青岛李沧区雅思考试课程培训基地

这篇雅思阅读素材是要跟大家分享关于器官移植的话题。器官移植可以延长人类寿命，然而目前可用的器官却越来越难得到。科学家们认为，猪的器官可以移植到人类，但随之而来的是细胞病毒问题。而这个问题，现在似乎正在得到解决。或许在不远的将来，人类有望解决移植中器官供应不足的问题了。一起看看文章怎么说的吧！

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(肾脏) or a cornea(眼角膜) does the same job, regardless of species(物种). And it works. In 1984 an American child lived for three weeks after receiving a baboon(狒狒) heart intended as a stopgap(临时替代) 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(胎儿) into rats.

Until now, though, two technical problems have stood in the way of (妨碍)routinely transplanting animal organs into people. One is that the recipient’s immune system (免疫系统)must be persuaded to tolerate a big chunk of (一大块)foreign tissue(人或动植物身体的组织). The other is that swapping(交换) tissues between species risks swapping diseases, too. This second problem may soon be addressed(解决), 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(消灭)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 (几千年)of experience mean they are easy to breed. But they are not perfect. In particular, their DNA is full of retroviruses(反转录病毒，一种致肿瘤病毒), known specifically as porcine endogenous retroviruses(内源性反转录病毒), or PERVS. The genes of these viruses hitch a lift(搭顺风车) 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，猪的DNA中携带大量反转录病毒，确切些是猪的内源性反转录病毒(PERVS)。携带这些病毒的基因随着遗传基因组在猪群中代代相传，在传递过程中就会引起感染。实验研究表明，只要给机会，这些病毒也可感染人体细胞。)

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(勇气;气概)of one of the rising stars of biotechnology(生物技术), 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 (稍微调整)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(删除) from genomes, and also allows new stretches to be inserted into the gap thus created. (CRISPR/Cas9能识别携带病毒的DNA序列，在感染病毒的地方将DNA分子与病毒DNA分开以防止病毒的侵害。通过实验稍做调整后，CRISPR/Cas9就能用于识别各种DNA序列，将特定的DNA序列从基因组中删除，并把新的DNA序列植入由此带来的空缺中。)

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们研究了一种PERVS基因序列，旨在用CRISPR/Cas9来攻击这些序列。他们发现，能让PERVS与主体的DNA结合的基因序列与使一类病毒与主体的DNA结合的基因序列是一样的。所以他们能够设计出一类CRISPR/Cas9 来识别这个特定的病毒基因序列并把它从基因组中去掉。)

单词记忆法之“滚动记忆法”

小编并不建议拿出大块时间集中背单词，因为那种方式效率并不高。根据艾宾浩斯遗忘曲线，随着时间的推移人们之前记住的东西会不断丧失，因此为了防止背过的单词被遗忘，同学们可以每天利用不同的时段反复记忆单词。

建议考雅思的同学们每天分4个时段背单词，每次背单词的时间不超过1个小时，具体的操作方法以每天背100个单词为例。

第1个时段可以选取在早上8：00-9：00之间：同学们可以在早上8点-9点的时候“刷”一遍，目的在于区分认识和不认识的单词，并且把不认识的单词进行第一次概念性的记忆。

第2个时段可以选取在中午13：00-14：00之间：在第2次背单词的过程中，重点针对的是第一遍筛选出来的不认识的单词，对这些单词进行深度记忆。

第3个时段可以选取在下午18：00-19：00之间：在第3次背单词的过程中，我们对于仍然不认识的单词进行重点记忆，对刚刚背下来的单词进行巩固记忆。

第4个时段可以选取在晚上11：00-12：00之间：在第4次背单词的过程中，主要查缺补漏，把白天背过的单词进行最后一遍记忆，把仍然没有背下来的单词继续背诵，并且做好重点标记。

最后，请大家务必注意一点：

100个单词通过一天4个时段的背诵之后，并没有结束。何为滚动?就是在第二天背另外100个新单词时，请务必把前一天没有背下来的旧单词加入到第二天的新单词当中，进行滚动记忆，直到完全背下来为止，这种“滚动记忆”的方法可以使单词像滚雪球一样越背越多，而且多次重复不易遗忘，最终实现单词的稳步提升。