PrepTest 121, Section 3, Question 23
The first thing any embryo must do before(467 more words not shown)
According to the passage, polarity(6 more words not shown)
- Aafter more stages(6 more words not shown)
- Bbefore the sperm(3 more words not shown)
- Cafter positional information(7 more words not shown)
- Dby the same(6 more words not shown)
- Ein the same(5 more words not shown)
Reading the passage
We don't know exactly how human polarity is established, but we do know something about its timing. Human embryos go through many stages of cell division without apparent asymmetries, and polarity develops later. The contrast is with the sperm-driven process discussed for frogs.
Working the answers
- ACorrect
After describing the frog mechanism, the writer says mammalian polarity develops “much later,” with many stages of cell division first. That supports more stages before polarity is established in humans than in frogs.
- B
Before sperm entry is the fruit fly's timing. Human polarity is described as developing after multiple stages of cell division.
- C
The gathering of p-granules is described for nematodes. The writer doesn't identify it as the mechanism in humans.
- D
The sperm-driven mechanism is apparently not the mammalian one. Calling human polarity an unresolved mystery doesn't permit us to substitute the nematode's explanation.
- E
This likewise assigns humans the simpler-vertebrate mechanism that the writer distinguishes from mammalian development. Unknown doesn't mean the same.
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