There are two upright struts of the ladder, one shown in blue here, the other shown black.

两条上下的主链作为支柱,一条以蓝色表示,一条以黑色表示

耶鲁公开课 - 生物医学工程探索课程节选

The corresponding half of the ladder that corresponds to any given ladder, let's say the black DNA molecule that corresponds to the blue one is not just a mirror image.

与任何单链配对的,另一条单链,比方说与蓝色DNA分子,配对的黑色DNA分子,并不是蓝色DNA分子的镜像

耶鲁公开课 - 生物医学工程探索课程节选