What is the symbolic reference?

Santiago carries the mast back to his shack, much as Christ carried the cross on his shoulders, falling several times (as Christ did on the Stations of the Cross) only to collapse on his bed to sleep "face down on the newspapers with his arms out straight and the palms of his hands up," recalling the crucifixion (122).

Those are the base pairs that make up the structure of DNA. The reason the pairing is so specific is because of their chemical structure. A and T easily form 2 hydrogen bonds with each other and C and G easily form 3 hydrogen bonds between each other. It is called complementary pairing. Adenine and guanine are called purines and thymine and cytosine are pyrimidines and purines are only compatible with pyrimidines. These bonds are formed because they are the most energetically favorable and stable based on the structure of these chemicals. I would recommend looking up things like this on Wikipedia because it is great for answers on everything.