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长难句分析
Today the most famous selfreplicating molecule is DNA (deoxyribonucleic acid), but it is widely thought that DNA itself could not have been present at the origin of life because its replication is too dependent on support from specialized machinery, which could not have been available before evolution itself began.
该句是一个由but连接的并列句。前一个分句较简单，主干为The most famous selfreplicating molecule is DNA... 。后一个分句结构复杂，嵌套了多层从句，其主干是it is widely thought that...；it为形式主语，真主语是that引导的从句；because引导了一个较长的原因状语从句，其中又嵌套了which...非限定性定语从句做 machinery 的后置定语；from... 介词短语后置修饰support。
