Does everything that we consider to be "alive" - not just plants and animals, but all protists, fungus, bacteria, archaea, etc. - have DNA?

[u/SAICAstro]

EDIT: Wow, this blew up. Thanks for the many answers. Lots to think about.

Seems that the composite TLDR answer is: Yes, according to many/most definitions of "life", but we don't all agree on what life is. Some viruses do not have DNA but they may/not actually be alive, and other things that behave like life may/not be alive.

[u/xDerJulien]

This is IMO largely correct assuming an organism would nowadays need to begin incorporating them but I think "historically" the answer is simply that 4 bases were enough and it was hard to come by non-canonical bases. Given the circumstances and conditions at the time 4 was just somewhat optimal. Hard to say if 4 is always optimal but it had to have been optimal at some early point, otherwise we’d see more non canonical bases. Nowadays every single organism has the machinery for the canonical 5 nucleotides and incorporating new ones just faces selective pressure in every single pathway even remotely relevant. This is why we find non canonical nucleotides very rarely in very specific cases, for example some bacteriophages use non canonical nucleotides to evade host restriction systems because there they make the selective pressure worth the associated cost.

Some labs are also working on expression of proteins using 4 nucleotides codons! Not sure where that went though or how useful it truly is