Viruses, infectious particles consisting of nucleic acid packaged in a protein coat (the capsid), are difficult to resist. Unable to reproduce outside a living cell, viruses reproduce only by subverting the genetic mechanisms of a host cell. In one kind of viral life cycle, the virus first binds to the cell’s surface, then penetrates the cell and sheds its capsid. The exposed viral nucleic acid produces new viruses from the contents of the cell. Finally, the cell releases the viral progeny, and a new cell cycle of infection begins. The human body responds to a viral infection by producing antibodies: complex, highly specific proteins that selectively bind to foreign molecules such as viruses. An antibody can either interfere with a virus’s ability to bind to a cell, or can prevent it from releasing its nucleic acid.
　　Unfortunately, the common cold, produced most often by rhinoviruses, is intractable to antiviral defense. Humans have difficulty resisting colds because rhinoviruses are so diverse, including at least 100 strains. The strains differ most in the molecular structure of the proteins in their capsids. Since disease-fighting antibodies bind to the capsid, an antibody developed to protect against one rhinovirus strain is useless against other strains. Different antibodies must be produced for each strain.
　　A defense against rhinoviruses might nonetheless succeed by exploiting hidden similarities among the rhinovirus strains. For example, most rhinovirus strains bind to the same kind of molecule (delta-receptors) on a cell’s surface when they attack human cells. Colonno, taking advantage of these common receptors, devised a strategy for blocking the attachment of rhinoviruses to their appropriate receptors. Rather than fruitlessly searching for an antibody that would bind to all rhinoviruses, Colonno realized that an antibody binding to the common receptors of a human cell would prevent rhinoviruses from initiating an infection. Because human cells normally do not develop antibodies to components of their own cells, Colonno injected human cells into mice, which did produce an antibody to the common receptor. In isolated human cells, this antibody proved to be extraordinarily effective at thwarting the rhinovirus. Moreover, when the antibody was given to chimpanzees, it inhibited rhinoviral growth, and in humans it lessened both the severity and duration of cold symptoms.
　　Another possible defense against rhinoviruses was proposed by Rossman, who described rhinoviruses’ detailed molecular structure. Rossman showed that protein sequences common to all rhinovirus strains lie at the base of a deep “canyon” scoring each face of the capsid. The narrow opening of this canyon possibly prevents the relatively large antibody molecules from binding to the common sequence, but smaller molecules might reach it. Among these smaller, nonantibody molecules, some might bind to the common sequence, lock the nucleic acid in its coat, and thereby prevent the virus from reproducing.
　　21. The primary purpose of the passage is to
　　(A) discuss viral mechanisms and possible ways of circumventing certain kinds of those mechanisms
　　(B) challenge recent research on how rhinoviruses bind to receptors on the surfaces of cells
　　(C) suggest future research on rhinoviral growth in chimpanzees
　　(D) defend a controversial research program whose purpose is to discover the molecular structure of rhinovirus capsids
　　(E) evaluate a dispute between advocates of two theories about the rhinovirus life cycle
　　22. It can be inferred from the passage that the protein sequences of the capsid that vary most among strains of rhinovirus are those
　　(A) at the base of the “canyon”
　　(B) outside of the “canyon”
　　(C) responsible for producing nucleic acid
　　(D) responsible for preventing the formation of delta-receptors
　　(E) preventing the capsid from releasing its nucleic acid
　　23. It can be inferred from the passage that a cell lacking delta-receptors will be
　　(A) unable to prevent the rhinoviral nucleic acid from shedding its capsid
　　(B) defenseless against most strains of rhinovirus
　　(C) unable to release the viral progeny it develops after infection
　　(D) protected from new infections by antibodies to the rhinovirus
　　(E) resistant to infection by most strains of rhinovirus
　　24. Which of the following research strategies for developing a defense against the common cold would the author be likely to find most promising?
　　(A) Continuing to look for a general antirhinoviral antibody
　　(B) Searching for common cell-surface receptors in humans and mice
　　(C) Continuing to look for similarities among the various strains of rhinovirus
　　(D) Discovering how the human body produces antibodies in response to a rhinoviral infection
　　(E) Determining the detailed molecular structure of the nucleic acid of a rhinovirus

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