Acceptor arm for the tRNA last universal common ancestor (LUCA). Shown in red is the triplet that, according to  and the dual complementarity hypothesis , was originally the duplicate of the anticodon. Yellow and blue mark the two modes of tRNA recognition by aaRSs, from the minor groove (5') and major groove (3') sides, respectively. The terminal NCCA, its presumable homolog GCCR and complementary YGGC quadruplets are framed. Note that the blue (3') half of the structure is a palindrome.