MEIOSIS, SPERMATOGENESIS, AND OOGENESIS
Meiosis
A. Process by which an organism reduces the chromosome number of its potential
gametes from diploid to haploid.
B. Cells that undergo meiosis pass through several stages similar to those observed in
mitosis.
1.
Meiosis I (reduction division)
a.
Interphase
(1)
Diploid cells destined to become gametes carry out the
usual cell processes observed during Interphase of mitosis.
(2)
The chromatin and its DNA duplicate
(3)
Centrioles duplicate
(4)
Centromeres duplicate
(5)
Cells are producing structures necessary for remaining
stages of meiosis
b.
Prophase I
(1)
Each chromosome composed of two chromatids becomes
more evident
(2)
Nucleolus disintegrates
(3)
Nuclear membrane disintegrates
(4)
Centrioles separate and move to opposite poles
(5)
Homologous chromosomes (synapse) forming tetrads
Each member of a homologous pair is a dyad (2
chromatids) 2 dyads = 1 tetrad (4 chromatids)
(6)
Crossing over between chromatids of some homologous
chromosomes often occurs
(7)
Spindle fibers form
c.
Metaphase I
(1)
Tetrads line up on the equator
(2)
One spindle fiber attaches to the centromere of each
dyad
d.
Anaphase I
(1)
The two dyads of each tetrad are pulled to opposite poles
(2)
Each dyad is still composed of two chromatids
e.
Telophase I
(1)
New nucleoli form
(2)
New nuclear membranes form
(3)
Each chromosome (dyad) uncoils becoming less
evident
(4)
Spindle fibers disappear
(5)
Cytokinesis produces two daughter cells
(6)
Each daughter cell is haploid
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2. Meiosis II (equational division)
a.
Prophase II
(1)
The dyads from each daughter cell of maiosis I
becomes more evident
(2)
Nucleolus disintegrates
(3)
Nuclear membrane disappears
(4)
Centrioles separate and move to opposite poles
(5)
Spindle fibers form
b.
Metaphase II
(1)
Dyads line up on the equator
(2)
One spindle fiber attaches to the centromere of each
chromatid
c.
Anaphase II
(1)
The chromatids of each dyad are pulled to opposite
Poles
d.
Telophase II
(1)
New nucleoli form
(2)
New nuclear membranes form
(3)
Each chromosome uncoils becoming less evident
(4)
Spindle fibers disappear
(5)
Cytokinesis produces two new daughter cells
(gametes)
(6)
Each daughter cell is haploid
(See illustration on page 3)
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Meiosis Illustrated
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II.
Spermatogenesis (male gametogenesis)
A. This is the meiotic process in many male animals, including the human, that
reduces the chromosome number of potential gametes from diploid to haploid.
B. Spermatogenesis in the human does not begin until the onset of puberty.
C. Cells located in the testes called spermatogonia undergo several mitotic divisions
to produce primary spermatocytes. Both spermatogonia and primary
spermatocytes are diploid cells
D. The primary spermatocytes undergo meiosis
E. Stages
1. The primary supermatocytes undergo the same processes during the same
stages as any other cell going through meiosis
2. Interphase – 2n
3. Prophase I – 2n
4. Metaphase I – 2n
5. Anaphase I – 2n
6. Telophase I
a. Completion of this stage yields two haploid cells call secondary
spermatocytes
b. EACH secondary spermatocytes continues through meiosis II
7. Prophase II – n
8. Metaphase II – n
9. Anaphase II – n
10.Telophase II – n
a.
Completion of this stage yields two haploid cells called
spermatids from each secondary spermatocytes
b.
Each spermatid matures into a functional sperm cell
III. Oogenesis (female gametogenesis)
A. This is the meiotic process in many female animals, including the human, that
reduces the chromosome number of potential gametes from diploid to haploid.
B. Oogenesis begins in the human female fetus BEFORE birth.
C. Cells called oogonia located in the ovaries of the fetus undergo several mitotic
divisions to produce primary oocytes. This process is generally completed three
months prior to birth, thus a new-born female infant already possesses all the
primary oocytes she will ever have.
D. Both oogonia and primary oocytes are diploid cells
E. The primary oocytes undergo meiosis
F. Stages
1. Interphase
a. Occurs in ovaries of female fetus
b. Primary oocytes are 2n
2. Prophase I
a. Occurs in ovaries of female fetus
b. Primary oocytes are 2n
c. Meiosis stops until the onset of puberty
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3. Metaphase I
a. Hormonal changes during puberty stimulate the continuation of meiosis by
the primary oocytes
b. Primary oocytes are 2n
4. Anaphase I 2n
5. Telophase I
a. Completion of this stage yields two haploid cells of unequal size. The
larger cell is secondary oocytes. The smaller cell is called a first polar body
b. The first polar body either disintegrates or continues through Meiosis II
c. The secondary oocytes is OVULATED. The secondary oocytes only goes
through meiosis II if a sperm penetrates. Sperm penetration stimulates the
secondary oocytes to undergo meiosis II and become a functional ovum
6. Prophase II (assuming sperm penetration) –n
7. Metaphase II - n
8. Anaphase II - n
9. Telophase II – n
a. The secondary oocytes divides UNEQUALLY to produce a larger functional
haploid ovum and a smaller second polar body
b. When nuclei of the sperm and ovum unite, a 2n zygote develops
c. The first polar body may have divided to form two polar bodies
(See illustration on page 6)
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GAMETOGENESIS
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