AACTGCT
Explanation:
Base-pairing of DNA is complementary. Pyrimidines (Cytosine & Thiamine) pair with purines (Adenine & Guanine).
Specifically, Adenine pairs with Thiamine while Guanine pairs with Cytosine. This strict base-pairing rule also helps DNA replication to be of high fidelity with very rare errors. The two complementary sequences are usually antiparallel to each other.