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
Crossing over – often, the sister homologous chromosomes of a cell exchange genetic material through recombination of genes.
Independent assortment/random fertilization – there are numerous variety of ways that the chromosomes of the gametes can combine, after meiosis, through fertilization to form a zygote
Mutation – on rare occasion, there is a slight and random change in the genes of a gametic cell caused by environmental factors.
