How does DNA polymerase know which nucleotide to add, I mean does it?
Its a good question. The answer is more or less YES. DNA polymerase has both 5-3 synthetase and 3-5 exonuclease activities. During replication, all DNA pol need to know is the incoming molecule is a deoxynucleotide which can fit in its synthetase site. Base pairing is the first step of synthesis and a rule of thumb Purines (A & G) pair with Pyrimidines (T & C). So, YES a new nucleotide RANDOMLY comes and sit in the active site, if its a match phosphodiester bond is formed else DNA pol waits for next nucleotide. Sometimes, however, if there is a mismatch, its exonuclease site detects the mismatch and removes it.