2019-04-24 18:02 三立在线
摘要：2019年3月亚太SAT真题及答案-阅读部分第五篇，双篇文章. 具体真题截图如下： 2019年3月亚太SAT真题及答案-阅读部分第五篇答案： 43：C 44、C 45、B 46、D 47、B 48、B 49、C 50、A 51、D 52、D 第一篇文章说的是作者认为RNA可以帮助找到生命的起源。之前人们一直依赖DNA和Protein，但是现在RNA的出现会让科学研究有重大的突破。DNA和Protein是互相依赖的，所
Few events have electrified the origin-of-life community as much as the early 1980s discovery of RNA ribozymes-strands of RNA that not only carry genetic information,but also act as catalysts。 Sidney Altman of Yale and Thomas Cech of the University of Colorado independently demonstrated that a particular segment of RNA can accelerate key biothemical reactions。This startling finding,which won Altman and Cech the Nobel Prize in 1989,inspired a new vision of life's origin。
Modern life relies on two complexly interrelated molecules ：RNA,which carries information,and proteins, which perform chemical functions。This interdependence leads to a kind of chicken-and-egg dilemma: Proteins make and maintain DNA, but DNA carries the instructions to make proteins。Which came first?RNA,it turns out,has the potential to do both jobs。
The RNA world theory quickly emerged following the discovery of ribozymes 。IT champions the central role of genetic material in the dual tasks of catalyst and information transfer。Over the years, “RNA World” has come to mean different things to different people, but three precepts are common to all versions of the theory: (1) Once upon a time, RNA rather than DNA stored genetic information;(2)ancient RNA replication followed the same rules as modern DNA replication by matching pairs of bases: A-U (the pyrimidine uracil, whose DNA equivalent is thymine) and C-G;and(3)ancient RNA played the same catelytic roles as modern protein enzymes。In this scenario,the first life-form was simply a self-replicating strand of RNA,perhaps enclosed in a protective lipid membrane。According to most versions og this hypothesis, modern metabolism emerged later, as a means to make RNA replication more efficient
"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。
"To think that these two Urzymes might have launched protein synthesis before there was life on Earth is totally electrifying," Carter said. "I can't imagine a much more exciting result to be working on, if one is interested in the origin of life."
The study leaves open the question of exactly how those primitive systems managed to replicate themselves -- something neither the RNA World hypothesis nor the Peptide-RNA World theory can yet explain. Carter, though, is extending his research to include polymerases -- enzymes that actually assemble the RNA molecule. Finding an Urzyme that serves that purpose would help answer that question.