That there are two colors per strut, so what's linking the two backbones together are two colored segments that come from the outside towards the middle, and that the colors occur only in certain combinations, red and green, yellow and blue, that's all you see.

每个梯阶有两个颜色,主链是由,两种不同颜色的片段由外向内连接起来的,这些颜色仅以某种特定方式配对,红连接绿,黄连接蓝,大家都能看到

耶鲁公开课 - 生物医学工程探索课程节选

Now DNA is a double helix and I've only shown you one part of the helix, right? I've shown you one upright strut and a base hanging off of it, but it forms a double helix because complementary strands of DNA strongly associate with one another and that's a very stable structure.

NA是双螺旋,我只是给你们讲了螺旋的一边,我给你们演示,一条链的支柱及碱基团,但其能构成双螺旋,是因为DNA是完全互补的两条链,彼此之间结合紧密,结构异常稳定

耶鲁公开课 - 生物医学工程探索课程节选