zoo 16th lecture part 17th

zoo 16th lecture part 17th - Week Four: October 22 26...

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Week Four: October 22 – 26 Defining Zygote Polarity - The first polar body forms – where this happens is the animal pole. - 180° opposite of this is the vegetal pole Activation of Protein Synthesis - Recall the Na:H antiport: as one sodium comes in, one hydrogen comes out – this causes a pH shift (6.8 to about 7.8) - Very shortly after fertilization, there is a burst of protein synthesis. This occurs before the first cleavage, but continues for a very considerable amount of time. - The red graph line represents the incorporation of 14 C valine in protein. There is a peak within about 5 hours after fertilization. It levels off at 5 hours and decreases a little bit. Then there is another burst at a much later time point. This is a special point where this new burst is initiated. - Protein synthesis appears to be strictly the result of proteins being translated. No or little new RNA is being made. It is relying on the stores of internal mRNA. This mRNA is probably masked until this point. - How do we know that no new RNA is being produced? Actinomycin D treated with seawater – protein synthesis still continues to come to a peak at about 5 hours. After this time, there is a rapid decrease in protein synthesis. Actinomycin D blocks transcription – the peak at 5 hours shows that translation is using stored mRNA. - Other animal systems may be producing RNA earlier than in the sea urchins.
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Cytoplasmic redistributions – amphibians - Example – certain types of molecules are distributed at specific ends of the cytoplasm (syncytial) - Prior to fertilization, amphibians have a very heavily pigmented cytoplasm in the animal hemisphere and a clear cytoplasm in the vegetal hemisphere. Sperm entry occurs relatively 30° above the equator in the animal hemisphere for these species. The sperm sets of a sperm aster made of microtubules. These microtubules interact with the cortical cytoplasm and shift it around the egg slightly so that a window opens up that reveals a grey cytoplasm (grey crescent). This section above the equator but part of the grey crescent will also be the site of the future dorsal lip of the blastopore. Cytoplasmic rearrangement in the Tunicate egg - Prior to fertilization, there is a chorion. The oocyte nucleus is actual a germinal vesicle. The sperm enters in the vegetal pole in this instance. The yellow cytoplasm is going to reconfigure itself as well. The polar body is going to be extruded from the material from the oocyte nucleus area (animal pole). Styela
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- Microtubule driven male pronucleus migration - The endpoint is where we have the wedge of yellow, clear, light grey, and dark grey cytoplasm. - If we treat with colchicine – inhibits microtubules, redistribution is blocked - If we treat with cytochalasin b – nothing happens, only inhibits actin - The yellow cytoplasm forms all mesoderm. The clear cytoplasm is going to become neural ectoderm. The light grey cytoplasm becomes epidermal ectoderm. The dark grey cytoplasm becomes mostly endoderm. -
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zoo 16th lecture part 17th - Week Four: October 22 26...

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