Hey Fyron... "The fully extended name of DNA" is deoxyribonucleic acid. Two words
I think you're saying what the link posted previously was saying, that the specific DNA spoken of is human mitochondrial DNA, and it's fully extended "name" is the longest word of English. Correct me if I'm wrong in that assumption, and disregard the rest of this posting if the assumption is wrong.
I'm still not convinced however, that this is a true word, and not the coded sequence of a specific strand of human mitochondrial DNA. The main reasons are 1) nobody has produced the actual word, only a reference to it and it's approximate length, and 2) the approximate length, IIRC, would coincide with the number of base pairs in human mitochondrial DNA. I can't really think of any other way that a DNA strand would be referred to with a string of such length.