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  阅读

  综合点评:本次考试阅读部分整体难度适中,文章易懂,考点比较常规,单词题也没有出现偏词。

  阅读第五篇

  Passage1: Genesis: The Scientific Quest for Life's Origin

  Passage2: Molecular fossils: findings challenge the attempts about origins of life

  BY University of North Carolina School of Medicine

  Now, research from UNC School of Medicine biochemist Charles Carter, PhD, appearing in the September 13 issue of the Journal of Biological Chemistry, offers an intriguing new view on how life began. Carter's work is based on lab experiments during which his team recreated ancient protein enzymes that likely played a vital role in helping create life onEarth. Carter's finding flies in the face of the widely-held theory that Ribonucleic Acid (RNA) self-replicated without the aid of simple proteins and eventually led to life as we know it.

  In the early 1980s, researchers found that ribozymes -- RNA enzymes -- act as catalysts. It was evidence that RNA can be both the blueprints and the chemical catalysts that put those blueprints into action. This finding led to the "RNA World" hypothesis, which posits that RNA alone triggered the rise of life from a sea of molecules.

  But for the hypothesis to be correct, ancient RNA catalysts would have had to copy multiple sets of RNA blueprints nearly as accurately as do modern-day enzymes. That's a hard sell; scientists calculate that it would take much longer than the age of the universe for randomly generated RNA molecules to evolve sufficiently to achieve the modern level of sophistication. Given Earth's age of 4.5 billion years, living systems run entirely by RNA could not have reproduced and evolved either fast or accurately enough to give rise to the vast biological complexity on Earth today.

  "The RNA world hypothesis is extremely unlikely," said Carter. "It would take forever." Moreover, there's no proof that such ribozymes even existed billions of years ago. To buttress the RNA World hypothesis, scientists use 21st century technology to create ribozymes that serve as catalysts. "But most of those synthetic ribozymes," Carter said, "bear little resemblance to anything anyone has ever isolated from a living system.

  Carter, who has been an expert in ancient biochemistry for four decades, took a different approach. His experiments are deeply embedded in consensus biology.

  Our genetic code is translated by two super-families of modern-day enzymes. Carter's research team created and superimposed digital three-dimensional versions of the two super-families to see how their structures aligned. Carter found that all the enzymes have virtually identical cores that can be extracted to produce "molecular fossils" he calls Urzymes -- Ur meaning earliest or original. The other parts, he said, are variations that were introduced later, as evolution unfolded.

  These two Urzymes are as close as scientists have gotten to the actual ancient enzymes that would have populated Earth billions of years ago.

  "Once we identified the core part of the enzyme, we cloned it and expressed it," Carter said. "Then we wanted to see if we could stabilize it and determine if it had any biochemical activity." They could and it did.

  Both Urzymes are very good at accelerating the two reactions necessary to translate the genetic code.

  "Our results suggest that there were very active protein enzymes very early in the generation of life, before there were organisms," Carter said. "And those enzymes were very much like the Urzymes we've made."

  The finding also suggests that Urzymes evolved from even simpler ancestors -- tiny proteins called peptides. And over time those peptides co-evolved with RNA to give rise to more complex life forms.

  In this "Peptide-RNA World" scenario, RNA would have contained the instructions for life while peptides would have accelerated key chemical reactions to carry out those instructions.

  文章大意:双篇文章有关基因科学。第一篇作者认为 RNA 可以帮助找到生命的起源。之前人们一直依赖基因和蛋白质,但是现在 RNA 的出现会让科学研究有重大的突破。基因和蛋白质是互相依赖的,所以科学家很难知道到底哪个先出现。RNA 可以帮助跳过这个难题,因为 RNA 可以起三个作用:第一,可以储存信息;第二,可以复制配对;第三,可以起到催化作用。

  第二篇作者反对 RNA 的重要性。主要基于两点:第一,RNA 没有足够的时间进化;第二,RNA 可能不会在那么早存在。Carter 教授使用最新技术进行了研究。人类基因密码由两大 modern day enzymes 族系转译。Carter 教授发现这两大族系由共有的 identical cores 来产生 molecular fossil, 教授将其命名为 Urzymes. 并推断出此物质可能是古时早期生命信息的的存在状态。

  以上内容由智课小编为大家整理的关于“2019年3月8日SAT真题回忆及考情分析阅读第五篇”,希望可以帮助到大家,小编在此预祝大家在考试中取得好成绩!

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