Though I haven't seen Prometheus,I must agree with you that this kind of thing happens a lot in movies/tv shows,and it's a pitty to see what might have been a relativly good movie ,turn in to a disapointment because of it!
And honnestly ,I don't have the knowledge that you obviously do about biology and chemistry,but I do know enough about it to realize that certain thing are just not possible!

Just like the "bad guy" that always stands up at least one more time at the end of a movie, no matter what they've done to get rid of him(stabbed or shot him with a machine gun mutiple times),he just doesn't die!

The Tommyknockers!!!Don't remind me,saw the movie and I found it rather gruesome!!!