翻译：The presumption on which human cloning rests is that all these cells，though now specialized，still contain exact copies of the original set of genetic instructions needed to make an entire individual and can do so if a way is found to switch them back on.

【答案提示】：句子的框架是The presumption is that all these cells…contain genetic instructions…and can do so…。其中介词on加which引导定语从句修饰主语the presumption; that从句为表语从句;此表语从句中contain和can do并列做all these cells的谓语;表语从句中又插入一个though引导的省略了主语all these cells的让步状语从句，补充完整为though these cells are now specialized; needed to make an entire individual为过去分词短语做genetic instructions的后置定语;并列谓语can do so又有if条件句修饰。rest on意为“依赖”。为避免重复do so代替前文提到的make an entire individual。

【译文】人类克隆赖以存在的假定是:虽然所有的细胞现已全部特化，但却仍然包含有发育为完整个体所需的与母体完全一致的全部遗传指令;如果能找到一个方法使失去作用的部分恢复作用，这些细胞就能发育成为完整的个体。