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Meiosis is the first step in gametogenesis: separation of homologous chromosomes into haploid daughter cells PowerPoint PPT Presentation

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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

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Meiosis is the first step in gametogenesis: separation of homologous chromosomes into haploid daughter cells

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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

Meiosis generates tremendous genetic diversity. How many different types of gametes can be generated by an individual (male or female) with 23 different chromosomes?

More than 223 or 8,000,000 different gametes

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.

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.

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.

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.

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.

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)

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.

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.

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).

What does a fly’s ovary look like?

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.

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?

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 isn’t 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 causesbreakdown of the nuclear envelopeby phosphorylating nuclear lamins (proteins stabilizing the envelope), it triggerschanges in the oocyte plasma membrane, it stimulatesovulation, and it causescondensation of chromosomes.

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.

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.

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