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新 SAT 官方指南阅读第三篇全解析
This passage is adapted from . Watson and . , “Genetical Implications of the Structure of
Deoxyribonucleic Acid.” ○C 1953 by Nature Publishing Group. Watson and Crick deduced the structure of DNA
using evidence from Rosalind Franklin and . Gosling’s X-ray crystallography diagrams of DNA and from Erwin
Chargaff’s data on the base composition of DNA.
The chemical formula of deoxyribonucleic acid (DNA) is now well established the molecule is
a very long chain, the backbone of which consists of a regular alternation of sugar and phosphate
groups. To each sugar is attached a nitrogenous base, which can be of four different types. Two of
the possible bases-adenine and guanine-are purines, and the other two-thymine and
5 cytosine-are pyrimidines. So far as is known, the sequence of bases along the chain is irregular.
The monomer unit, consisting of phosphate, sugar and base, is known as a nucleotide.
The first feature of our structure which is of biological interest is that it consists not of one
chain, but of two. These two chains are both coiled around a common fiber axis. It has often
been assum