The reason why is because A only pairs with T, and G only pairs with C. So, if you have a strand with GGTCTA, the only possible pairing for that to replicate is CCAGAT.
DNA consists of two base pairs one is purine and another is purine. The A and G are purines and T, C are pyrimidines. During the DNA replication A combined with T, and C combines with G. These are called complementary pairing. Therefore, a DNA strand that has bases GGTCTA produces a strand with the bases CCAGAT
