Nearly a century ago, biologists found that if they separated an invertebrate animal embryo into two parts at an early...
GMAT Reading Comprehension : (RC) Questions
Nearly a century ago, biologists found that if they separated an invertebrate animal embryo into two parts at an early stage of its life, it would survive and develop as two normal embryos. This led them to believe that the cells in the early embryo are undetermined in the sense that each cell has the potential to develop in a variety of different ways. Later biologists found that the situation was not so simple. It matters in which plane the embryo is cut. If it is cut in a plane different from the one used by the early investigators, it will not form two whole embryos.
A debate arose over what exactly was happening. Which embryo cells are determined, just when do they become irreversibly committed to their fates, and what are the "morphogenetic determinants" that tell a cell what to become? But the debate could not be resolved because no one was able to ask the crucial questions in a form in which they could be pursued productively. Recent discoveries in molecular biology, however, have opened up prospects for a resolution of the debate. Now investigators think they know at least some of the molecules that act as morphogenetic determinants in early development. They have been able to show that, in a sense, cell determination begins even before an egg is fertilized.
Studying sea urchins, biologist Paul Gross found that an unfertilized egg contains substances that function as morphogenetic determinants. They are located in the cytoplasm of the egg cell; i.e., in that part of the cell's protoplasm that lies outside of the nucleus. In the unfertilized egg, the substances are inactive and are not distributed homogeneously. When the egg is fertilized, the substances become active and, presumably, govern the behavior of the genes they interact with. Since the substances are unevenly distributed in the egg, when the fertilized egg divides, the resulting cells are different from the start and so can be qualitatively different in their own gene activity.
The substances that Gross studied are maternal messenger RNA's—products of certain of the maternal genes. He and other biologists studying a wide variety of organisms have found that these particular RNA's direct, in large part, the synthesis of histones, a class of proteins that bind to DNA. Once synthesized, the histones move into the cell nucleus, where section of DNA wrap around them to form a structure that resembles beads, or knots, on a string. The beads are DNA segments wrapped around the histones; the string is the intervening DNA. And it is the structure of these beaded DNA strings that guide the fate of the cells in which they are located.
The passage is most probably directed at which kind of audience?
1. Passage Analysis:
Progressive Passage Analysis
| Text from Passage | Analysis |
|---|---|
| Nearly a century ago, biologists found that if they separated an invertebrate animal embryo into two parts at an early stage of its life, it would survive and develop as two normal embryos. | What it says: In the 1920s, scientists cut early-stage animal embryos in half and got two normal animals instead of one. What it does: Opens with a historical experiment that establishes our starting point Source/Type: Historical fact about scientific discovery Connection to Previous Sentences: This is our foundation - no prior sentences to connect to Visualization: Imagine cutting a tiny developing sea urchin embryo in half → Result: two complete sea urchins instead of two broken pieces Reading Strategy Insight: This is a clean, simple opening. Authors often start RC passages with concrete examples before diving into complexity. What We Know So Far: Early embryos can be split to make two normal animals What We Don't Know Yet: Why this works, what determines cell fate |
2. Passage Summary:
Author's Purpose:
To explain how recent molecular biology discoveries have finally solved a century-old debate about what controls cell development in embryos.
Main Point:
Cell fate in developing embryos is controlled by a chain reaction that starts before fertilization - maternal RNA molecules are spread unevenly in eggs, they make proteins that wrap around DNA differently in different cells, and these different wrapping patterns determine whether a cell becomes part of the heart, brain, or other body parts.
Why It's Wrong:
- The passage focuses on scientific discovery rather than funding considerations
- No discussion of costs, benefits, or practical applications that legislators would need
- Far too technical and detailed for a legislative audience making budget decisions
Why It's Wrong:
- Too basic and explanatory for experts who specialize in molecular genetics
- Specialists wouldn't need definitions of terms like "cytoplasm" or analogies like "beads on a string"
- Covers broad historical context rather than cutting-edge research details that specialists require
Why It's Wrong:
- Alumni newsletters typically feature lighter, more personal content about the institution
- This passage is too technical and educational for a general alumni audience
- No connection to Paul Gross's college or institutional affiliation mentioned
Why It's Wrong:
- Marine biologists studying speciation would focus on evolutionary processes, not embryonic cell determination
- The passage emphasizes molecular mechanisms rather than species formation
- While sea urchins are mentioned, they're used as a research model, not as the focus of marine biology
Why It's Right:
- The explanatory tone and systematic progression from basic to complex concepts matches undergraduate education
- Technical terms are defined and explained, appropriate for students learning molecular biology
- Comprehensive historical context helps students understand how scientific knowledge develops
- The "beads on a string" analogy and other clarifications indicate an educational purpose