MEIOSIS
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Meiosis is a process of nuclear division that
reduces the number of chromosomes in new
cells to half the number in the original cell. The
halving of the chromosome number counteracts
a fusion of cells later in the life cycle of the
organism. In animals , meiosis produces
haploid reproductive cells called GAMETES.
Human gametes are sperm and egg cells, each
which contains 23(1n) chromosomes. The
fusion of sperm and egg results in a zygote that
contains 46 (2n) chromosomes
MEIOSIS IS DIFFERENT FROM
MITOSIS
A. The process of meiosis results in the production of
Daughter Cells that have HALF THE NUMBER OF
CHROMOSOMES OF THE PARENT CELL (HAPLOID
CELL).
B. Daughter Cell produced by meiosis ARE NOT ALL
ALIKE. THE DAUGHTER CELLS MAY HAVE
DIFFERENT CHROMOSOMES FROM EACH OTHER.
C. The NUMBER OF CELLS PRODUCED BY MEIOSIS
IS DIFFERENT.
(1) Mitosis - One Parent Cell PRODUCES TWO
DIPLOID DAUGHTER CELLS.
(2) Meiosis - One Parent Cell PRODUCES FOUR
HAPLOID DAUGHTER CELLS.
STAGES OF MEIOSIS
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Like Mitosis – Cells go through G1, S, G2
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Cells begin with 2N # of Chromosomes
THE PROCESS OF MEIOSIS SEPARATES THE
PAIRS OF CHROMOSOMES IN A DIPLOID CELL
TO FORM HAPLOID CELLS.
ONE PARENT CELL DIVIDES TWICE TO
PRODUCE FOUR HAPLOID DAUGHTER CELLS.
DURING MEIOSIS, THE NUMBER OF
CHROMOSOMES IN EACH CELL IS REDUCED
FROM DIPLOID TO HAPLOID BY SEPARATING
HOMOLOGOUS PAIRS OF CHROMOSOMES.
MEIOSIS I
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AT THE START OF MEIOSIS I EACH
CHROMOSOME CONSIST OF TWO STRANDS OF
SISTER CHROMATIDS CONNECTED AT THE
CENTROMERE.
HOMOLOGOUS PAIRS OF CHROMOSOMES
COME TOGETHER BEFORE MEIOSIS BEGINS,
AN EVENT THAT DOES NOT OCCUR IN MITOSIS.
THIS EVENT IS CALLED SYNAPSIS .
Each Pair of Homologous Chromosomes is called a
TETRAD .
PROPHASE I
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Chromosomes become thick and visible, the chromosomes of
each homologous pair are tangled together.
Portions of Chromatids may break off and attach to adjacent
Chromatids on the homologous Chromosome - a process called
CROSSING-OVER (genes are exchanged between homologues)
Crossing-Over results in Genetic Recombination by
producing a new mixture of genetic material.
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Chiasmata – location on homologue where
crossing over occurs
PROPHASE I cont.
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Each pair consists of FOUR CHROMATIDS, BECAUSE EACH
CHROMOSOME IN THE PAIR HAD REPLICATED BEFORE
MEIOSIS BEGAN.
The centrosomes move to opposite poles, Nucleolus and the
Nuclear Envelope disappear and the spindle fibers form.
METAPHASE I

Homologous pairs (Tetrads) are still together
and arrange along the equator of the cell
(metaphase plate)

Kinetichore fibers attach to each chromosome
Ensures tetrad will
be separated
ANAPHASE I
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The homologous pairs of chromosomes separate from each
other, spindle fibers pull one member from each pair to opposite
ends of the cell. The random separation of the homologous
chromosomes is called INDEPENDENT ASSORTMENT.
(random separation of Male and Female chromosomes)
TELOPHASE I AND CYTOKINESIS I
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Cytokinesis takes place; each new cell is
haploid, containing one chromosome
from each pair.
Results of Meiosis I
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Formation of 2 new daughter cells – but
unlike mitosis, these daughter cells are
haploid cells (1N) not diploid cells (2N)
The cells have ½ the # of chromosomes as
parent cell.
MEIOSIS II
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OCCURS AFTER MEIOSIS I (may go through
a short interphase – cells grows slightly)
CHROMOSOMES DO NOT REPLICATE
BEFORE BEGINNING THE SECOND
PHASE MEIOSIS II WILL DIVIDE
CHROMOSOMES INTO HAPLOID CELLS
CALLED GAMETES.
Meiosis II just like the phases of Mitosis
Each new haploid Cell from Meiosis I will go
through a second division, forming the FOUR
GAMETES  HAPLOID CELL.
PROPHASE II
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Spindle fibers form again (if cell goes through
brief interphase) and begin to move the
chromosomes toward the midline of the
dividing cell
METAPHASE II
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The chromosomes move to the midline of the
dividing cell, with each chromatid facing
opposite poles of the dividing cell.
ANAPHASE II
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The chromatids separate and move to
opposite poles of the cell
TELOPHASE II AND CYTOKINESIS II
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A nuclear membrane forms around the
chromosomes in each of the four new cells,
resulting in four new cells, each with half of
the original number of chromosomes.
FORMATION OF GAMETES
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In Animals, meiosis produces haploid
reproductive cells called GAMETES.
Meiosis occurs within the Reproductive
Organs, in the TESTES or OVARIES
SPERMATOGENESIS
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In the Testes, meiosis is involved in the production
of Male Gametes known as Sperm Cells or
Spermatozoa.
In the development of Sperm Cells, a Diploid
Reproductive Cell divides Meiotically to form FOUR
Haploid Cells called SPERMATIDS.
Each Spermatid then develops into a Mature Sperm
Cell.
The production of Sperm Cells is called
SPERMATOGENESIS .
OOGENESIS
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OOGENESIS is the production of Mature Egg Cells
or OVA.
Notice that the Female only produces ONE EGG
(OVUM) under normal circumstances.
Although creating 4 Haploid Cells through meiosis,
only One Becomes the Egg, the other Three
products of meiosis are called POLAR BODIES
,and Degenerate. This is due to the unequal
dividing of the cytoplasm during Cytokinesis I & II.
Meiosis I
Meiosis II