Term
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Definition
| describes an egg that has been fertilized by more than one sperm |
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Term
| What are polyspermic zygotes? |
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Definition
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Term
| What happens when more than one sperm feltilizes an egg? |
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Definition
| The sperms have multiple centrioles and cleave aberrantly at the first division, leading to embryonic death. |
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Term
| How does the egg stop polyspermy form occuring? |
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Definition
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Term
| What types of blocking polyspermy do sea urchins have? |
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Definition
| the fast block and the slow block. |
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Term
| Explain the fast block to polyspermy in the sea urchin. |
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Definition
1. There is a changing electic potential of the egg membrane
2. This is caused by an influx of Na2+ ions into the egg form the environment
3. The membrane potential changes from -70mV to +20mV
4. This is transient and only lasts a minute or so |
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Term
| Explain the slow block to polyspermy in the sea urchin. |
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Definition
1. There is an increase in intracellular Ca2+ conc. form calcium stores within the egg
2. This stimulates the cortical granules to bind to the cell membrane
3. They release proteases that cleave protein bridges between the vitelline membrane and the plasma membrane
4. This forms a fertilization membrane and hyaline layer where the cortical granules were.
4. This is the cortical granule reaction |
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Term
| How does the MPF arrest the cell cycle- how does it continue? |
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Definition
1. Maturation promoting factor (MPF) phosphorylates a number of protein targets by the kinase activity.
2. For the meiosis to continue the cell must lower the levels of MPF
3. The calcium wave results in the activation of CaMK2 that degrades the MPF, allowing the cell to continue meiosis.
4. Levels of MPF rise and fall before each successive cell division |
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Term
| What are the main features of sea urchin fertilization? |
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Definition
Jelly coat
Vitelline membrane
Acrosomal reaction at the jelly coat
Highly-species specific
Acrosomal process
Sperm tip contacts egg
Egg haploid at time of fertilization |
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Term
| What are the main features of mammal fertilization? |
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Definition
Ovarian follicular cells (oocyte/granulosa/thecal)
Zona pelludica
Acrosomal reaction at zona pelludica
Species specifity?
No acrosomal process
Side of sperm head contacts egg membrane
Egg arrested in metaphaseII of meiosis |
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Term
| Explain the general outline of cleavage. |
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Definition
A series of mitotic divisions transform the egg cytoplasm into many small nucleated cells called blastomeres.
Cleavage divisions have a unique cell cycle.
Divisions reslut in the progressive reduction of cytoplasmic volume.
The rates of cell division are under control of mRNAs and proteins in the oocyte (ex. mammals).
This is very fast. |
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Term
| What are the differences between early blastomere and somatic cell mitosis? |
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Definition
Somatic cells have G1 and G2 phases to grow.
They also have a rest phase which is G0.
Early blastomeres only have M (mitosis) and S (DNA synthesis) phases with no growth phases.
This is due to spacial restrictions. |
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Term
| How does the nuclear/cytoplasmic ratios vary between somatic cell division and blastomere cell divisions? |
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Definition
Somatic cells- Ratio remains constant
Blastomere cell- Nuclea/cytoplasmic ration increases with each cell division. |
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Term
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Definition
The division of the nucleus.
Mechanical agent: Mitotic spindle (determines position of two daughter blastomeres)- produces mictotubules made of tubulin. |
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Term
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Definition
The division of the cell cytoplasm.
Mechanical agent: Contractile ring (determines cytoplasm distribution) produces microfilaments made of actin.
Produces cleavage furrow. |
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Term
| How are the mechanical agents of cytokinesis and karyokinesis related? |
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Definition
| The mitotic spindle and the contractile ring are perpendicular to one another. |
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Term
| Describe the basic structure of the centriole. |
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Definition
A pair of centrioles in centre (L-shaped).
Encased by centriosome matrix with nucleating sites (y-tubulin ring complexes or holes in the walls.
Microtubules grow from the nucleating sites. |
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Term
| Explain how the distribution of yolk is a main parameter affecting cleavage patterns. |
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Definition
It determines where cleavage should occur and the relative size of the blastomeres.
Yolk generally inhibits cleavage.
If yolk distribution is asymetric, the egg pole rich in yolk is the vegetal pole and the region low in yolk is the animal pole. |
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Term
| Explain how specific factors in the egg cytoplasm can be a main parameter affecting cleavage. |
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Definition
| Influence the position and angle of the mitotic spindle and the timing of formation. |
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Term
| When yolk distribution is sparce and even, what patterns are produced? |
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Definition
| Isolecithal: Radial cleavage, spiral cleavage, bilateral cleavage and rotational cleavage. |
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Term
| When there is moderate vegetal yolk disposition, what cleavage patterns are produced? |
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Definition
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Term
| When there is dense yolk throughout most of the cell, what types of cleavage patterns are produced? |
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Definition
| Telolecithal: Bilateral cleavage, Discoidal cleavage. |
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Term
| When there is yolk in the centre of the egg, what cleavage patterns are produced? |
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Definition
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Term
| What phyla have radial cleavage? |
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Definition
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Term
| What phyla have spiral cleavage? |
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Definition
| Annelids, molluscs, flatworms |
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Term
| What phyla have isolecithal bilateral cleavage? |
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Definition
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Term
| What phyla have rotational cleavage? |
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Definition
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Term
| What phyla have mesolecithal cleavage? |
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Definition
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Term
| What phyla have telolecithal bilateral cleavage? |
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Definition
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Term
| What phyla have discoidal cleavage? |
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Definition
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Term
| Which phyla have centrolecithal cleavage? |
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Definition
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Term
What are the mechanisms underlying the asymetric division in C. elegans?
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Definition
- Sperm pronucleus at posterior end- egg pronucleus at anterior end. Underneath cell membrane (near cortical granules) PAR-3 is present and MEX-5 is even;y spread throughout the cell.
- Asters bind to the sperm pronucleus and the membrane- spreading out along it and causing a wave of PAR-2 to displace PAR-3 (up untill 2/3 of the cell)
- MEX-5 now forms a sharp gradient across this 2/3 line- with none in the posterior end
- The egg pronucleus moves to the middle of the cell and then the sperm and undergo asymetrical mitotic division
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