Meiosis and Gametogenesis
10/15/2008 6:13:00 AM
Meiosis only occurs in germ line cells (cells that produce gametes (haploid
reproductive cells)
 Male meiosis occurs in the seminiferous tubules of the testis
 Female meiosis occurs in the ovary
Meiosis reduces the chromosomal count (in humans) from 46 to 23 (22
autosomal, 1 sex chromosome)
 Chromosomal count is reduced in Meiosis I. Tetrads (homologous
pairs) are separated
Two unique things happen during meiosis that do not occur in mitosis
 Genetic recombination

General
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o Homologous pairs get up close and personal and even swap
information…its very intimate. There is at least one crossing
over event per homologous pair.
Reduction in chromosomal count. Homologous pairs line up at the
metaphase plate and are then separated.
Meiosis
Two cell divisions take place right after another. There is no clear
interphase between meiosis I and Meiosis II.
The prophase of Meiosis I is significantly longer than mitosis. This
allows for crossing over and thereby genetic recombination.
 The centromeres divide during meiosis II, when sister chromatids
are separated. Once the centromeres have divided and each
chromatid has its own centromere, it is then referred to as a
chromosome.
 Spindle from Meiosis I and Meiosis II are formed at right angles to
each other.
 Meiosis produces 4 daughter cells (not in female meiosis)
During the elongated prophase I, there are some specific characteristics of
the homologous pairs that give us an idea of progress.
 Leptotene-Homologous pairs have both undergone DNA synthesis
but are only loosly associated with each other (if at all).
 Zygotene-Homologous pairs are starting to get close to each other.
The synaptonemal complex is forming. The recombination process
is initiated in this stage
o Synaptonemal complex is a proteinaceous structure that
brings the homologous pairs close together.
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o The chromatines are bound to the lateral elements. These
lateral elements are bound to the transverse filaments which
all attached to the central element.
o Lack of a synaptonemal complex can lead to polypoidy,
anuploidy, non-recombination. (generally bad things)
Pachytene
o The synaptonemal complex is fully formed and the
homologous pairs are as close as they will get. Crossing over
events occur in this phase.
Diplotene
o The synaptonemal comples is breaking down and the
homologous pairs are moving apart. The chiasmata are still
attached.
 Diakinesis
o The four chromatids in the tetrad are visible.
Compare and contrast Mitosis and Meiosis
MITOSIS
MEIOSIS
1. Somatic Cells
Germ Line Cells
2. One divisiontwo daughter cells
Two divisionsfour daughter cells
3. Diploid
Haploid
4. Normal prophase
Lengthened prophase
5. Homologous pairs don’t pair up
Homologous pairs to indeed pair
6. No crossing over
Crossing over, genetic exchange
7. 2 cell divisions, one centromere
division
1 cell division, one centromere
division
8. Interphase always between
divisions
No specific interphase between
Meiosis I and Meiosis II
9. DNA synthesis between every
No DNA division between Meiosis I
division
and Meiosis II.
MALE MEIOSIS
Occurs in the seminiferous tubules of the testis.
Starts at puberty and stops at death
Spermatogenesis happens in layers or groups. There are layers of different
stage cells.
 The basement layer is spermatagonia. These are stem cells as they
can continually reproduce. They undergo mitosis and give rise to
primary spermatocytes.
 The second layer, which is separated from the spermatogonia by


tight junctions formed by the seritoli cell, is comprised of primary
spermatocytes. These will undergo Meiosis I and become secondary
spermatocytes.
The Third layer is comprised of Secondary spermatocytes. These
will undergo Meiosis II and become spermatids.
The final layer is the development and maturation of the spermatids
to become spermatozoa.
o Golgi will become the acrosomal process.
o The mitochondrian will aggregate around the neck of the
sperm
o One of the centrioles will enlongate and form the flagella
o The head of the sperm will become more elongated and
arrowhead like (more effiencient when swimming)
Supporting cells for male gametogenesis
 Seritoli cells
o very large cells in the seminiferous tubules that provide the
major support for spermatogenesis
o Blood Testis barrier by forming tight junctions around the
spermatagonia
o Secrete Androgen Binding Protein which is necessary for
testosterone affects on developing sperm
o Nutritional assitance for developing sperm
 Leydig cells
o Produce Testosterone (are the endocrine cells of testis)
o Stimulated by Lutinizing Hormone (LH)
FEMALE MEIOSIS
Oogenesis occurs in the ovary and only produces one egg per oogonia
(different then males)
Starts before birth and then STOPS in Prophase I in the diplotene stage of
development (Embryonic Phase).
Restarts at puberty, then every month a few primary oocytes will come out
of hibernation and complete Meiosis I. One will be ovulated.
Meiosis II is not completed until after fertilization. (Postnatal Phase)
Postnatal phase
 Primary Oocyte will come out of hibernation and finish Meiosis I.
Now called the Secondary Oocyte
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Major changes in the follicle ensue
o Primary FollicleMultilaminar primary follicle
o Multilaminar Primary FollicleSecondary Follicle
 Secondary follicle has the start of the antrum
o Secondary FollicleGraffian Follicle
 Ready to BURST.
 Pronounced antrum. Oocyte surrounded by corona
radiata (follicular cells)
The Egg undergoes major changes, just not in size
o Unequal cytokinesis—formation of polar bodies
o Cortical Granules are formed (important for slow block.)
o Only three “daughter cells” are produced. One egg, two polar
bodies (first polar body does not undergo meiosis II)
Prevention of Polyspermy
 Cortical Reaction
o Fertilization results in the exocytosis of the cortical granules
which causes a major change is vitelline membrane
morphology. It become hard. After the slow block its called
the fertilization envelope.
Objectives
10/15/2008 6:13:00 AM
1. Recognize the different stages of meiotic division
a. Meiosis is broken into two phase: Meiosis I and Meiosis II. Meiosis I
is the reduction step where the number of chromosomes is reduced
by half. Meiosis II is the stage where the sister chromatids are
separated.
b. Meiosis I has an extended prophase. During the extra time,
crossing over events occur and there are specialized chromosome
morphologies
i. Leptotene- Chromosomes have been duplicated and are
condensed
ii. Zygotene—Homologous pairs are starting to associate with
each other. The synaptonemal complex is starting to form.
Crossing over events are initiated.
iii. Pachytene—Synaptonemal complex is formed and the
homologous pairs will be the closest they will ever be.
Crossing over is well on its way.
iv. Diplotene—Chiasmata are visible (areas of crossing over)
v. Diakinesis—separation of the homologous pairs in preparation
for separation.
2. Recognize the differences between mitosis and meiosis
a. Both have one centromere division, but meiosis has two cell
divisions
b. Mitosis has 2 daughter cells and meiosis has four.
c. Mitosis occurs in somatic cells and meiosis only occurs in the germ
line cells.
d. Recombination occurs in meiosis but not in mitosis.
e. Lengthened prophase I in meiosis with no interphase between
meiosis I and meiosis II.
f. DNA synthesis only occurs before Meiosis I, there is no DNA
synthesis between Meiosis I and Meiosis II.
3. Recognize the male organ where meiosis occurs
a. Male Meiosis occurs in the seminiferous tubules of the testis.
4. Recognize the concept of mitotic and meiotic division during
spermatogenesis
a. Mitosis is the first division from spermatagonia to primary
spermatocyte. Meiosis I will make secondary spermatocytes.
Spermatids are the product of meiosis II. Spermatids will mature
into spermatozoa.
5. List the types of nuclear division in spermatogonia, spermatocytes, and
spermatids
a. Spermatagonia undergo mitosis. They are a continually renewing
cell population
b. Spermatocytes undergo meiosis.
c. Spermatids do not have nuclear division, but rather they mature
into spermatozoa. The golgi apparatus will be come the acrosomal
cap, the mitochondrian will migrate to the neck region, which is just
posterior to the nucleus. The nucleus will undergo some
morphological changes to become more “aerodynamic.” It will
become a more efficienty swimmer. One of the centrioles will
elongate to become the flagella.
6. List the functions of Seritoli Cells and Leydig Cells
a. Seritoli Cells are the large cells that provide support for developing
spermatozoa. They create the blood testis barrier by having tight
junctions around the spermatogonia. They provide food for the
developing spermatocytes/tids/zoa. They also produce androgen
bindning factor so the developing gametes may utilize the
testoterone produced by the Leydig Cells
b. Leydig cells respond to Lutinizing Hormone and produce
Testosterone. They are not in the seminferous tubules, but are in
the interstitial space.
7. Recognize the female organ where meiosis occurs
a. Female meiosis occurs in the ovary.
8. Describe the two phases of female meiosis
 Phase one occurs during fetal development. All of the primary
oocytes produced by a female will be made in the first 3 months of
fetal development. Mitosis STOPS during prophase I in the
diplotene state. They will stay in this state until menarche.
 Phase 2 occurs after puberty. Each month a few primary oocytes
will come out of dormancy and finish meiosis I. They will not
undergo meiosis II unless they are fertilized.
9. List the types of nuclear division in oogonia, oocytes, and zygote
a. Oogonia undergo mitosis
10.
11.
b. Oocytes undergo meiosis with unequal cytokinesis. There is equal
division of chromosomes, but unequal division of cytoplasm. This
makes polar bodies (not very useful. They really do nothing.)
c. Zygotes undergo mitosis as they have been fertilized and the cell is
now diploid again. This will become the fetus.
Recognize the events during fertiization
a. Fertilization will cause the secondary oocyte to go through meiosis
II. The penetration of the sperm into the oocyte will cause the
repulsion by depolarizing the membrane. The cortical granules will
exocytose there contents and cause the vitelline membrane to
harden creating the fertilization envelope.
Describe the embryonal development
10/15/2008 6:13:00 AM