请大家在本帖回复：
1. 文章大概结构 2. 自己写的答案
解析+文章翻译明晚微信群里公布
报名活动，加微信号killgmat
|
Scientists recognize semiconservative replication as the mechanism by which DNA replicates prior to cell division. In this process, the bases of the double helix are broken, and the molecule "unzips" into two strands. Each strand then joins with a new strand, so that each copy is an equal mix of old and new DNA. Prior to the Meselson-Stahl experiment of 1958, other methods of DNA replication were suggested. In the conservative replication model, a completely new double helix would be formed, leaving the original intact. Dispersive replication, by contrast, breaks the DNA molecule into small segments, each of which joins with a complimentary segment. Then, the segments reform into two strands of DNA that contain pieces of the old and pieces of the new DNA.
In the Meselson-Stahl experiment, the scientists grew E. coli bacterial cultures, some with heavy nitrogen isotopes, and some with lighter nitrogen isotopes. This gave the scientists a way to distinguish between old and newly synthesized DNA. The bacteria grown in heavy nitrogen were then transferred to medium containing the lighter density DNA, which would incorporate into any newly synthesized DNA. After one generation, the DNA was of a density exactly intermediate to the heavy and light DNA. This eliminated the possibility of conservative replication, which would have resulted in equal amounts of the two separate densities. After two generations, half of the DNA was intermediate and half was light. This eliminated the possibility of dispersive replication, which would have produced DNA that was all of a density between the light and exactly intermediate DNA. E. coli are bacteria, and are thus prokaryotes, organisms that lack complex cell structures, but semiconservative replication also occurs in eukaryotes, organisms with complex cells such as animals and plants. Since each DNA molecule in a eukaryote is incorporated in one chromosome, human chromosome replication is also semiconservative.
When Watson and Crick deciphered the structure of DNA in 1953, they intuited that the semiconservative model was correct. However, this still amounted to a clever guess, and so it remained until Meselson and Stahl devised a way to differentiate between the original strands of DNA and future generations. Since then, autoradiographic images have confirmed the correctness of Watson and Crick's intuition and the effectiveness of Meselson and Stahl's elegant experiment.
Q1. The author mentions that eukaryotes replicate through semiconservative replication in order to make the point that
A)the choice of E. coli bacteria was a good one for the Meselson-Stahl experiment
B) semiconservative replication depends on an organism having a complex cell structure
C) Watson and Crick realized that their work would have implications for humans
D) the Meselson-Stahl had implications for forms of life beyond simple bacteria
E) each DNA molecule in a eukaryote is incorporated in one chromosome
Q2. According to the passage, which of the following is the reason that, had the conservative model been correct, the same number of heavy and light strands of DNA would have resulted?
A) Conservative replication breaks up the original DNA into small segments.
B) An equal mix of old and new DNA is formed using the conservative model.
C) Conservative replication would keep the original strands untouched.
D) The double helix of DNA is separated only in the conservative model of replication.
E) The conservative replication model splits the original DNA into four discrete strands.
Q3. It can be inferred from the passage that in the Meselson-Stahl experiment, a third generation of bacteria would produce bacteria with
A) some light density and some intermediate density DNA
B) some light density and some heavy density DNA
C) some heavy density and some intermediate density DNA
D) only light density DNA
E) only intermediate density DNA
参考答案： DCA
| 
京公网安备11010202008513号 京ICP证101109号 京ICP备12012021号
发表于 2021-6-4 09:11:47
收藏
收藏
