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Key Concepts:
•What happens during the process of meiosis?
•How is meiosis different than mitosis?
Vocabulary• Homologous—chromosome from mom and
corresponding chromosome from dad
Vocabulary• Diploid—Cells with two
sets of chromosomes
(2N)
– Code for same genes
in somatic
• Haploid—Cells with one
set (N)
– In the sex cells
(gametes)
– These will join with
sex cells (ex: ova
joins with pollen)
Eye Color is not as simple as it appears in this figure!
Each Species contains a characteristic number of chromosomes
Organism Body Cell (2N) Gamete (N)
Human 46 23
Garden Pea 14
Fruit Fly 8
Dog 39
Chimpanzee 24
Leopard Frog 13
Corn 20
Apple 17
Adder’s Tongue Fern 1260
7
4
78
48
26
10
34
630
• Meiosis is a specialized type of cell division that occurs in the formation of gametes such as egg and sperm.
• Meiosis appears much more complicated than mitosis…
– It is really just two divisions in sequence
• Recall that the purpose of mitosis was for growth
and repair of damaged tissues.
• What is the purpose of meiosis?
To make sex cells!!!!
Aka- gametes
Aka sperm or egg
Aka ova or pollen
INTERPHASE• Interphase in meiosis is identical
to interphase in mitosis
• Meiotic division will only occur in cells associated with male or female sex organs.
MEIOSIS I
• Meiosis I, the first of the two divisions, is often called
reduction division
– It is here that the chromosome number is reduced from
2N (diploid) to 1N (haploid).
• Ex: Humans: diploid number = 46 (haploid = 23)
PROPHASE I• Prophase I is similar to
prophase in mitosis
– Chromatin condenses into
chromosomes
– Spindle begins to develop
– Breakdown of the nuclear
envelope
Prophase I—Important Events
• Homologous chromosomes pair up to form a tetrad
• The sister chromatids press together at points along their length
• It is during this alignment that chromatid arms may overlap and temporarily fuse resulting in crossing over
Result: chromosomes (consisting of 2 sister chromatids) are no longer just from “mom” or “dad”—the chromosomes are a mixture!!!
Vocabulary
• Tetrad—each chromosome pairs with
homologous chromosome
– 4 sister chromatids!
• Crossing over– when chromosomes are in tetrads, they
may exchange parts
This happens in Prophase I
Result–increased genetic variety in offspring (a good
thing)
METAPHASE I
• Here is where the critical difference occurs
between Metaphase I in meiosis and
metaphase in mitosis.
METAPHASE I
• In Mitosis, all the chromosomes line up on the metaphase plate in no particular order.
• In Metaphase I (of Meiosis), the homologous pairs are alignedon either side of the metaphase plate.
ANAPHASE I
• During Anaphase I the
spindle fibers contract
– The homologous pairs
are pulled away from
each other
TELOPHASE I• Cleavage is followed by cytokinesis, but…
• The nuclear membrane (envelope) usually is not reformed and the chromosomes do not disappear.
At the end of Telophase I
• Each daughter cell has a single set of chromosomes
• Half the total number in the original cell where the chromosomes were present in pairs.
• While the original cell was diploid (2n), the daughter cells
are now haploid (1n).
• This is why Meiosis I is often called reduction division.
– Diploid = 4 Haploid = ?
– Diploid = 36 Haploid = ?
It was 4, now it will be 2 in
each cell!!
MEIOSIS II
• Meiosis II is quite simply a mitotic division of each of
the haploid cells produced in Meiosis I.
• There is no Interphase between Meiosis I and Meiosis
II and the latter begins with…
PROPHASE II
• A new set of spindle fibers
forms
• The chromosomes begin to
move toward the equator
of the cell.
METAPHASE II
• All the chromosomes in
the two cells align with the
metaphase plate. (Like in
mitosis….)
ANAPHASE II
• The centromeres split
• The spindle fibers shorten
– Drawing the chromosomes
toward each pole of the cell.
– (Like in mitosis….)
TELOPHASE II
• Cleavage is followed by cytokinesis
• The nuclear membrane (envelope) reforms
• The chromosomes begin to fade, replaced by the granular chromatincharacteristic of interphase.
• Total of four daughter cells, each with half the total number of chromosomes as the original cell.
• Male structures
– All four cells will eventually
develop into typical sperm
cells.
• Female life cycles in “higher”
organisms
– Three of the cells will typically
abort
– Leaving a single cell to develop
into an egg cell
• Usually much larger than a
typical sperm cell.
Gametes
Meiosi
s II
http://www.biologycorner.com/bio4/notes/meiosis.php
NOVA video
http://www.pbs.org/wgbh/nova/baby/divi_flash.html
https://www.youtube.com/watch?v=qCLmR9-YY7o