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passage 4
P1
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”
Contrast species ·
Different species' embryos use different processes to establish polarity
P2
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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
Contrasting example ·
Nematode
Unlike fruit flies, nematode eggs don’t have all the polarity information they need; sperm also provides information
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
P3
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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
23.
According to the passage, polarity ██ ███████████ ██ █ █████ ██████
a
after more stages ██ ████ ████████ ████ ██ █████
b
before the sperm ██████ ███ ███
c
after positional information ██ ████████ ██ ███ ███████ ██ ██████████
d
by the same ████████████ █████████ ██ ██ ███ ████████
e
in the same ███ ██ ██ ███████ ███████████