2013年雅思考试阅读模拟练习及答案五.docVIP

2013年雅思考试阅读模拟练习及答案五 Standard museum practice can wash away DNA. 1. Washing， brushing and varnishing fossils — all standard conservation treatments used by many fossil hunters and museum curators alike — vastly reduces the chances of recovering ancient DNA. 2. Instead， excavators should be handling at least some of their bounty with gloves， and freezing samples as they are found， dirt and all， concludes a paper in the Proceedings of the National Academy of Sciences today. 3. Although many palaeontologists know anecdotally that this is the best way to up the odds of extracting good DNA， Eva-Maria Geigl of the Jacques Monod Institute in Paris， France， and her colleagues have now shown just how important conservation practices can be. This information， they say， needs to be hammered home among the people who are actually out in the field digging up bones. 4. Geigl and her colleagues looked at 3，200-year-old fossil bones belonging to a single individual of an extinct cattle species， called an aurochs. The fossils were dug up at a site in France at two different times — either in 1947， and stored in a museum collection， or in 2004， and conserved in sterile conditions at -20 oC. 5. The teams attempts to extract DNA from the 1947 bones all failed. The newly excavated fossils， however， all yielded DNA. 6. Because the bones had been buried for the same amount of time， and in the same conditions， the conservation method had to be to blame says Geigl. “As much DNA was degraded in these 57 years as in the 3，200 years before，” she says. Wash in， wash out 7. Because many palaeontologists base their work on the shape of fossils alone， their methods of conservation are not designed to preserve DNA， Geigl explains. 8. The biggest problem is how they are cleaned. Fossils are often washed together on-site in a large bath， which can allow water — and contaminants in the form of contemporary DNA — to permeate into the porous bones. “Not only is the authentic DNA getting washed