Because it takes three units where there's only four choices at each position to have at least 20 unique combinations.

因为每个位点上仅有四种可能性,只有采用三个碱基一组,才能取得至少二十种不同的组合方式

耶鲁公开课 - 生物医学工程探索课程节选

It's only got one of four possible choices at each position.

每个位点上都只有四种可能性

耶鲁公开课 - 生物医学工程探索课程节选