Are the only ‘rungs’ of the gene ‘ladder’ AT, TA, GC or CG?

Question: Are the only 'rungs' of the gene 'ladder' AT, TA, GC or CG?
- Keywords:
  gene
Asked by Newt to Carmen, Daniel, Laura, Noel, Steph on 16 Mar 2015.
- Carmen Denman answered on 16 Mar 2015:
  
  Yep, as far as I know we haven’t discovered any new nucleotides!
  Carmen
- Noel Carter answered on 16 Mar 2015:
  
  Yes. Although sometimes the bases can be modified by chemicals to change the way the DNA used by the cell as the instructions for the way the cell works

Question: Are the only 'rungs' of the gene 'ladder' AT, TA, GC or CG?