PrepTest 121 - Section 3 - Passage 4 - Question 26

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The first thing any embryo must do before it can develop into an organism is establish early polarity—that is, it must set up a way to distinguish its top from its bottom and its back from its front. ███

Intro topic · Embryo polarity

How an embryo develops different “sides”

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Contrast species · Different species' embryos use different processes to establish polarity

Example of polarity process · Fruit fly

Fruit fly eggs already have all the information needed for polarity even before fertilization; “fertilize-and-go” approach

Consequences · of fruit fly polarity process

Tradeoff: eggs take a long time to “set up” ahead of time, but embryo grows very quickly

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Contrasting example · Nematode

Unlike fruit flies, nematode eggs don’t have all the polarity information they need; sperm also provides information

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Elaborate on example · Details of process in nematode egg

When sperm enters egg, proteins collect on one side of egg, signaling front/back

Similarity/contrast · Nematode-like process used in some vertebrates (frogs) but not others (humans)

Continue contrast · Mammal embryos establish polarity much later

But underlying process is unknown

New contrast · between establishing polarity and growing specific body parts

Species differ widely in how they signal polarity in the embryo, yet they’re very similar in how they signal specific body parts to grow

Main point · Developmental mechanisms differ between species early on, but are similar later in development

Passage Style

Single position

Spotlight

26.

According to the passage, which ███ ██ ███ █████████ ██ █ █████ ██████████ ███████ ███ █████████████ ██ ████████ ██ ███ █████ ███ ███ ██ ███ █████████

Question Type

Stated

In P2 the author tells us that one major difference between polarity in the fruit fly and the nematode is that in the fruit fly the information necessary for polarity is already contained in the cytoplasm of the egg before fertilization, whereas for the nematode the information occurs after the sperm enters the egg.

The fruit fly ██████ █████ ██████ ██ █████████ ████████ ████ ████ ███ ████████ ███████

This is anti-supported. Polarity is established after the sperm enters the egg for the nematode, whereas polarity information is already contained in the cytoplasm of an egg before fertilization in the fruit fly.

The mechanisms that █████████ ████████ ███ ████ ██████ ████████████ ██ ███ ████████ ████ ██ ███ █████ ████

Not supported, because the author never compares the ease of identification of polarity mechanisms in the nematode and fruit fly.

Polarity signals for ███ █████ ███ ██████ ███ █████████ ████████ ██ ███ ███ ███ █████ ███████ ███ ███ ████████ ██████ ███ █████████ ████████ ██ ███ ██████

Not supported, because the author doesn’t say that the information for polarity in the nematode is inscribed “entirely” in the sperm. Rather, some information is connected to the point at which the sperm enters the egg.

17%

Polarity in the █████ ███ █████ ████ ██████ ██ ████ ████████ ██ ██████ ███████████ ████ ██ ███ █████████

Not supported, because the author doesn’t comment on stages of cell division in connection with fruit fly and nematode polarity development.

Polarity is established ███ ███ █████ ███ ██████ █████████████ ███ ███ ███ ████████ ███████ ██████████████

Supported by the beginning of P2.

74%

PrepTest 121 - Section 3 - Passage 4 - Question 26

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