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  3. Nearly six years after the sequence of the human genome was sketched out, one might assume that researchers had worked out what

Nearly six years after the sequence of the human genome was sketched out, one might assume that researchers had worked out what

admin2012-07-11  37
问题     Nearly six years after the sequence of the human genome was sketched out, one might assume that researchers had worked out what all that DNA means. But a new investigation has left them wondering just how similar one person’s genome is to another’s.
    Geneticists have generally assumed that your string of DNA "letters" is 99. 9%identical to that of your neighbor’s, with differences in the odd individual letter. These differences make each person genetically unique — influencing everything from appearance and personality to susceptibility to disease.
    But hold on, say the authors of a new study published in Nature. They have identified surprisingly large chunks of the genome that can differ dramatically from one person to the next. "Everyone has a unique pattern," says one of the lead authors Matthew Hurles at the Wellcome Trust Sanger Institute in Cambridge, U.K.
    The differences in question — made up of stretches of DNA that span tens to hundreds of thousands of chemical letters — are called "copy-number variants", or CNVs. Within a given stretch of DNA, one person may carry one copy of a DNA segment; another may have two, three or more. The region might be completely absent from a third person’s genome. And sometimes the segments are shuffled up in different ways.
    These variable regions received short shrift (承认) for many years. When the human genome sequence was pieced together, they were largely glossed over, because researchers were focused on finding one overarching reference sequence — and because the repetitive nature of the segments makes them hard to sequence. "It was swept under the rug," says Michael Wigler, who is also mapping CNVs at Cold Spring Harbor Laboratory, New York.
    The new study, led by Hurles and Stephen Scherer of the Hospital for Sick Children in Toronto, Canada, and their colleagues is the most detailed attempt to find how CNVs are scattered across the whole human genome. To do this, they compared genome chunks from 270 people of European, African or Asian ancestry. They found nearly 1,500 such regions, taking up some 12%of the human genome. That doesn’t mean that your DNA is 12%different from mine (or 88%similar), because any two people’s DNA will differ at only a handful of these spots.
    According to the team’s back-of-the-envelope calculations, one person’s DNA is probably 99. 5%similar to their neighbor’s, or a bit less. "I’ve tried to do the calculation and it’s very complicated," says Hurles. "It all depends on how you do the accounting."
    The answer is also unclear because researchers think that there are many more variable blocks of sequence that are 10,000 or 1,000 letters long and were excluded from the current study.
According to the passage, CNVs refer to______.
选项 A、the differences in human genome
B、stretches of DNA
C、different chemical letters
D、the copy numbers of variants
答案C
解析 文章第四段首句的主干为The differences are called“copy-number variants”，or CNVs(这些差异叫作密码变体，也称CNVs)。[A]意为“人类基因的差异”，与原文意思相符，故为答案。第四段首句还提到，这种差异(The differences)是由DNA的延伸组成的，而非这种延伸本身，故[B]错误。
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