The nucleotide has a directionality, there's an up and a down to it and it's going to turn out the chain that's formed by polymerizing these has a directionality as well and that's important in defining the structure.

核苷酸有方向性,有上端和下端,核苷酸长链,是由核苷酸沿一个特定方向聚合而成的,这在定义结构时很重要

耶鲁公开课 - 生物医学工程探索课程节选