這篇雅思閱讀素材是要跟大家分享關(guān)于器官移植的話題。器官移植可以延長人類壽命，然而目前可用的器官卻越來越難得到?？茖W家們認為，豬的器官可以移植到人類，但隨之而來的是細胞病毒問題。而這個問題，現(xiàn)在似乎正在得到解決。或許在不遠的將來，人類有望解決移植中器官供應(yīng)不足的問題了。一起看看文章怎么說的吧！

Transplanting(移植)organs brings life to the dying. But most donor organs are harvested from the dead. Shortfalls in the number of volunteer donors, the difficulty of gaining the consent of grieving relatives, and a reduction in most countries of the rate of fatal road accidents (the most reliable source of healthy organs), mean that there is a constant lack of them. Thousands die each year while on waiting lists for transplants. Researchers have, therefore, long sought ways to boost supply.

One idea is to harvest animal organs. That is less mad than it sounds. A liver, a kidney(腎臟) or a cornea(眼角膜) does the same job, regardless of species(物種). And it works. In 1984 an American child lived for three weeks after receiving a baboon(狒狒) heart intended as a stopgap(臨時替代) until a human donor could be found (unfortunately, one was not found in time). Conversely(相反的), human organs have been transplanted into animals for the purpose of research. Earlier this year, for example, a paper in the American Journal of Transplantation described moving kidneys from human fetuses(胎兒) into rats.

Until now, though, two technical problems have stood in the way of (妨礙)routinely transplanting animal organs into people. One is that the recipient’s immune system (免疫系統(tǒng))must be persuaded to tolerate a big chunk of (一大塊)foreign tissue(人或動植物身體的組織). The other is that swapping(交換) tissues between species risks swapping diseases, too. This second problem may soon be addressed(解決), if George Church of the Harvard Medical School has his way. For, as he and his colleagues describe this week in Science, genetic engineering can now be used to eliminate(消滅)one of the most worrying types of pathogen (病菌)that might be spread via transplants.

The animal most commonly suggested as a donor is the pig. Pigs are roughly the size of human beings. They are reasonably well understood. And millennia (幾千年)of experience mean they are easy to breed. But they are not perfect. In particular, their DNA is full of retroviruses(反轉(zhuǎn)錄病毒，一種致腫瘤病毒), known specifically as porcine endogenous retroviruses(內(nèi)源性反轉(zhuǎn)錄病毒), or PERVS. The genes of these viruses hitch a lift(搭順風車) from one pig generation to another as an integral part of the porcine genome, whence they can break out and cause infection. And tests in laboratories suggest that, given the opportunity, they can infect human cells as well. (尤其是，豬的DNA中攜帶大量反轉(zhuǎn)錄病毒，確切些是豬的內(nèi)源性反轉(zhuǎn)錄病毒(PERVS)。攜帶這些病毒的基因隨著遺傳基因組在豬群中代代相傳，在傳遞過程中就會引起感染。實驗研究表明，只要給機會，這些病毒也可感染人體細胞。)

The existence of PERVs, then, has been one of the main obstacles to transplanting pig organs into people.

Dr Church and his colleagues thought PERVs ideal candidates to test the mettle(勇氣;氣概)of one of the rising stars of biotechnology(生物技術(shù)), CRISPR/Cas9. This is a gene-editing technique derived from bacteria, which use it as a sort of immune system. In nature, it recognises specific sequences of viral DNA and chops the DNA molecule (分子) apart at these points, protecting the bacterium from harm. Tweaked (稍微調(diào)整)a bit in the laboratory, it can be made to recognise any DNA sequence and do likewise. This permits specific stretches of DNA to be deleted(刪除) from genomes, and also allows new stretches to be inserted into the gap thus created. (CRISPR/Cas9能識別攜帶病毒的DNA序列，在感染病毒的地方將DNA分子與病毒DNA分開以防止病毒的侵害。通過實驗稍做調(diào)整后，CRISPR/Cas9就能用于識別各種DNA序列，將特定的DNA序列從基因組中刪除，并把新的DNA序列植入由此帶來的空缺中。)

Dr Church and his fellow researchers analysed the genetic sequences of one family of PERVs,with a view to (旨在)attacking them with CRISPR/Cas9. They found that the sequence of the gene which lets the virus integrate itself into its host’s DNA is the same from one strain of virus to another. That allowed them to program a CRISPR/Cas9 system to look for this particular sequence and chop it out of the genome. (丘奇博士和他的同事們研究了一種PERVS基因序列，旨在用CRISPR/Cas9來攻擊這些序列。他們發(fā)現(xiàn)，能讓PERVS與主體的DNA結(jié)合的基因序列與使一類病毒與主體的DNA結(jié)合的基因序列是一樣的。所以他們能夠設(shè)計出一類CRISPR/Cas9 來識別這個特定的病毒基因序列并把它從基因組中去掉。)

單詞記憶法之“滾動記憶法”

小編并不建議拿出大塊時間集中背單詞，因為那種方式效率并不高。根據(jù)艾賓浩斯遺忘曲線，隨著時間的推移人們之前記住的東西會不斷喪失，因此為了防止背過的單詞被遺忘，同學們可以每天利用不同的時段反復記憶單詞。

建議考雅思的同學們每天分4個時段背單詞，每次背單詞的時間不超過1個小時，具體的操作方法以每天背100個單詞為例。

第1個時段可以選取在早上8：00-9：00之間：同學們可以在早上8點-9點的時候“刷”一遍，目的在于區(qū)分認識和不認識的單詞，并且把不認識的單詞進行第一次概念性的記憶。

第2個時段可以選取在中午13：00-14：00之間：在第2次背單詞的過程中，重點針對的是第一遍篩選出來的不認識的單詞，對這些單詞進行深度記憶。

第3個時段可以選取在下午18：00-19：00之間：在第3次背單詞的過程中，我們對于仍然不認識的單詞進行重點記憶，對剛剛背下來的單詞進行鞏固記憶。

第4個時段可以選取在晚上11：00-12：00之間：在第4次背單詞的過程中，主要查缺補漏，把白天背過的單詞進行最后一遍記憶，把仍然沒有背下來的單詞繼續(xù)背誦，并且做好重點標記。

最后，請大家務(wù)必注意一點：

100個單詞通過一天4個時段的背誦之后，并沒有結(jié)束。何為滾動?就是在第二天背另外100個新單詞時，請務(wù)必把前一天沒有背下來的舊單詞加入到第二天的新單詞當中，進行滾動記憶，直到完全背下來為止，這種“滾動記憶”的方法可以使單詞像滾雪球一樣越背越多，而且多次重復不易遺忘，最終實現(xiàn)單詞的穩(wěn)步提升。