It's a twisted ladder and there's a couple of things to notice about this familiar structure.

它犹如扭曲的梯子,对于这众所周知的结构有几件事必须注意

耶鲁公开课 - 生物医学工程探索课程节选

One is that there are two backbones right here in the light blue, so this would be the upright parts of the ladder that are twisted.

其一是浅蓝色所代表的两条主链,这是构成,扭曲梯子的垂直部分

耶鲁公开课 - 生物医学工程探索课程节选