Mitosis Meiosis
Number of divisions 1 2
Number of daughter cells
2 4
Genetically identical? Yes No
Chromosome # Same as parent Half of parent
Where Somatic cells Germ cells
When Throughout life At sexual maturity
Role Growth and repair Sexual reproduction
