1) Nucleotide 2) The smallest unit of DNA 
In genetics, the smallest unit of a chromosome in which alteration can be effective in causing a mutation. ... Origin: mutation + -on ... (05 Mar 2000) ... 
Type: Term Pronunciation: myū′ton Definitions: 1. In genetics, the smallest unit of a chromosome in which alteration can be effective in causing a mutation (a single nucleotide change).
[n] - the smallest unit of DNA where a mutation can occur
noun the smallest unit of DNA where a mutation can occur