Every unit in the backbone of this ladder has the same sugar unit, the same pentose; it's either RNA or DNA and has ribose or deoxyribose, so you don't need to describe the whole thing.

主链的每个单位,都由相同的糖单位构成,同样的戊糖,RNA里是核糖,DNA里是脱氧核糖,所以你不需要所有细节都描述

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