The grammatical rule for all DNA: junk and coding sequences.

Selfish DNA, coding sequences, and junk DNA in the genome are no stranger to each other; rather, they represent three phases in the life cycle of DNA. Accordingly, they all obey the same grammatical rule of TG/CA/CT excess and CG/TA deficiency. On the one hand, it is this very rule which keeps isoelectric points of most proteins near the neutral range. On the other hand, this rule creates numerous palindromes, thus maintaining symmetry between complementary strands. Many of these palindromes encode identical oligopeptides on both strands.