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

And remember for this class, we always define z as the internuclear or the bond axis.

记住在我们的课堂上，我们总是把z方向定义为核间轴的方向。

麻省理工公开课 - 化学原理课程节选

So in terms of the carbon hydrogen bond, it's a sigma bond, - because we define it -- any time we are bonding to an atom, we have to keep redefining our bond axis to whatever two atoms we're talking about.

对于碳氢键，它是sigma键，因为我们定义它-,任何时候我们有原子成键，我们要重新定义键轴方向,为我们所讨论的两个原子方向。

麻省理工公开课 - 化学原理课程节选