This is potentially a very stupid question: I'm guessing ДНК is pronounced дээнка́. As such, is it treated grammatically as a feminine noun, or is it invariable? In other words, do you say в его дээнке́ , инопланетяне взяли мою дээнку́ , etc?
I ask this because I've only ever seen it in writing, I've never heard anyone actually say it :)
This very initialism is invariable.
However, there are some acronyms like ЗАГС, БОМЖ, ТЭН etc. which do decline as any other noun would, and in fact many Russian speakers are unaware those are initialisms.
This is more common for initialisms which would have been masculine if they were nouns, however, ЧК used to decline as well in early XX century (забрали в чеку, арестован чекой etc.), and the acronym з/к gave birth to the (masculine) word зек "inmate".
There is even a vernacular word касемсот "a heavy tractor", which originates from К-700, a heavy tractor manufactured in Leningrad.
It declines and is in every other way treated as a noun, you can find ads saying продам касемсот модели К-701.
As with most initialisms, it is invariable.
В его дээнка
Инопланетане взяли мою дээнка
I listened lectures by antropologist Станислав Дробышевский - he makes many witty remarks in his speech. He (and some other scientists) change ДНК in his speech (maybe because it sounds fun, or, maybe, because it is some kind of jargon): "Просеквенировали дээнку неандертальцев", "нашли в дээнке современных людей следы дээнки кроманьонцев", etc. (example: Расшифровка генома древнего человека) So, in fact, you can hear someone declining ДНК as if it is feminine noun.
But, anyway, this was the only time I have heard such usage. I have never heard such usage on TV, in popular science movies or in speech of any people I met in person.
ДНК is pronounced дээнка́Because it's just the names of the corresponding letters, like Dee-En-Ey. It is never written like this. On the other hand, "normal" words like "загс" differ in pronounce and do decline.