Maybe I should answer you NO! Whatever that they be they are not, in general, Arabic or Semitic roots! Some of such Mughatta'aat's only comprise one single letter, like "Qaaf" (ق) in [50:1], "Nun" (ن) in [68:1] and "Sad" (ص) in [38:1]. Do you think these be roots? What about "Kaf Ha Ya 'Ain Sad" (کهیعص) in [19:1]?
However, you may like to think about another subject, why a single alphabet may be read like a word, e.g. why do we say Qaf to mean "ق", from where do "a" and "f" come into its name? Why its name differs from its sound? Also a same thing can be asked about all the Arabic alphabets and even alphabets in many and maybe all the other languages. Why we call it an alphabet if it is already read as a word? Maybe they and their combinations are only acronyms for words which are actually sound like how we read them now for Mughatta'aat's? Maybe there are words that we don't know and there is no entrance for them in any dictionary but they exist? It may be the case! Yes, some Shia Muslims believe e.g. the alphabet "Qaf" (ق) is name of a mountain, the name of the mountain itself is Qaf, not Q alone, that is, قاف instead of only ق ! The mountain is introduced in Shia Ahadith as a mountain which is the root for every earthquake, the blue colour of sky is due to its reflection of sun light and it is located in the fourth Earth (see here to know more about the Seven Skies and Seven Earths according to the teachings of Alhlol-Beyt --peace be upon them--). Anyway, I only know that we have a science of numbers and a science of alphabets and these you can find in Shia Ahadith from Ahlol-Beyt --peace be upon them-- whom the Shia Muslims believe to be the firmly-grounded-in-knowledge introduced in Quran and then by the holy prophet --peace be upon them--. Then you may be able to judge a single word, not only Mughatta'aat's in Quran, as a acronym representing a sentence! This is particularly the case for the name of the prophet David --peace be upon him--, actually Davood. Then you may find a hidden strength in the language Arabic and may understand better why Allah has chosen this language for His last and most complete divine book.