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

It's the phosphate and the pentose that make up the backbone - that make up the upright struts of the ladder and it's the bases that make up the connecting struts, so the bases are the colors.

磷酸基和戊糖,共同构成了主链,主链又构成了梯子上下的支柱,而碱基,则构成连接支柱的梯阶,碱基是用各种颜色表示的

耶鲁公开课 - 生物医学工程探索课程节选

The backbone's on the outside of the molecule like the upright struts of a ladder on the outside of a ladder.

主链位于DNA分子结构外侧,像梯子两侧上下的支柱,记住是梯子外侧

耶鲁公开课 - 生物医学工程探索课程节选