Sentence:  All life depends on <WSD>nitrogen</WSD>; it is the building block from which nature assembles amino acids, proteins, and nucleic acids; the genetic information that orders and perpetuates life is written in <WSD>nitrogen</WSD> ink. 
Meaning of the word marked with <WSD> tags:  (Q627) The chemical element ("symbol" N) with an atomic number of 7 and atomic weight of 14.0067. It is a colorless and odorless gas.