Meiosis is the first step in gametogenesis: separation of homologous

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Meiosis is the first step in gametogenesis
separation of homologous chromosomes into haploid
daughter cells
Spermatogonia and oogonia are the germ cells that
will eventually develop into the mature sperm or
egg Primary spermatocyte or oocyte the first
step in this development is the duplication of
homologous chromosomes to get ready for meiosis
Secondary spermatocyte or oocyte the first
meiotic division separates the homologous
chromosomes from each parent Spermatids or eggs
the second meiotic division separates the 2
chromatids and creates 4 haploid cells In males,
this eventually produces 4 sperm cells by the
process of spermiogenesis. In females, it
produces 1 egg and 3 polar bodies. This allows
the egg to retain more cytoplasm to support early
stages of development
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Meiosis generates tremendous genetic diversity.
How many different types of gametes can be
generated by an individual (male or female) with
23 different chromosomes?
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More than 223 or 8,000,000 different gametes
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The timing of meiosis differs in females and males
In males, the spermatogonia enter meiosis and
produce sperm from puberty until death. The
process of sperm production takes only a few
weeks. Each ejaculation has 100 to 500 million
sperm. In females, this process is more complex.
The first meiotic division starts before birth
but fails to proceed. It is eventually completed
about one month before ovulation in humans. In
humans, the second meiotic division occurs just
before the actual process of fertilization
occurs.
Thus, in females, the completion of meiosis can
be delayed for over 50 years. This is not always
good. Only I egg produced In addition, all
meiosis is ended in females at menopause.
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Homologous chromosomes form the synaptonemal
complex which facilitates crossing over and
genetic diversity
During meiosis, homologous chromosomes join
together in pairs to form the synaptonemal
complex. Each pair of chromatids is connected by
axial proteins. The 2 homologous chromosomes are
held together closely by central element
proteins. A recombination nodule forms that
contains enzymes for cutting and splicing DNA.
Chromosomes are cut and joined crosswise at
points called chiasmata, seen when they
separate. The exchange of genetic material is
evident when the chromosomes separate This
process is dangerous as it leads to deletions and
duplications of genetic material. However, it is
also valuable because it increases genetic
diversity and facilitates evolution.
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In older women, failure of the synaptonemal
complex to separate properly can cause genetic
disease
Down syndrome is trisomy 21. It results in short
stature, round face and mild to severe mental
retardation. This is the failure of the 2
chromatids to separate during meiosis 2. It
results in one oocyte receiving 2 instead of 1
chromatid. In older women, long term association
of chromatids (i.e., over 50 years) results in
the axial proteins failure to separate. Down
syndrome occurs with a frequency of 0.2 in women
under 30 but at 3 in those over 45 years of age.
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Spermatogenesis occurs in the seminiferous tubules
The mammalian testes are divided into many
lobules, and each lobule contains many tiny
seminiferous tubules. Sperm develop in an ordered
fashion in these tubules. Cells start to mature
on the outside and move inward (towards the
lumen) as the become mature sperm.
Spermatogonia are the most primative cells. They
differentiate as primary spermatocyte ? secondary
? spermatid ? sperm are released into
lumen. Sertoli cells are supporting cells that
stretch from the lumen to the edge of the tubule.
They nourish the developing sperm. They form a
blood-testis barrier to control spermatogenesis
(similar to the blood-brain barrier). These cells
also inhibit spermatogenesis before puberty and
stimulate the process after puberty.
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Spermiogenesis is the maturation process into
sperm
The golgi vesicles combine to form an acrosomal
vesicle that lies over the nucleus. Its full of
enzymes
Centosomes start to organize microtubules into
long flagella
Mitochondria start to localize next to the
flagella to provide ready energy
The nucleus condenses in size and is stabilized
by special proteins called protamines
The excess cytoplasm is pinched off as a residual
body (no need for organelles and cytoplasmic
proteins)
Sperm are tiny, but highly specialized missiles
for delivering the male genome Microfilaments
shoot the acrosome into the egg to harpoon it
and pull it in. The acrosome has enzymes for
breaking into the egg. The midpiece has large
numbers of mitochondria for horsepower. The tail
has a powerful flagellum for driving the sperm
into the proximity of the egg (in humans, through
the uterus and up into the oviduct.
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Spermatogonia and oogonia are stem cells
What is a stem cell? Stem cells have 3
properties 1. They are undifferentiated
cells 2. They have potential for
self renewal 3. They are able to
undergo differentiation to form committed
progenitor cells (a fancy
word for all types of differentiated
adult cells such as muscle, bone,
skin, etc)
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The goal of oogenesis is to produce one egg with
massive amounts of cytoplasm
In many organisms, such as frogs and birds, the
egg must contain all the nutrients to support the
entire process of embryonic development In
humans, the egg does not need to grow so large
because the fertilized egg only needs to support
growth until it implants in the uterus. The
placenta then nourishes development.
In some organisms, such as frogs, oocytes grow to
extremely large size and they have very active
chromosomes that synthesize large amounts of RNA.
In contrast to sperm which are tiny cells,
oocytes are among the largest cells in the
body. Oocytes contain Lampbrush chromosomes
look like brushes that were used years ago to
clean lamps. Frog oocytes can contain 200,000
times as many ribosomes as a normal cell.
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Oocytes have a very small nucleus / cytoplasm
ratio
Most normal cells have several times as much
cytoplasm as nucleus. This allows the nucleus to
make enough mRNA and rRNA to keep up with the
cytoplasm and cell needs. In some species,
oocytes have a tremendously tiny nucleus to
cytoplasm ratio. They must have a large amount of
cytoplasm and ribosomes to make all of the
proteins needed for embryonic development. The
nucleus is just not large enough to keep up and
maintain enough transcription to generate all of
the needed components. However, oocytes have
developed specializations to deal with this
problem. 1. Ribosomal RNA genes are often
amplified in oocytes. This allows more templates
to transcribe more rRNA.
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Specializations allow the egg to accumulate
cytoplasm nurse cells allow oocytes of
insects to produce massive amounts of RNA
In Drosophila melanogaster, the oogonia are
called ctyoblasts, and they undergo an unusual
specialization They undergo multiple mitotic
divisions, but fail to undergo cytokinesis (cell
division). Thus, they all remain connected to the
original cell as cytocytes One of the lucky
cytocysts becomes the oocyte The other 15 become
nurse cells. They make large amounts of RNA and
nutrients but they send it all to the oocyte.
This allows the oocyte to accumulate massive
amounts of cytoplasm to support development (15
nuclei instead of 1).
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What does a flys ovary look like?
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Vitellogenesis is the process of producing the
major yolk proteins
Yolk animal eggs contain large amounts of
protein, lipid, and glycogen to nourish the
embryo. These materials are collectively called
yolk. Yolk is minimal in animal eggs that
sustain only the first portion of embryogenesis
(humans and many mammals that have a placenta
need only support cleavage for several days
before implantation into the uterus). However,
yolk is stored in large amounts in the eggs of
birds and reptiles because their eggs have to
support the entire process of development. Yolk
proteins are synthesized in the liver in
vertebrates, or in the fat body of insects (an
analogous organ)
Animal vegetal polarity In eggs that have a
lot of yolk, the yolk is concentrated in the
vegetal pole. The animal pole contains the
nucleus and relatively little yolk. The yolk in
the vegetal pole interferes with cytokinesis
during the process of cleavage leading to
incomplete cleavage.
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Maturation processes prepare the oocyte for
ovulation and fertilization
Most oocytes of different species are arrested in
the first meiotic division. Oocyte maturation
begins officially when this block is removed and
meiosis starts once again. 1. The nuclear
membrane breaks down and DNA starts to condense
into chromosomes 2. The permeability of the
oocyte plasma membrane changes so it can function
outside of the ovary. 3. The plasma membrane
develops receptors to interact with the
sperm Fertilization occurs at different stages
of oocyte maturation
How is oocyte maturation initiated?
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Control of oocyte maturation has been studied
extensively in frogs
Oocyte maturation is controlled by hormone
interactions between the pituitary and follicle
cells. Pituitary ? gonadotropin hormone ?
stimulates follicle cells ? progesterone ?
triggers oocyte maturation by activating c-mos
expression C-mos activates maturation promoting
factor, the same activity as M-phase promoting
factor, that is composed of cyclin B and cyclin
dependent kinase 1
The exact
mechanism isnt understood.
If c-mos is inactivated by antisense
oligonucleotides, no oocyte maturation occurs. On
the other hand, if extra c-mos is injected it
triggers oocyte maturation before it is
ready. MPF does many things, although the exact
pathways have yet to be found. It causes
breakdown of the nuclear envelope by
phosphorylating nuclear lamins (proteins
stabilizing the envelope), it triggers changes in
the oocyte plasma membrane, it stimulates
ovulation, and it causes condensation of
chromosomes.
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Development of mammalian oocytes occurs within
the ovary
In the mammalian ovary, the oocytes are closely
associated with somatic cells called granulosa
cells which aid oocyte maturation and ovulation.
The timing of oocyte maturation and ovulation
varies in different mammals. Ovulation can be
stimulated by seasonal cues, the process of
mating, or in primates, by the monthly cycle
regulated by hormones such as estradiol, produced
by the granulosa cells.
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Eggs are protected by elaborate envelopes
Vitelline envelope a glycoprotein layer covers
the plasma membrane of all eggs. This acts to
protect the egg. Eggs that are deposited in
water have a jelly-like coating that surrounds
the egg (frogs eggs) Eggs that are deposited on
land have particularly elaborate envelopes. The
eggs of birds have a vitelline envelope, a
fibrous layer, an outer layer of albumin (egg
white), and a shell composed of calcium
carbonate. The outer envelopes are synthesized in
the oviduct after the egg has been fertilized.