#27 - Must Be True, Purpose/Function

[mN2mmvf]

Hi,

Can you help me understand why (B) is the correct answer? In the passage, the writer discusses out the "sets of essential genes...are remarkably similar among all life forms" that lead to the establishment of polarity, but the expressions of this differ by organism. Answer choice (B), however, says that the "genetic mechanisms used are disparate" and notes that the disparity is surprising.

How are the genetic mechanisms disparate? The same set of genes control the process in every organism; only the results differ, not the mechanism (i.e., DNA). I thought that (D) was a better choice, because it acknowledged that the same genetic process occurs in all life forms but with different results.

Thanks!

[AthenaDalton]

Great question!

The disparity in how polarity develops is surprising since there is so much similarity in how embryos of all species develop, except for the very early polarity stage. Starting at line 45, the author tells us that after polarity has been established, there are "remarkably similar" sets of genes to develop eyes, brains, and extremities. So flies, mice, and humans all develop eyes the same way, but don't establish polarity the same way. This is odd since it's an early phase of development where (one would think) inter-species differences would be minimal as compared to later stages.

Answer choice (D) isn't as good of an answer because the author briefly addresses the fact that the process of establishing polarity does occur in all lifeforms in lines 1-8, but then spends the rest of the article talking about the differences in how polarity is established. Answer choice (D) describes only a part of the author's argument, not his main point.

I hope this helps clear things up. Good luck studying!

Athena

[mN2mmvf]

Thanks Athena. I guess I meant not so much why the disparity is surprising (I get that)...it's more that, why can the genetic mechanisms used be fairly called "disparate"? The mechanisms (DNA) seem to be the same.

[AthenaDalton]

Hi mN2,

The passage gives a few examples of mechanisms which differ from each other, and speaks about each "mechanism" as being distinct from the others:

Lines 5-10: "The mechanisms that establish the spatial configurations in an embryo are far less similar across life forms ... the signals the fruit fly uses to know its front end from its back end turn out to be radically different from those than the nematode .... and both appear to be quite different from the polarity signals in the development of humans."

Lines 15-20: Fruit flies establish polarity through signals inscribed in the cytoplasm of the egg

Lines 23-30: Nematodes establish polarity when bundles of proteins cluster at one end of the yolk, which are then separated after it divides

Lines 39-45: Human embryos establish polarity at a much later stage than other embryos, and the mechanism behind this is unknown

Since the author treats these as separate mechanisms, we should take him at his word and consider them distinct for the purposes of this passage.

Best,

Athena

[mN2mmvf]

Thanks Athena; I missed those references. Makes sense.

[afulbright.2000@gmail.com]

Can you explain why answer choice "C" is wrong?

I can understand how "B" is correct and that it encompasses the entirety/primary purpose of the passage best.

However, I had selected answer choice "C" because its specific mention of polarity seemed more appealing than "C"s broader wording and I felt it still highlighted the main purpose of the passage. Perhaps a better reason why "C" is not the best answer choice is because of the word "classification" utilized in the verbiage? Perhaps the comparisons that the author of the passage was making are not necessarily qualified as classifications?

[afulbright.2000@gmail.com]

Can you explain why answer choice "C" is wrong?

I can understand how "B" is correct and that it encompasses the entirety/primary purpose of the passage best.

However, I had selected answer choice "C" because its specific mention of polarity seemed more appealing than "C"s broader wording and I felt it still highlighted the main purpose of the passage. Perhaps a better reason why "C" is not the best answer choice is because of the word "classification" utilized in the verbiage? Perhaps the comparisons that the author of the passage was making are not necessarily qualified as classifications?

*Sorry for the typo:
However, I had selected answer choice "C" because its specific mention of polarity seemed more appealing than "B"s broader wording and I felt it still highlighted the main purpose of the passage.

[Robert Carroll]

afulbright,

I think the very breadth of answer choice (B) is a good thing about that answer. Answer choice (C) is not correct because it gives a very misleading impression of what the passage is doing. The passage is doing both more and less than classifying the different mechanisms by which embryos establish early polarity. More, because the author is bringing up different mechanisms in order to point out how strange it is that organisms that will end up extremely different will also start out with extreme different mechanisms (so far, this isn't strange!) but, in between, will use very similar mechanisms for elaborating parts. If organisms that ended up very different had different mechanisms throughout, that would make some sense, but, no, the early mechanisms, when all lifeforms are pretty rudimentary, are the ones that are different, and the later mechanisms, when lifeforms are already starting to become different from each other, are similar! That's odd. Answer choice (B) gets at that, while answer choice (C) is too neutral in its description, giving no idea there's anything unusual about the situation.

I also said that the passage is doing less than answer choice (C) says it is. The passage isn't trying for a classification of early polarity mechanisms at all. It brings up some examples and never claims that it's classifying the whole field.

For these reasons, answer choice (C) doesn't express the main point of the passage.

Robert Carroll