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With James Watson and Francis Crick's landmark article on the double helical structure of DNA now more than a half-century old, the sheer volume of knowledge we have since amassed regarding the regulation and expression of genetic material is staggering, and continues to expand daily. Yet for all that has been accomplished in05 the study of genetics, there comes now and again a discovery to underscore just how many mysteries we have yet to unravel. Historically, we have defined a gene as a region of DNA responsible for encoding and regulating the expression of a discrete, heritable trait. The use of?regulating?here is of no small importance, as the protein-coding sequence itself represents only a10 fraction of the DNA contained within a given gene. A "promoter" region, for instance, does not directly contribute to the mRNA transcript, but instead provides binding sites for transcription factor proteins, and functions as sort of an "on" or "off" switch for the expression of the gene's corresponding trait. Similarly, "silencer" and "enhancer" regions can also bind regulatory proteins, and help to fine-tune the precise degree to15 which a gene will be expressed under various circumstances and in response to varying stimuli. However, perhaps the most implicitly fascinating non-coding regions of DNA are those embedded within the protein-coding region itself. During transcription, nucleotides are polymerized into a strand of mRNA whose sequence is complementary to that of the template DNA. This "pre-mRNA" typically20 contains several regions of non-coding material, or "introns," that must be excised prior to translation of the protein-coding regions, which are referred to as "exons." In a complex process known as?splicing,?the introns are removed and degraded, while the adjacent ends of exons are adjoined, and trafficked out of the nucleus to the endoplasmic reticulum, where protein synthesis can at last begin.25 Predictably, mutations that affect a gene's splicing pattern may precipitate severe functional impairments to its encoded protein, and some studies have estimated that as many as half of all disease-causing mutations in humans—including those responsible for Alzheimer's disease, Parkinson's disease, and certain forms of cystic fibrosis— are ultimately a result of altered splicing. Furthermore, an increasing body of research30 has also reported patterns of altered splicing in a wide variety of cancer cells, though it remains to be seen as to whether these changes contribute to oncogenesis, or are simply symptomatic of dysregulated growth. With such grim potential for genetic misstep, one might wonder how evolution could have ever favored the development of such a precarious and seemingly superfluous35 system of gene expression in the first place. The answer to this lies in the fact that alternative splicing patterns are not exclusively pathological, but can and do occur under physiological circumstances as well. That is to say, through tightly controlled changes to the differential removal of introns and retention of exons, two identical strands of pre-mRNA can, ultimately, code for two entirely different proteins.40 Calcitonin gene-related peptide, or CGRP, was among the first proteins identified as a product of physiological alternative splicing. Whereas calcitonin is a well-known hormone produced by the medullary cells of the thyroid gland, and is involved in the regulation of calcium levels in the blood, CGRP is believed to mediate pain sensations within central and peripheral neurons. Despite their unique structures and vastly differing45 functions, both proteins are nonetheless encoded by the same gene. The discovery of physiological alternative splicing came as a challenge to our traditional understanding of genes, which held that each coding region was responsible for the expression of a single protein. Today, of course, we know this line of thought to be an elegant but erroneous oversimplification. Scientists have demonstrated that50 the vast majority of animal genes participate in alternative splicing to one extent or another; far from a mere biochemical curiosity, it is a vital biological strategy to maximize the economy of genetic material, which must be laboriously reproduced with each cell division, while maintaining an immense diversity in the protein-encoding capacity of a genome. In an extreme example, the genome of the insect species55 Drosophila melanogaster contains about 15,000 genes. Yet, through alternative splicing, one single D melanogaster gene—known as DSCAM—has been shown to encode about 38,000 different proteins.
The table illustrates the corresponding exons for the pre-mRNA transcipt and nine alternative splicing isoforms of the the α-tropomyosin gene.
mRNA Splicing Isoforms of α-tropomyosin
1. What is the overall purpose of this passage?
2. The first paragraph of the essay (lines 1-6) primarily serves to
3. As used in line 3, the word "amassed" most closely means
4. As used in line 20, the word "excised" most closely means
5. Lines 29-32 ("Furthermore . . . growth.") most strongly suggest that the author of the passage believes that scientific thinking regarding the contribution of altered splicing to cancer is
6. Based on lines 54-57, which expression gives the most likely range of values for the total number of proteins in a Drosophila melanogaster genome?
7. Which of the following statements accurately describes the relationship between the pre-mRNA and all the other isoforms portrayed in the table?
8. Which option gives the best evidence for the answer to the previous question?
9. Scientists who thought along the lines outlined in lines 49-54 ("Scientists . . . genome") would most likely have what opinion about the information in the given table?
10. Based on the passage, what would someone who believed in the first historical theory of DNA most likely think would be the number of unique protein transcripts that would result from the replication of the α-tropomyosin gene outlined in the table?
11. Which option gives the best evidence for the answer to the previous question?
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