The possibility of an alternative to DNA life forms
If you think that DNA is the only genetic material, think again. For it is widely believed among biochemists and molecular biologists that this predominantly DNA coded world that we currently live in was preceded by an RNA world.
RNA is very much like DNA in that it also is a string of nucleotides containing a base, a sugar and phosphodiester bonds to hold the sugars together. The primary difference is that the ribose sugar in RNA has a hydroxyl group (-OH) at its 3' position, which has been replaced by a hydrogen (H) in DNA - thus the name deoxy-' ribonucleotide, meaning DNA.
One of the major reasons why RNA based life forms are believed to have preceded DNA based life forms is that RNA is capable of catalytic activity, unlike DNA. Thus in the primeval biochemical soup, RNA had the advantage of enzymatic activity and thereby self-assembly.
RNA could act on itself in early evolutionary phases, thereby splicing it into novel combinations, acting as adaptor molecules that bound specifically to amino acids and finally arranging amino acids on RNA templates with the help of the core RNA factories of ribosomes to form the first polypeptides and proteins.
Finally reverse transcription of RNA led to the formation of DNA and RNA took up the stance of second fiddle.
However life forms that have their genetic material encoded in RNA, like the deadly retroviruses, still exist to this day. This makes a biochemist like me wonder about the broad scheme of things.