This is an explanation for passage 1 of LSAT preptest 70, the October 2013 LSAT. This passage is about pathogens, and how prions appear to be a new type of pathogen. Prions are special because they lack genetic material.
This section has paragraph summaries and an analysis of the passage, links to the explanations for the questions are below.
- Scientists used to assume that pathogens had DNA in their cell structure.
- CJD is caused by prions. Prions are a pathogen composed mostly of protein. They have no nucleic acid or genetic material in their cell structure.
- First half of 3rd paragraph: Prions are normal cells. But they can go weird, become dangerous, and replicate themselves. This replication creates a plaque that kills nerve cells.
Second half of 3rd paragraph: The immune system can’t fight prions, as they are normal cells. So there’s no way to stop prions, and CJD is fatal.
- Scientists were skeptical of prions, but now they accept that prions cause CJD and maybe other diseases. But we don’t really understand the mechanisms by which prions replicate and destroy cells.
This is a very dense passage. If you dislike science then you probably weren’t happy to see this first.
If you find scientific language off-putting, I recommend checking out ~20 back issues of the Economist from the library. Each issue has about three pages of well written science articles. Reading them is an excellent way to become familiar with scientific language and concepts.
Another good idea is to reread any sections of the passage that you don’t understand. I’ve timed students, and they greatly overestimate how much time it takes to reread a paragraph. When you read something twice you understand it much better and you can go faster on the questions.
Prions Are Pathogens Without DNA
The gist of this passage is that prions are a new type of pathogen. Pathogen just means something that causes disease.
We used to think that all pathogens had DNA (also known as nucleic acid). But prions don’t have DNA (second-to-last sentence of 2nd paragraph). This is the most important fact in the passage, so I’ll repeat it: Prions lack DNA.
Prions sound pretty nasty. They’re ordinary proteins in your body, so your immune system won’t attack them (second-to-last sentence of 3rd paragraph). Unfortunately, sometimes prions go wonky and get a strange shape (first sentence of 1st paragraph).
When prions take this new shape, they start reproducing. Scientists aren’t really sure how prions reproduce (last sentence of 4th paragraph), but the result is quite deadly.
The immune system can’t stop the prions, and we haven’t found therapies to stop them, either. So prions just keep reproducing, and eventually kill you (last sentence of 3rd paragraph).
CJD is the main disease mentioned in the passage, and it’s caused by prions. When prions in the brain reproduce, they create a plaque that kills nerve cells (second-to-last sentence of 3rd paragraph).
Prions Probably Aren’t Contagious
A few answer choices talk about prions being contagious. This is a red herring. The passage never says if prions can spread from person to person. The second sentence of 3rd paragraph mentions that prions are “infectious”, but this just means that prions spread quickly within your body. If prions spread to someone else, that person’s immune system would likely attack the prions, because immune systems attack things that come from outside the body.
It’s important to note that we’re not sure about much. The theory of prions as disease agents is fairly well supported (paragraph 4), but we don’t fully understand prions (paragraph 4). We’re not even 100% certain that prions cause CJD. The passage is not clear on this point, but the first sentence of 1st paragraph say that the prion theory has merely challenged conventional wisdom. Researchers haven’t yet definitively proven that conventional wisdom is wrong.
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