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Meiosis
Division of Sex Cells
Meiosis
 A process of reduction division in which the
number of chromosomes per cell is cut in half
through the separation of homologous
chromosomes in a diploid cell.
 Diploid – 2 sets of chromosomes (2N)
 Haploid – 1 set of chromosomes (N)
 Homologous pair – chromosomes that each
have a corresponding chromosome from the
opposite sex parent
Karyotyping
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Homologues
 Chromosomes exist in homologous
pairs in diploid cells.
Exception: Sex chromosomes (X, Y).
Other chromosomes are known as autosomes, they have
homologues.
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Meiosis creates genetic variation
 During normal cell growth, mitosis produces
daughter cells identical to parent cell (2n
to 2n)
 Meiosis results in genetic variation by
shuffling of maternal and paternal
chromosomes and crossing over.
No daughter cells formed during meiosis
are genetically identical to either mother or
father
During sexual reproduction, fusion of the
unique haploid gametes produces truly
unique offspring.
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Stages of Meiosis
 Meiosis usually involves 2 distinct stages
 Meiosis I (PI, MI, AI, TI)
 Meiosis II (PII, MII, AII, TII)
Interphase
Meiosis in preceded by a preparatory phase
known as INTERPHASE (G1,S,G2)
(G1-first growth,S-DNA is copied,G2-growth)
*cell grows (matures)
*chromosomes replicate before Meiosis
begins, directly before Prophase I
Prophase I
 Chromatin condenses so that chromosomes
become visible
 Nuclear envelope disassembles
 Centrosome (with centrioles) move to opposite
ends of the cell
 Spindle fibers (cytoplasmic fibers) begin to appear
 Each chromosome pairs with its corresponding
homologous chromosome, through a process
called synapsis, to form a tetrad. (This step does
not occur in Mitosis) Crossing-over (when
chromosomes overlap and exchange portions of
their chromatids) may occur, which leads to
genetic recombination, or genetic variation.
Crossing over
Chiasmata (chiasma) – sites
of crossing over, occur in
synapsis. Exchange of genetic
material between non-sister
chromatids.
Crossing over produces
recombinant chromosomes.
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Metaphase I
 Homologous pairs of chromosomes (tetrads) line up
along the midline of the cell, often called the
metaphase plate
 Spindle fibers attach to the centromere (kinetochore)
of chromosomes (some say this happens in
“prometaphase”
Anaphase I
 The homologous chromosomes move toward
opposite ends (poles) of the cell.
Telophase I & Cytokinesis I




Chromosomes move to opposite ends of cell
Nuclear membranes form for 2 new nuclei
Spindle apparatus disappears
The cell separates into 2 “daughter” progeny cells through
cytokinesis
 Cytokinesis: division of the cytoplasm of the cell that finalized
the division of the entire cell.
 Meiosis I results in two haploid (N) cells, having half the
genetic information as the parent cell.
Prophase II
 In prophase II, Spindle fibers form;
chromosomes begin to move to the midline of
the cell
Metaphase II
 The chromosomes line up at the metaphase
plate w/ each sister chromatid facing
opposite poles of the cell.
Anaphase II
 Sister chromatids separate and move to
opposite ends of the cell.
Telophase II & Cytokinesis II
 Nuclear envelope forms around each of
the 4 new cells; cytokinesis II occurs,
resulting in 4 haploid cells.
Gamete Formation
 In males, meiosis results in 4 sperm cells
 In females, meiosis results in 1 egg cell
and three polar bodies, which are not
used in reproduction.
Mitosis vs Meiosis
Results in
Cells are
Occurs in
Mitosis
2 Diploid Cells
(2N)
Genetically
Identical
Somatic (Body)
Cells
Meiosis
4 Haploid Cells
(N)
Genetically
Different
Sex Cells
Mitosis vs. meiosis
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Mitosis
Interphase (G1,S,G2)
G1-first growth period
S phase, new DNA is synthesized by the cell
resulting in each chromosome with two molecules
of DNA. These are attached together at a structure
called the centromere.
G2-second growth period
Remember that interphase is not part of mitosis!
Prophase
*chromosomes becoming visible
*nuclear membrane disintegrates
*paired centrioles(centrosomes) move to opposite
ends of the cell
*mitotic spindle forms
*Late prophase is sometimes given a special name Prometaphase because of the
two distinct events that happen then, namely, the disintegration of the nuclear
membrane and, the attachment of the chromosomes to the spindle. Some would
argue that the attachment of the chromosomes to the spindle happens during
metaphase.
Metaphase
*chromosomes are lined up at the midline
*fibers are attached to kinetochore (according to
some)
Remember that the chromosomes are still duplicated
chromosomes during metaphase
Anaphase
*chromatids of each duplicated chromosome separate
*chromosomes travel toward the centrosomes
When the chromosomes reach the end of the spindles,
telophase begins
Telophase
*membranes for the two new nuclei form
*spindles are broken down by the cell
*unduplicated chromosomes begin to unravel and stretch out
At the end of telophase the nuclei have taken on the
appearance of interphase nuclei, and the cell cycle is ready to
begin again in each of the two cells, assuming cytokinesis has
occurred.
The basic point of cytokinesis is to divide the cytoplasm of the original cell
such that each of the nuclei. from mitosis gets roughly equal amounts of
cytoplasm and the organelles each new cell is going to need.
Mitosis & Meiosis links
http://staff.jccc.net/pdecell/celldivision/mito
Mitosis Stages
sis1.html#prophase
(good description)
http://www.cellsalive.com/mitosis.htm?iframe=true&width=90%&height=90%
Mitosis and Meiosis Animation
Mitosis Animation
Mitosis Video
Mitosis Stages
(basic)
http://www.johnkyrk.com/mitosis.html
http://www.youtube.com/watch?v=VlN7K1-9QB0
http://library.thinkquest.org/C0118084/Gene/Chromoso
mal_Inheritance/StagesMitosis.htm