Self-study
An organism is considered to have an infection when a disease-causing agent, called a pathogen, establishes a viable presence in the organism. This can occur only if the pathogenic agent is able to reproduce itself in the host organism. The only agents believed until recently to be responsible for infections—viruses, bacteria, fungi, and parasites—reproduce and regulate their other life processes by means of genetic material, composed of nucleic acid (DNA or RNA). It was thus widely assumed that all pathogens contain such genetic material in their cellular structure.
This assumption has been challenged, however, by scientists seeking to identify the pathogen that causes Creutzfeldt-Jakob disease (CJD), a degenerative form of dementia in humans. CJD causes the brain to become riddled with tiny holes, like a sponge (evidence of extensive nerve cell death). Its symptoms include impaired muscle control, loss of mental acuity, memory loss, and chronic insomnia. Extensive experiments aimed at identifying the pathogen responsible for CJD have led surprisingly to the isolation of a disease agent lacking nucleic acid and consisting mainly, if not exclusively, of protein. Researchers coined the term "prion" for this new type of protein pathogen.
Upon further study, scientists discovered that prions normally exist as harmless cellular proteins in many of the body's tissues, including white blood cells and nerve cells in the brain; however, they possess the capability of converting their structures into a dangerous abnormal shape. Prions exhibiting this abnormal conformation were found to have infectious properties and the ability to reproduce themselves in an unexpected way, by initiating a chain reaction that induces normally shaped prions to transform themselves on contact, one after another, into the abnormal, pathogenic conformation. This cascade of transformations produces a plaque, consisting of thread-like structures, that collects in the brain and ultimately destroys nerve cells. Because prions, unlike other pathogens, occur naturally in the body as proteins, the body does not produce an immune response when they are present. And in the absence of any effective therapy for preventing the cascade process by which affected prions reproduce themselves, CJD is inevitably fatal, though there are wide variations in pre-symptomatic incubation times and in how aggressively the disease progresses.
Although the discovery of the link between prions and CJD was initially received with great skepticism in the scientific community, subsequent research has supported the conclusion that prions are an entirely new class of infectious pathogens. Furthermore, it is now believed that a similar process of protein malformation may be involved in other, more common degenerative neurological conditions such as Alzheimer's disease and Parkinson's disease. This possibility has yet to be fully explored, however, and the exact mechanisms by which prions reproduce themselves and cause cellular destruction have yet to be completely understood.
1.
Which one of the following most accurately expresses the main point of the passage?
a
Although most organisms are known to produce several kinds of proteins, the mechanism by which isolated protein molecules such as prions reproduce themselves is not yet known in detail.
Although the author does not at the end that we don’t have an exact understanding of prions yet, this isn’t the main point, because it doesn’t capture the view that prions are a new kind of pathogen without DNA. The point isn’t merely to describe the limits of our knowledge; it’s to present what we do know about prions.
b
Research into the cause of CJD has uncovered a deadly class of protein pathogens uniquely capable of reproducing themselves without genetic material.
This best captures the main point. The author describes how CJD research discovered prions in P2, describes what we know about how prions destroy nerve cells in P3, and describes further support for the “conclusion” that prions are a new kind of pathogen in P4.
c
Recent research suggests that prions may be responsible not only for CJD, but for most other degenerative neurological conditions as well.
The author doesn’t suggest that prions may be responsible for “most” other degenerative neurological conditions. She merely indicates that it may be involved in at least some other common degenerative neurological conditions. Since (C) is not supported, it can’t be the main point. In any case, the main point isn’t about what else prions might cause, but on prions’ status as a new kind of pathogen.
d
The assertion that prions cause CJD has been received with great skepticism in the scientific community because it undermines a firmly entrenched view about the nature of pathogens.
This doesn’t capture the view that there is evidence prions are a new class of pathogens. Although this view “was initially received with great skepticism,” the author indicates that there’s support for the view. The main point is focused on the support for the view, not on the fact there’s skepticism.
e
Even though prions contain no genetic material, it has become clear that they are somehow capable of reproducing themselves.
This is too narrow. The author describes prion replication in P3 as part of support for the view that prions are a new kind of pathogen that can replicate without DNA.