Passage 36
Protein synthesis begins when the gene1 encoding aprotein is activated2. The genes sequence of nucleotides istranscribed3 into a molecule4 of messenger RNA （mRNA），which reproduces the information contained in that（5） sequence. Transported outside the nucleus5 to the cyto-plasm，the mRNA is translated into the protein itencodes by an organelle known as a ribosome， which strings6 together amino acids in the order specified7 by thesequence of elements in the mRNA molecule. Since the（10）amount of mRNA in a cell determines the amount of thecorresponding protein， factors affecting the abundance of mRNAs play a major part in the normal functioningof a cell by appropriately regulating protein synthesis.For example， an excess of certain proteins can cause cells（15） to proliferate8 abnormally and become cancerous； a lack of the protein insulin results in diabetes9.
Biologists once assumed that the variable rates at which cells synthesize different mRNAs determine the quantities of mRNAs and their corresponding proteins（20） in a cell. However， recent investigations11 have shown that the concentrations of most mRNAs correlate best， not with their synthesis rate， but rather with the equally vari-able rates at which cells degrade the different mRNAs in their cyTOPlasm. If a cell degrades both a rapidly and（25） a slowly synthesized mRNA slowly， both mRNAs will accumulate to high levels.
An important example of this phenomenon is the development of red blood cells from their unspecialized parent cells in bone marrow13. For red blood cells to accu-（30）mulate sufficient concentrations of hemoglobin （which transports oxygen） to carry out their main function， the cells parent cells must simultaneously14 produce more of the constituent15 proteins of hemoglobin and less of most other proteins. To do this， the parent cells halt synthesis（35） of nonhemoglobin mRNAs in the nucleus and rapidly degrade copies of the nonhemoglobin mRNAs remaining in the cyTOPlasm. Halting synthesis of mRNA alone would not affect the quantities of proteins synthesized by the mRNAs still existing in the cyTOPlasm. Biologists now（40） believe that most cells can regulate protein production most efficiently16 by varying both mRNA synthesis and degradation17， as developing red cells do， rather than by just varying one or the other.
1. The passage is primarily concerned with discussing the
（A） influence of mRNA concentrations on the development of red blood cells
（B） role of the synthesis and degradation of mRNA in cell functioning
（C） mechanism18 by which genes19 are transcribed into mRNA
（D） differences in mRNA concentrations in cell nuclei20 and in the cyTOPlasm
（E） way in which mRNA synthesis contributes to the onset21 of diabetes
2. The passage suggests that a biologist who held the viewdescribed in the first sentence of the second paragraphwould most probably also have believed which of thefollowing？
（A） The rate of degradation of specific mRNAs has little effect on protein concentrations.
（B） The rate of degradation of specific mRNAs should be studied intensively.
（C） The rates of synthesis and degradation for any given mRNA are normally equal.
（D） Different mRNAs undergo degradation at widely varying rates.
（E） Most mRNAs degrade very rpaidly.
3. Which of the following best describes the relationshipbetween the second and third paragraphs of the passage？
（A） The second paragraph presents arguments in support of a new theory and the third paragraph presents arguments against that same theory.
（B） The second paragraph describes a traditional view and the third paragraph describes the view that has replaced it on the basis of recent investigations.
（C） The third paragraph describes a specific case of a phenomenon that is described generally in the second paragraph.
（D） The third paragraph describes an investigation10 that was undertaken to resolve problems raised by phenomena22 described in the second paragraph.
（E） Both paragraphs describe in detail specific examples of the phenomenon that is introduced in the first paragraph.
4. The accumulation of concentrations of hemoglobin inred blood cells is mentioned in the passage as anexample of which of the following？
（A） The effectiveness of simultaneous variation of the rates of synthesis and degradation of mRNA
（B） The role of the ribosome in enabling a parent cell to develop properly into a more specialized12 form
（C） The importance of activating23 the genes for particular proteins at the correct moment
（D） The abnormal proliferation of a protein that threatens to make the cell cancerous
（E） The kind of evidence that biologists relied on for support of a view of mRNA synthesis that is now considered obsolete24
5. To begin to control a disease caused by a proteindeficiency， the passage suggests that a promising25experimental treatment would be to administer a drugthat would reduce
（A） only the degradation rate for the mRNA of the protein involved
（B） only the synthesis rate for the mRNA of the protein involved
（C） both the synthesis and degradation rates for the mRNA of the protein involved
（D） the incidence of errors in the transcription of mRNAs from genetic26 nucleotide sequences
（E） the rate of activity of ribosomes in the cyTOPlasm of most cells
6. According to the passage， which of the following best describes the current view on the relationship betweenthe synthesis and the degradation of mRNA inregulating protein synthesis？
（A） Biologists have recently become convinced that the ribosome controls the rates of synthesis and degradation of mRNA.
（B） There is no consensus27 among biologists as to the significance of mRNA degradation in regulating protein synthesis.
（C） The concept of mRNA degradation is so new that most biologists still believe that the vital role in protein regulation belongs to mRNA synthesis.
（D） Degradation of mRNA is now considered to be the key process and mRNA synthesis is no longer believed to play a significant role.
（E） Degradation of mRNA is now considered to be as important as mRNA synthesis has been， and still is，believed to be.
7. According to the passage， which of the following canhappen when protein synthesis is not appropriatelyregulated？
（A） Diabetes can result from errors that occur when the ribosomes translate mRNA into protein.
（B） Cancer can result from an excess of certain proteins and diabetes can result from an insulin deficiency.
（C） A deficiency of red blood cells can occur if bone marrow cells produce too much hemoglobin.
（D） Cancer can be caused by excessively rapid degradation of certain amino acids in the cyTOPlasm of cells.
（E） Excessive synthesis of one protein can trigger increased degradation of mRNAs for other proteinsand create severe protein imbalances.
8. The passage suggests that a biologist who detected high levels of two proteins in a certain type of cell would be likely to consider which of the following as a possibleexplanation？
（A） The rate of mRNA degradation for one of the proteins increases as this type of cell develops a more specialized function.
（B） The two proteins are most likely constituents28 of a complex substance supporting the cells specialized function.
（C） The cells are likely to proliferate abnormally and possibly become cancerous due to the levels of these proteins.
（D） The mRNAs for both proteins are being degraded at a low rate in that type of cell.
（E） The mRNAs for the two proteins are being synthesized at identical rates in that type of cell.
