MIIM - 532 - Topic 5: Sexual Reproduction
The four-chromatid structure formed during prophase I is called?
Bivalent
Binding of the sperm to the egg results in?
Activation of the egg by calcium-mediated signaling mechanisms
What is the overview of the pathway of meiosis?
After DNA replication two nuclear (and cell) divisions are required to produce the haploid gametes. The duplicate homologs each consisting of tightly bound sister chromatids, pair up and are segregated into different daughter cells in Meiosis I. The sister chromatids separate only in meiosis II. Homolog pairing in meiosis leads to genetic recombination (crossing over) during Meiosis I. Each diploid cell the enters meiosis therefore produces four genetically different haploid cells.
In animals the gametes develop from diploid germ-line cells in the gonads, all the rest of the cells in the organism are?
Somatic cells
Spermatogonia divide by mitosis at the periphery of the seminiferoud tubule and enter what process?
Some enter meiosis I and become primary spermatocytes
Improper homolog separation results in?
Some of the haploid gametes produced lacking a particular chromosome while others have more than one copy.
Some of the cohesin subunits that operate during meosis are the same as those involved in mitosis, while there are others that are?
Specific for meiosis
Spermatids undergo dramatic morphological changes as they differentiate into?
Sperm
The primary spermatocytes give rise to the the secondary spermatocytes which differentiate into?
Spermatids and sperm
The secondary spermatocytes then complete meiosis II to become?
Spermatids which differentiate into spermatozoa
In the formation of the synaptonemal complex, the presynaptic alignment of a hoolog is followed by?
Synapsis
The first stage of the formation of the synaptonemal complex is called interphase I. What occurs in this phase?
The 2 paternal sister chromatids more closer to each other, as do the maternal sister chromastides
On average how may crossovers occur in meiosis?
Between two and three crossovers occur between each pair of human homologs
A fundamental difference between meiosis I and mitosis in that?
Meiosis I homologs, rather than sister chromatids, separate and then segregrate into two daughter cells.
The second division of meiosis is termed?
Meiosis II
After meiosis I what are the next steps of meiosis?
Meiosis II, prophase II, metaphase II, anaphase II, telophase II, and finally cytokinesis
Genes that regulate spermatogenesis are highly conserved and include?
The Daz genes which encode RNA-binding proteind located in a cluster on the Y chromosome
When binding the zona induces the sperm to undergo?
The acrosome reaction
In the formation of the synaptonemal complex, what occurs during synapsis?
The axial core of a homolog becomes tightly linked to the axial core of the partner by a closely packed array of transverse filaments to create a synaptonemal complex
what is a chiasma?
The connection between the homologs that can be seen microscopically at the end of prophase I
What happens in the acrosome reaction?
The contents of the acrosome are leased by exocytosis
Before the two gametes fuse at fertilization what kinds of randomizing genetic reassortment occur in meiosis I?
The random distribution of maternal and paternal homologs and crossing-over
Signals are required for female-specific organs to develop such as?
The secreted signal protein Wnt4 that is required for normal mammalian ovary development
The second stage of the formation of the synaptonemal complex is called leptotene phase. What occurs?
The sister chromatids become tightly juxtapsed due to binding of the chromatids to the axial core by the cohesion complex
primordial germ cells development depends on?
The suppresion of expression of somatic genes as well as the inhibition of translation of specific mRNAs by nanos RNA-binding proteins
The fifth and final stage of the formation of the synaptonemal complex is called diplotene phase. What occurs?
The synaptonemal complex disassembles
In egg cells what dictates which cells become primordial germ cells
signals from eighboring cells
Meiosis and sperm production begin when?
During puberty
The Sry gene encodes the Sry protein that does what?
Binds to DNA and influences the transcription of other genes involved in Sertoli cell development
When do sperm mature?
In the female genital tract
The first division of meiosis is termed?
Meiosis I
What are the progenitors of the gametes?
Primordial germ cells
Sperm are stored where?
The epididymis
Meiosis generates haploid cells that are genetically different from one another and from the two haploid cells that formed the organism in the first place. The genetic differences are by what mechanisms?
1 - An individual gamete contains either the maternal or paternal versions of each chromosome, because the choice of maternal or paternal occurs independently and randomly for each pair of homologs, the original maternal and paternal chromosomes are reshuffled into novel combinations in the haploid cell. 2 - Although the maternal and paternal versions of each chromosome have similar DNA sequences, they are not identical, and they undergo genetic recombination during meiosis - a process called crossing-over to produce novel hybrid versions of each chromosome.
What are three features of meiosis I that are distinct from mitosis?
1 - The kinetochores on the sister chromatids of a homolog attach to microtubules emanating from the same pole of the meiosis I spindle. The kinetochores segregate together into the same daughter call at anaphase I 2 - A trong physical linkage is maintained between the homologs that resists the pulling forces of the meiosis I spindle until the bivalents are aligned at the equator of the spindle and the homologs separate at anaphase I. 3 - The arms of the sister chromatids separate at anaphase I, resolving the chiasmata and allowing the homologs to separate
What are the two mechanisms that operate to ensure that only one sperm fertilizes an egg?
1 - a change in the egg plasma membrane caused by the fusion of the first sperm prevents other sperm from fusing 2 - The cortical reaction, which releases enzymes that change the structure of the zona pellucida so that sperm cannot penetrate it - these changes include the inactivation of ZP3 so that it can no longer bind sperm or induce the acrosome reaction
What happens at fertilization?
A haploid sperm fuses with a haploid egg to form a diploid cell, called the fertilized egg or zygote that contains a new combination of chromosomes distinct from either parent
A capacitated sperm must penetrate the layers of granulosa cells using?
A hyaluronidase enzyme on the surface of the sperm
When homologs are tightly connected along their entire length they are usually joined together by?
A protein complex called the synaptonemal complex
Many genes influence the development of the genital ridge into a gonad but what has an especially important role?
A single gene on the Y chromosome - Sry
Sperm capacitation is associated with?
An increase in cytosolic pH, thyrosin phosphorylation of various sperm proteins, and the unmasking of cell-surface receptor that help bind the sperm to the zona pellucida
Some developing follicles acquire a fluid-filled cavity (antrum) and become?
Antral follicles
Immature germ cells (spermatogonia) are located where?
Around the outer edge of the seminiferous tubules
Sexually reproducing organisms inherit two full sets of chromosomes, one from each parent. Each set contains?
Autosome, which are common to all members of the species, and sex chromosomes which are differently distributed according to the sex of the animal.
The cohesin complexes that assemble on DNA during the S phase of meiosis and bind sister chromatids together during meiosis are major components of the?
Axial core
Once a sperm finds an egg, it must first migrate through the layers of the granulosa cells that surround the egg and then do what?
Bind to and cross the zona pellucida and then bind to and fuse with the egg plasma membrane
In the formation of the synaptonemal complex, the recombination complex is proposed to bring homologs together this assembles on a ds-break in a chromatid, and does what?
Binds matching DNA sequence on the nearby homolog
How does ZP3 trigger the acrosome reaction?
By activating a lectin-like receptor on the sperm surface - receptor ativation leads to an increase in calcium in the sperm cytosol which initiates exocytosis
How does meiosis produce gametes with half as many chromosomes as their diploid precursor cells?
By modifying the mitotic program so that a single round of DNA replication is followed by two successive rounds of chromosome segregation
What aneuploid?
Cells with an abnormal number of chromosomes
What are euploid?
Cells with the correct number of chromosomes
What is the most common cause of spontaneous abortion?
Chromosome segregation errors during egg development
At the beginning of meiosis the chromosomes have replicated their DNA in meiotic S phase. The two copies are tightly bound together by?
Cohesin complexes along their entire length and are called sister chromatids
In human oocyte development, a single layer of follicle cells surrounds most of the primary oocytes in newborn girls, these are the primordial follicles. A small portion of these then begin to grow to become?
Developing follicles which have multiple layers of follicle cells (granulosa cells) surrounding the growing oocyte
Individual chromatids separate during what phase of meiosis?
Diakinesis phase
Cells in most animals usually proliferate in what phase to form a multicellular organism?
Diploid
The Y chromosome influences the sex of an individual by?
Directing somatic cells of the genital ridge to develop into a testis instead of an ovary
Replicated chromosomes undergo major rearrangements within the nucleus during prophase I. The chromosome ends (telomeres) are tightly bound to the inner surface of the nuclear envelope - initially distributed diffusely, followed by tight clustering. This is followed by?
Dispersion again which are thought to make prophase I faster and more efficient.
A small number of the spermatogonia serve as stem cells which do what?
Divide slowly by mitosis throughout life producing daughter cells that either remain stem cells or commit to maturation
Mitosis involves cell division in which both daughter cells are?
Each diploid
After fertilization the cells divide repeatedly to produce the?
Early embryo - certain cells inherit characteristics that allow them to become primordial germ cells
Sperm contributes centrioles during fertilization that do what?
Enter the egg along with the sperm nucleus and tail, and a centrosome forms around them
For sperm capacitation, the sperm undergoes?
Extensive biochemical and functional changes, including changes in glycoproteins, lipids, and ion channels in the sperm plasma membrane
If more than one sperm fuses to an egg what happens?
Extra mitotic spindles are formed, resulting in faulty segregation of the chromosomes during the first mitotic cell division and aneuploid cells are produced,
Mammalian embryos are programmed to develop into what sex?
Female unless prevented from doing so by the testes, which direct the embryo into a male
How do Sertoli cells influence development of the embryo?
First by inhibiting meiosis and development along the pathway to produce eggs, secretion of anti Mullerian hormone that suppresses development of female reproductive tract, stimulation of endothelial and smooth muscle cells in mesenchymal tissue to migrate to gonads, induction of other somatic cells in the the gonad to Leydig cells,
In the developing ovary wothout the influence of Sry, what develops?
Follicle cells as the supporting cells which will influence the sex hormone-producing cells to secrete estrogen
The two secondary spermatocytes proceed through meiosis II to produce?
Four spermatids
After primordial germ cells proliferate in the gonads they undergo meiosis and differentiate into mature haploid gametes and then what occurs?
Fusion of the egg and sperm that initiates embryogenesis
Most developing cells will not become?
Germ cells in the developing embryo
Unlike mitosis, meiosis has to produce gametes with?
Half as many chromosomes as their diploid precursor cells
What does retinoic acid do?
Induces the proliferating germ-line cells to enter meiosis and start the egg production development pathway
In animals, gametes or somatic cells fuse at fertilization to form a diploid zygotes which develops?
Into a new individual
What impact does crossing over have on genetic diversity?
It exponentially increases the genetic diversity generated in gametes as multiple crossover events occur between non-sister homologous chromatids
Why is the acrosome reaction required?
It exposes the hydrolytic enzymes that help sperm tunnel through the zona pellucida and it alters the sperm surface to allow it to bind and fuse with the plasma membrane of the egg
Haploid germ cells arise from a special kind of cell division in which the normal complement of chromosomes is precisely halved. This type of division is termed?
Meiosis
Once the cortical reaction has taken place what occurs in the egg?
Metaphase II resuems producing a second polar body and a zygote
What causes the increase in IP3 during sperm and egg fusion?
Most likely PLC-zeta that cleaves PIP2 to make IP3 and DAG
There is a cell cycle check point in sperm development that leads to cell death if meiosis goes wrong is this available in female meiosis?
No
In the formation of the synaptonemal complex, the paired homologs are brought into closer position with their structural axes by a mechanism that depends?
On the programmed ds-DNA breaks that occur in sister chromatids
During menstration the pituitary secretes a surge of follicle stimulating hormone which accelerates the growth of 10-12 antral follicles causing?
One follicle to become dominant
Germ cell determinants are found in?
P granules
During meiosis, each chromosome must communicate with its unique homologous partner by?
Physically pairing and undergoing genetic recombination.
What are the steps of egg development (through meiosis I)?
Promordial germ cells migrate to the gonad and become oogonia which proliferate by mitosis before entering meiosis I. The cells remain arrested after diplotene of prophase I - the primary oocytes synthesize a coat and cortical granules this is followed by oocyte maturation which occurs duing sexual maturity. The cells progress through meiosis I and the replicated homologous chromosomes segregate at anaphase I into two sets each containing half of the original number of chromosomes
The progeny of a single maturing spermatogonium do what?
Remain connected to one another by cytoplasmic bridges throughout differentiation
The zygote develops into a new multicellular organism through?
Repeated rounds of mitosis with subsequent cell specialization, including the production of gametes
The dominant primary oocyte completes meiosis I and then enlarges and?
Ruptures the surface of the ovary releasing the secondary oocyte triggering the complete of meiosis II if sperm fertilizes it within 1 to 2 days
Primary spermatocytes complete meiosis to become?
Secondary spermatocytes
In mitosis and meiosis II kinetochores on the two sister chromatids of a chromosome attach to opposite poles of the spindle and therefore?
Separate into different daughter cells at anaphase
Sry is expressed ina subpopulation of somatic cells of the developing gonad these cells differentiate into?
Sertoli cells which are the main supporting cells in the testis
Development of gonad into sperm or egg production is dependent on?
Signaling by retonic acid
What are the steps of egg development (through meiosis II)?
The cytoplasm of the cells divide asymmetrically to produce a small polar body and a large secondary oocyte. The cytoplasm of the large secondary oocyte divides symmetrically to produce the mature egg (ovum) and a second small polar body each with a haploid set of chromosomes. The polar bodies degenerate and oocyte maturation proceeds to metaphase of meiosis II where arrest occurs until ovulation when secondary oocytes are relased for fertilization and the cell then completes meiosis becoming the mature egg
During the diplotene phase unjoining (desynapsis) begins with?
The disassembly of the synaptonemal complex and the concomitant condensation and shortening of the chromosomes
What occurs in meiosis I?
The duplicated paternal and maternal homologs (including the two replicated sex chromosomes) pair up alongside each other and exchange genetic information through genetic recombination. They line up at the equator of the meiotic spindle after which the duplicated homologs, rather than sister chromatids, are pulled apart and segregated into two daughter cells.
After puberty sperm production is continuous in?
The epithelial lining of very long, tightly coiled tubes (seminiferous tubules
A form of sexual reproduction occurs in some simple eukaryotes. What occurs in this reproduction?
The fission of yeast occurs as the haploid cells proliferate. During mating, they fuse to form a diploid zygote, which exists only transiently after mating. The diploid zygote undergoes meiosis to form new haploid cells which then continue to proliferate under mitosis, forming new haploid organisms
The primordial germ cells migrate to where to develop?
The gonads that form the ovaries and the testes
What is the full process of fertilization?
The haploid eff pronucelus and haploid sperm pronucleus are separate units within the egg cytoplasm. The nuclear envelopes of the haploid pronuclei interdigitate. The chromosomes have already duplicated and the centrosome is replicated. The nuclear envelopes of both pronuclei breakdown and the chromosomes of the egg and sperm align on a single metaphase spindle prior to mitosis. This is followed by cell division to produce two diploid daughter cells.
What occurs during prophase I in meiosis?
The homologs begin to associate along their length. Stable pairing leads to genetic recombination between the homologs. The homologs become more juxtaposed, forming a four-chromatid structure.
During mitosis, homologous chromosomes behave independently of each other. In contract during meiosis I what occurs?
The homologs recognize each other and associate physically for the maternal and paternal homologs to undergo genetic recombination
In meiosis, the first event is the replication of cellular DNA. What phase is this?
The meiotic S phase
The third stage of the formation of the synaptonemal complex is called zygotene phase. What occurs?
The paternal and maternal sister chromatid homolods are brought closer to each other by the assembly of the transverse filaments
The fourthstage of the formation of the synaptonemal complex is called pachytene phase. What occurs?
The transverse filament array is complete and the homologs are held tightly together and crossing over occurs
In fertilized mammalian eggs, the two pronuclei do not fuse directly what happens instead?
They approach each other but remain distinct until after the membranes of each pronucleus has broken down in preparation for the zygote's first mitotic division.
Specialized cells involved in sexual reproduction are haploid which means?
They contain only a single set of chromosomes
What occurs during meiosis II?
This stage occurs without further DNA replication. The sister chromatids are pulled apart and segregated to produce haploid daughter cells at anaphase II
During prophase I only two types of sperm are normally produced which are?
Those containing one Y chromosome, which will give rise to male embryos, and those containing one X chromosome which will give rise to female embryos
The meiotic S phase results in the generation of?
Two copies of each paternal and maternal chromosome homolog
Fertilization is not complete until what happens?
Two haploid nuclei (called pronuclei) one from the egg and one from the sperm have come together and combined their chromosomes into a single diploid nucleus.
The primary spermatocytes complete meiosis I to produce?
Two secondary spermatocytes each containing 22 duplicate autosomal chromosomes and either duplicated X or duplicated Y chromosomes
What is the cortical reaction?
When the sperm fuses to the egg, cortical granules from the sperm are released by exocytosis
The development of the primordial germ cells into eggs of sperm is determined by?
Whether the genital ridge is in the process of developing into an ovary or testis
The sex chromosomes in the somatic cells of the genital ridge determine?
Which type of gonad the genital ridge will become
Male mammals have what type of chromosome which are not homologous to each other but must pair and undergo crossing-over during prophase I if they are to segregate normally at anaphase I?
X and Y chromosomes
Females mammals have two what chromosomes that pair and segregate like other homologs during prophase I?
X chromosomes
The binding of sperm to the zona pellucida is complex and involves both?
ZP3-dependent and ZP3-independent mechanisms, in addition to proteins on the sperm surface
During menstruation the dominant primary oocyte completes meiosis I and the secondary oocyte?
arrests at metaphase II
When the sperm fuses with the egg plasma membrane is causes a local increase in cytosolic calcium which causes?
calcium channels to open allowing for more calcium to enter the cytosol and the continuation of the fertilization
Genetic recombination begins during pairing in early prophase I with production of programmed double-strand breaks in chromatid DNA - some of these recombinations will later resolve into?
crossovers, where a fragment of maternal chromatid is exchanged for a corresponding fragment of homologous paternal chromatid
In mammalian femlaes oocytes begin meiosis in the fetal ovary and arrest after?
diplotene after the synaptonemal complex has disassembled in meiosis I and meiosis I only completes after the female has become sexually mature and the oocyte is released from the ovary
Sry gene also encodes the Sox9 protein that does what?
directly activates the transcription of some Sertoli cell-specific genes, including the gene encoding the anti-mullerian hormone
The proteins that form the transverse filaments that bridge between the axial cores of the homologs form?
homodimers that interact with each other
Sexual reproduction occurs in diploid organisms which means?
each cell contains two sets of chromosomes, one inherited from each parent
Sexual reproduction requires a specialized type of cell division called meiosis- during meiosis, a diploid precursor cell gives rise to?
haploid progeny cells
In humans the centrosome duplicates and the two resulting centrosomes do what?
help to organize the assembly of the first mitotic spindle in the zygote
The two copies of each autosome, one from the mother and fater, are called?
homologous chromosomes or homologs
How does the process of mitosis differ from meiosis?
homologs do not pair up and the sister chromatids separate during the single division. Each diploid cell that divides by mitosis produces two genetically identical diploid daughter cells
Calcium increase during fusing of the sperm and egg is caused by?
increases in IP3 which releases calcium from the endoplasmic reticiulum and initiates the calcium wave and oscillations
Capacitation alters two crucial aspects of sperm behavior which are?
it greatly increases motility of the flagellum, and it makes the sperm capable of undergoing the acrosome reaction (binding and fusing with the ovum)
Most of the spermatogonia divide a liminited number of times by mitosis before they sopt proliferating and begin what called what?
meiosis I called primary spermatocytes
Proteins that form transverse filaments are important for crossing over because?
mutations that result in the absence of expression of these proteins fail to form crossovers
The specialized haploid cells generated during meiosis are termed gametes - in animals, these are?
the egg (ova) and sperm (spermatozoa)
