Flashcards in Meiosis, Germ Cells And Fertilization Deck (88):
How many sets of chromosomes are in a haploid cell?
What are gametes?
Specialized reproductive cells.
They are either sperm or egg generated through meiosis.
What is fertilization?
When haploid gametes fuse to form a diploid cell (zygote).
What is a zygote?
A fertilized diploid cell that divides by mitosis to become a multicellular organism.
What are germline cells?
Gametes and their precursors.
What are somatic cells?
Cells from the rest of the body (excluding germline cells) that leave no progeny.
What are autosomes?
Chromosomes common to both sexes; there is one from each parent in each diploid nucleus.
What are the sex chromosomes?
X and Y
How does meiosis create genetic diversity?
From random segregation of homologs during meiosis
From crossing over
What is the end product of meiosis?
One round of DNA synthesis that produces half the number of chromosomes.
When do chromosomes replicate?
In the S phase
How are two sister chromatids abound?
By cohesin complexes
When do homologs pair, recombine and separate?
In meiosis I.
What occurs during meiosis II?
Sister chromatids are divided to produce 4 daughter haploid cells.
When do homologs begin to pair?
During prophase I.
This can last days/weeks.
What is the term for a 4 chromatid structure?
What does the term "chiasma" refer to?
Crossoing over of chromosomes.
Homologs are joint together by what protein structure?
What is the pseodoautosomal region?
A small region of homology between X and Y. It allows the two chromosomes to pair.
What is the first stage of prophase?
This is when homologs begin to condense and pair.
What are the five phases of prophase?
What occurs during zygotene?
Homologs pair and synaptonemal complexes form.
What occurs during pachytene?
What occurs during diplotene?
Synaptonemal complex begins to break down; homologs begin to separate, but remain attached at the chiasmata.
What happens during diakinesis?
Maximum condensation is reached.
Homologs separate and transition into metaphase.
When do cohesin complexes assemble on DNA?
During S phase.
They bind sister chromatids.
What role do cohesin complexes play in meiosis I?
They play an important role in segregating homologs in meiosis I.
Kinetochores on two sister chromatids attach to microtubules coming from the ___ pole.
Kinetochores on two sister chromatids attach to microtubules coming from the SAME pole.
When do the arms of sister chromatids separate?
During anaphase I.
This resolves the chiasmata and allows homologs to separate.
When do sister chromatids fully separate?
During anaphase II.
It occurs when separase cleaves cohesin complexes.
You are examining an oocyte from a female donor and note that the cell is frozen in meiosis. Meiosis in oocytes is arrested for years after what stage of prophase?
When is meiosis I completed in human female oocytes?
Only at ovulation
When is meiosis II completed in female oocytes?
What is nondisjunction?
When homologs fail to separate properly.
Trisomy 21/Down syndrome is an example of an aneuploid disorder. What does the term "aneuploid" mean?
An aneuploid is a cell with abnormal chromosome number.
What is the chromosome count in euploid cells?
Euploid cells have normal numbers of chromosomes.
What are two ways in which genetic diversity can be generated?
Random distribution of maternal/paternal homologs
Variation increased by crossing over
When does meiosis of male gametes begin?
What is the percentage of eggs and sperm that are aneuploid?
20% of eggs
3 - 4 % of sperm
25% of all human conceptions are aneuploid. What is this mainly due to?
Nondisjuction in oocytes at meiosis I
Are male or female gametes more prone to new DNA mutations?
They are more prone as a result of undergoing more mitotic cell divisions.
What are primordial germ cells?
Cells that give rise to gametes.
They migrate to developing gonads.
How do primordial germ cells (PGCs) proliferate?
Through mitosis and then meiosis in order to differentiate into mature haploid gametes (sperm/egg).
In mammals, what dictates whether a cell will become a PGC or not?
Neighboring cell signaling.
In mammals, a dividing egg is totipotent for the first few rounds of cell division. What does it then differentiate into?
Are PGCs totipotent or pluripotent?
They can be removed and cultured with signaling proteins to form cell lines for any cell type for the body except extraembronic cells.
What determines if a gonad is an ovary or testes?
Sex chromosomes in the genital ridge
What determines the sex of an embryo?
The presence or absence of Y determines the sex of an individual.
Y directs the genital ridge to develop into testis.
What gene is necessary for sufficient testis development in mammals?
You do genetic testing on a mouse and determine that has XX sex chromosomes, as well as active expression of the Sry gene. What is the gender of the mouse?
You conduct a genetic test on a mouse. You determine that the mouse has XY sex chromosomes and also has a mutation that inactivates the Sry gene. What is the gender of the mouse?
Expression of Sry causes cells to differentiate into Sertoli cells. What are Sertoli cells?
Testis support cells
What are two functions of the Sry gene?
They cause cells to differentiate into sertoli cells.
They also encode a DNA binding protein that regulates transcription of genes involved in Sertoli development.
What is the function of Sox9?
It activates Sertoli-specific genes, including anti-Mullerian hormone, which prevents development of females.
You conduct a genetic test on a mouse and discover that the mouse has expression of Sox9, but no expression of Sry. What is the gender of the mouse?
Sry gene induces Leydig cell differentiation in somatic cells. What is the function of Leydig cells?
They secrete testosterone, which is responsible for secondary sexual characteristics.
What occurs to a developing organism in the absence of Sry?
Genital ridge becomes an ovary
PGC becoms an egg
Somatic cells differentiate into
Follice cells and theca cells, which are estrogen-producing cells.
Why can mammals not undergo parthogenic activation (activation in the absence of sperm?)
Due to imprinting. Mammals need two sets of chromosmes in order to develop.
Where is the nutritional reserve found in eggs?
In the yolk. It is rich in lipids, proteins and polysaccharides.
What is the function of the egg coat?
It protects the egg from mechanical damage and acts as a species-specific barrier to sperm (zona pellucida).
When an egg is activated, specialized secretory cells called corticle granules release contents. What is the function of the contents?
They enter an egg coat and block polyspermy (more than one sperm fertilizing an egg).
What are the stages of oogenesis?
PGCs migrate to the gonad
Dpiloid oogonia under mitotic and then meiotic cell divisions.
Primary oocytes arrest at prophase I
Cytoplasm divides assymetrically to produce polar body and secondary oocyte.
Secondary oocyte is arrested in metaphase II and completes meiosis only after fertilization.
Why do eggs have such a large size?
They have extra gene copies in the cell.
They also have yolk proteins, accessory cells, and follicle cells.
What is a primordial follicle?
The most primitive follicle. It is an oocyte surrounded by a single layer of follicle cells.
What are follicle cells?
Cells that form the outer layer of an oocyte.
What does a developing follicle look like?
It has multiple layers of follicle cells surrounding a growing oocyte.
What is antrum?
A fluid-filled cavity found in antral follicles during puberty.
Its creation is initiated by FSH.
What hormones trigger ovulation and the primary oocyte's completion of meiosis I?
FSH and LH
A secondary oocyte is arrested in what stage of cell division?
What two hormones trigger maturation and the rupture of the secondary oocyte from the dominant atntral follicle?
FSH and LH
What is contained in the acrosomal vesicle of a sperm?
Enzymes to help the sperm fertilize an egg.
When does meiosis of sperm begin?
Where does spermatogenesis occur?
In the seminiferous tubules.
They maintain close contact with Sertoli cells here.
True or false: the progeny of spermatogonium do not complete cytokinesis.
They remain connected to cytoplasm bridges in order to have access to the diploid genome for their development.
What are the requirements for fertilization?
Albumin (helps extract cholesterol from the membrane, increasing ability to fuse with acrosome membrane)
Ca 2+ and H2O (they activate cAMP)
CAMP (helps to initiation capacitation-associated changes).
What enzyme do sperm cells use to penetrate granulosa cells?
What occurs once sperm binds to the zona pellucida?
The sperm is altered so it can bind and fuse with the plasma membrane of an egg.
What membrane proteins are crucial to binding?
ZP1 (cross links filaments)
ZP2, 3 (form long filaments)
What does egg fusion from a sperm induce?
A cortical reaction in which the contents of granules are released. Meiosis resumes in egg.
What triggers fusion of an egg and sperm?
Increased Ca 2+ concentration in the cytosol.
True or false: fusion of sperm causese change in the egg plasma membrane so other sperm cannot fuse.
This is known as inhibition of polyspermy.
Which membrane protein is inactivated during egg activation?
ZP3. This prevents sperm binding to an egg or an induction of another acrosome reaction.
ZP2 is also cleaved so that the zona pellucida is impenetrable.
After fertilization, what do sperm contribute to that are not found in the eggs?
Sperm contribute to centrosomes and centrioles.
What is one consequence of polyspermy?
Extra mitotic spindles are present in the egg. This leads to faulty segregation of chromosomes and anueploidy.
What is a risk involved in in vitro fertilization?
What is involved in ICSI?
Egg is injected with a single sperm.
How have clones from many tpes of malles been made?
By transferring the nucleus of a somatic cell to an unfertilized egg lacking a nucleus.