PrepTest 121, Section 3, Question 26
The first thing any embryo must do before(467 more words not shown)
According to the passage, which(21 more words not shown)
- AThe fruit fly(11 more words not shown)
- BThe mechanisms that(14 more words not shown)
- CPolarity signals for(23 more words not shown)
- DPolarity in the(15 more words not shown)
- EPolarity is established(12 more words not shown)
Reading the passage
In the fruit fly, the positional information is already in the egg before fertilization. In the nematode, the place where sperm enters supplies a crucial cue, followed by changes in the distribution of p-granules. Keep that contrast in mind while checking the details each choice adds.
Working the answers
- A
We aren't given comparable durations for establishing polarity in the two organisms. The fly's one-day development into a larva isn't a measurement of how long its polarity takes to form.
- B
Both mechanisms are described. Nothing compares how easy they are to identify.
- C
The fly half fits, but the nematode half goes too far. A signal from the point of sperm entry doesn't mean all positional information is inscribed in the sperm. The process also involves p-granules already distributed through the egg's cytoplasm.
- D
No comparison of the number of cell divisions needed in these two organisms is stated.
- ECorrect
The fruit fly's polarity signals are in place “before fertilization.” For the nematode, sperm entry provides crucial positional information, so fertilization plays a role in establishing polarity. That is the timing-and-trigger difference we predicted.
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