【Native Speaker每日综合训练】 【38-B】 一周精选 - Cloning

AceJ

本期选文来自本期选文来自ffffionabear的38-01越障部分
原文链接：http://forum.chasedream.com/thread-908793-1-1.html

Part I: 原文回顾 + Part II: 思路梳理，难词难句

Will Cloning Ever Save Endangered Animals?

In 2009 the Brazilian Agricultural Research Corp. (Embrapa) and the Brasilia Zoological Garden began scavenging and freezing blood, sperm and umbilical cord cells from roadkill and other wild animals that had died, mostly in the Cerrado savanna—an incredibly diverse collection of tropical forest and grassland ecosystems home to at least 10,000 plant species and more than 800 species of birds and mammals, some of which live nowhere else in the world. Specimens were collected from the bush dog, collared anteater, bison and gray brocket deer, among other species.
老样子，还是把第一段单独拿出来，看看这篇文章主要讲了什么。2009年，BARC和BZG开始搜集和冷冻死亡野生动物的血液，精子和脐带细胞。这是一次异常多样的收集活动，包括生活在热带雨林及草地环境的10000种植物和800多种鸟类及哺乳动物，其中有一些已经从地球消失了。
Scavenge: v,(of a person, an animal or a bird) search through wastes for things that can beused or eaten

The idea was to preserve the genetic information of Brazil's endangered wildlife. One day, the organizations reasoned, they might be able to use the collected DNA to clone endangered animals and bolster dwindling populations. So far the two institutions have collected at least 420 tissue samples. Now they are collaborating on a related project that will use the DNA in these specimens to improve breeding and cloning techniques. Current cloning techniques have an average success rate of less than 5 percent, even when working with familiar species; cloning wild animals is usually less than 1 percent successful.

Many researchers agree that, at present, cloning is not a feasible or effective conservation strategy. First of all, some conservationists point out, cloning does not address the reasons that many animals become endangered in the first place—namely, hunting and habitat destruction. Even if cloning could theoretically help in truly desperate situations, current cloning techniques are simply too ineffective to make much of a difference. Compared with cloning domestic species—particularly cattle, which have been successfully cloned for years to duplicate desirable traits—cloning endangered species is far more difficult for a number of reasons.

Successful cloning generally involves at least three essential components: DNA from the animal to be cloned; a viable egg to receive that DNA; and a mother to gestate the resulting embryo. Often, hundreds of embryos and attempted pregnancies are needed to produce even a few clones. Scientists usually have a poor understanding of endangered animals' reproductive physiology, which makes it too risky to extract a sufficient number of eggs from that species or rely on females of that species to give birth to clones. Legal protections sometimes preclude threatened species from such procedures as well. To compensate, researchers fuse the DNA of an endangered species with eggs from a closely related species and select mothers from the latter. Such hybrid embryos often fail to develop properly.
二，三段在说出大规模收集的目的之外，重点强调了目前克隆技术的成功率很低，克隆家养动物才不到5%的成功率，野生动物更是低至不到1%。况且这也不是保护濒危动物最简单有效的方式，原因很简单，克隆并不能解决导致动物濒危的原因--即环境破坏和污染。
第四段则说了相当专业性的内容。克隆需要3个要素：被克隆动物的DNA，接受DNA的卵子和产生胚胎的代孕妈妈。一次克隆往往需要上百次胚胎受孕，而科学家对濒危物种的繁衍习性又经常缺乏了解（这个很好理解，濒危物种嘛，自然没地方去了解他们的生理习性），再加之法律的因素，使得科学家最终只能选择相近物种来做克隆。这种混合的胚胎效果自然不如原配好。
gestate: v,(biology or medical) to carry a young human or animal inside the womb until itis born
Although they are keenly aware of these problems, Martins and his colleagues, as well as a few other scientists around the world, think that efforts to archive the genetic information of endangered wildlife are worthwhile. Some researchers remain optimistic that cloning will become a useful tool for conservation in the future. Optimists point to recent successes cloning wild mammals using closely related domestic species, improved techniques for preventing developmental abnormalities in a cloned embryo, better neonatal care for newborn clones and in vitro fertilization made possible by stem cells derived from frozen tissue.

One species that might benefit from cloning is the northern white rhinoceros, which is native to Africa. In 1960 the global northern white rhino population was more than 2,000 strong, but poaching has reduced their numbers to as few as 11 today. By last count, three live in zoos—two in San Diego and one in the Czech Republic—four live in the Ol Pejeta Conservancy in Kenya and as few as four individuals may still live in the wild based on unconfirmed reports, but they have not been spotted in several years. Most of the captive animals are uninterested in mating or infertile, although two rhinos mated in the summer of 2012.

Right now, though, cloning is unlikely to help the white rhino or any other threatened species. To date, the story of cloning endangered animals is one of a few high-profile successes and many, many failures. Since the early 2000s, using the same technique that produced Dolly, researchers have cloned several endangered and even extinct mammals, including a mouflon sheep and a bovine known as a gaur in 2001; a kind of wild cattle called a banteng in 2003; a wild goat known as the Pyrenean ibex in 2009; and wild coyotes in 2012. In each case many more clones died before birth than survived; in most cases none of the clones survived into adulthood.
我把5，6，7这三段放在一个部分。因为这里有一个很明显的科技文的写作思路，那就是在陈述了困难之后，立马来点正能量的东西。尽管在克隆上困难重重，但是科学家对克隆依旧乐观，凭借选择合适的受孕物种，更好的避免胚胎发育异常，对新宝宝无微不至的neonatal care和试管受孕，克隆取得了很大的进步。紧接着就是举例子--白犀牛。在描述了白犀牛目前是多么多么的稀少，克隆尽管目前还没有直接帮助白犀牛但依旧十分重要之后，就是一连串眼花缭乱的成功案例，或者可以叫成功的尝试。有兴趣的同学们可以自行搜索这其中的故事。不过像Dolly这么出名的相信大家应该都有所耳闻吧。这是世界上第一只用分化成熟的体细胞克隆出的羊。
Mismatched

All those attempted clones of endangered or extinct animals died in different ways for different reasons, but they all shared one fundamental problem—they were not exact replicas of their counterparts. In most cases, researchers have combined DNA from the threatened species with eggs from a related domestic species. Each surrogate mother is often implanted with dozens of hybrid embryos in order to achieve at least a few pregnancies, a strategy that requires extracting hundreds of eggs. Because the reproductive physiology of most endangered animals is so poorly understood, researchers are often unsure when the animals ovulate and how best to acquire their eggs. In some cases legal protections prevent scientists from harvesting eggs from threatened species. For all these reasons, they turn to more familiar domestic species instead.

Injecting the DNA of one species into the egg of another species—even a closely related one—creates an unusual hybrid embryo that often fails to develop properly in the womb of a surrogate mother. Hybrid embryos have the nuclear DNA of the cloned species and the mitochondrial (mtDNA) DNA of the donor egg. This mismatch becomes problematic as the embryo develops. Nuclear DNA and mtDNA work together; they both contain genetic recipes for proteins with which cells extract energy from food. In a hybrid embryo these proteins do not always fit together properly, which leaves cells starved for energy. Complicating matters further, the surrogate mother often rejects the hybrid embryo because she recognizes some of the embryo's tissues, particularly the placenta, as foreign.

Another problem—and the most intractable so far—is that a hybrid embryo created via nuclear transfer is not a genetic blank slate like most embryos. All vertebrates begin life as hollow balls of embryonic stem cells, which can become almost any type of adult cell. Each of those stem cells contains a copy of the exact same genome packaged into chromosomes—tight bundles of DNA and histone proteins. As the embryo develops, the stem cells begin to take on their adult forms: some become skin cells, others heart cells and so on. Different types of cells begin to express different patterns of genes. Inside each cell an assortment of molecules and enzymes interacts with DNA and histones to change gene expression. Some molecules, such as methyl groups, physically block cellular machinery from reading the genetic instructions in certain segments of DNA; some enzymes loosen the bonds between histones and DNA, making particular genes more accessible. Eventually, each cell type—skin cell, liver cell, brain cell—has the same genome, but a different epigenome: a unique pattern of genes that are actively expressed or effectively silenced. Over time, an adult cell's epigenome can change even further, depending on the animal's life experiences.

So when researchers inject an adult cell's nucleus into an empty egg, the nucleus brings its unique epigenome with it. As Gurdon's early experiments in the 1950s and subsequent studies have shown, an egg is capable of erasing the epigenome of introduced nuclear DNA, wiping the slate clean—to some extent. This process of "nuclear reprogramming" is poorly understood, and the egg often fails to complete it properly, especially when the egg is from one species and the nuclear DNA from another. Incomplete nuclear reprogramming is one of the main reasons, scientists think, for the many developmental abnormalities that kill clones before birth and for the medical issues common to many survivors, such as extremely high birth weight and organ failure.
8，9，10，11段则是对上文描述的克隆中碰到的困难又做了进一步的描述，解释了为什么用相似的物种做为受孕载体会导致胎盘不正常，提前死亡或者发育不良等问题。其中8，9段大致意思与前文描述相差不大，这里就不多做累赘。而10，11段这两段，我认为是全文最难理解，也是最值得我们细细品味的两段。能够顺利读懂这两段，GMAT阅读一定不在话下。
第10段，目前碰到最棘手的问题，就是通过核细胞转移而产生的混合胎盘并不像大多数胎盘一样是遗传空白 (genetic blank slate)。所有的脊椎动物都是以胎盘干细胞空心球（hollow balls实在不知道怎么翻译啊，有了解的同学请帮忙提供一下）开始生命的，这种干细胞可以变成任何一种成年细胞。每一个干细胞都包含了完全相同的被打包成染色体的基因组。随着胎盘的发育，干细胞开始呈现出成年细胞的形态：皮肤细胞，心脏细胞等等。不同的细胞有不同的基因模式（patterns of genes）。而在每一个细胞内部，分子和酶又会和DNA和组蛋白的相互作用从而改变基因表现。例如，一些分子会阻止细胞在特定的DNA阶段读取基因指令；一些酶则会松散组蛋白和DNA的连接，使得特定的基因更加容易读取。最终，每一种细胞--皮肤细胞，肝细胞，脑细胞--都有相同的基因组，但有不同的表观基因组(epigenome)：一种独特的基因构造。而成年细胞的表观基因组会随着年龄而改变，改变程度取决于寿命。
在第10段描述了发育过程中基因组和表观基因组这一独特的关系之后，11段就在此基础上，提出了克隆目前最棘手的问题：当一个成年细胞核注入进一个空的卵子后，成年细胞是带有已经形成的表观基因组的。尽管卵子会在一定程度上清除这种已经被携带的遗传信息，这一过程被称为细胞核重组，但研究者对这种重组知之甚少。卵子经常会无法全面清除遗传信息，尤其细胞核和卵子来自于不同物种时。而无法完全清除的遗传基因就是导致克隆各种失败的主要原因之一。

Some researchers see ways around these problems. Pasqualino Loi of the University of Teramo in Italy was part of a team that successfully cloned endangered mouflon sheep in the early 2000s; the clones died within six months of birth. Loi and his colleagues think they can increase the chances of a hybrid embryo surviving in a surrogate mother's womb. First, they propose, researchers could nurture a hybrid embryo for a short time in the lab until it develops into what is known as a blastocyst—the ball-shaped beginnings of a vertebrate composed of an outer circle of cells, the trophoblast, surrounding a clump of rapidly dividing stem cells known as the inner cell mass. Eventually, the trophoblast becomes the placenta. Researchers could scoop out the inner cell mass from the hybrid blastocyst, Loi suggests, and transplant it into an empty trophoblast derived from the same species as the surrogate mother. Because the surrogate mother is far less likely to reject a trophoblast from her own species, the developing embryo within has a much better chance of surviving.

Scientists have also figured out how to encourage nuclear reprogramming by bathing the egg in certain compounds and chemicals, such as trichostatin A, which stimulate or inhibit the enzymes that determine a cell's epigenome. Most recently, Teruhiko Wakayama of the RIKEN Center for Developmental Biology in Kobe, Japan and his colleagues produced 581 cloned mice from a single donor mouse over 25 generations, using trichostatin A to achieve success rates as high as 25 percent in some but not all generations. To solve the mismatch of mtDNA and nuclear DNA, Loi suggests simply removing the egg's native mtDNA and replacing it with mtDNA from the species to be cloned—something that researchers tried in the 1970s and '80s, but have not attempted recently for reasons that are unclear.

Some of the most successful attempts to clone endangered animals in recent years have involved two of the most beloved domestic species—cats and dogs.
最后三段，依旧是老套路。不能总是讲些负面的东西，积极向上的必须再次出现。针对胎盘排斥的问题，研究者将混合胎盘在实验室中培育直至变成胚包(blastocyst) -- 一个包括了滋养层(trophoblast)的球形脊椎动物生命开始体，最终，滋养层变成胎盘。然后研究者从混合胚包中提取内细胞团，并将其转移至代孕妈妈本身的胚包中。科学家同样也找到了解决遗传信息无法完全清除的办法，将卵子浸在特殊的混合剂和化学成分中，从而刺激或者抑制决定表观基因组的酶的产生。针对mtDNA和nuclear DNA不匹配的问题，科学家也同意建议可以将卵子本身的mtDNA换成被克隆物种的mtDNA。文章的最后告诉了我们一个好消息，我们喜欢的猫猫狗狗也开始被克隆了。
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本期精选就到这里啦，希望大家喜欢，有什么问题，给我留言~

wangkun1119

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douzhu1015

ganxielouzhu

JennyF

GOOD GOOD!!

puptna

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ffffionabear

占座~~待会再重做一遍~~谢谢AceJ桑！！！
10'31
话说之前没留意这是科技文的套路（只是觉得这文像女主一样一会要一会不要的~~）再读一遍才发现打击--圆回来----再打击----再圆回来~~结论是未来还是美好的是科技文章的内在逻辑套路诶~~受益良多好么~！！

jhyzxxy

打卡报到！！！

疏离无罪

占～～～～～～～

醒醒Shine

Ace的分段真的很赞，很能帮助到清晰掌握文章结构。
读完后回一下：
1- Problem and key factors: cloning difficult, what to conquer
2- some progress: environment envolve
3- still hard:  incomplete reprogramming
4- some solution: 2 examples

robot青

thx~~~~~~~