The DNA base sequence is usually read down one side of the molecule or the other. The sequence is usually read with reference to the bases and their corresponding identifying letter. Black is the original DNA and red is the newly formed DNA.

e.g.(1a)ATG CTC ATT  TTA  GGG CCC ATA CTC

           = 24 bases thus we can write the complementary sequence of the other helix as:

12(2a)qTAC GAG TAA AAT CCC GGG TAT GAG

 

In DNA replication (1a) will act as the template for a new complementary sequence of bases (1b):

copy 1:

          (1a)ATG CTC ATT  TTA  GGG CCC ATA CTC

          (1b)TAC GAG TAA AAT CCC GGG TAT GAG

 

and (2a) will act as a template for the other new complementary sequence (2b)

Copy 2:

            (2a)TAC GAG TAA AAT CCC GGG TAT GAG

            (2b)ATG CTC ATT  TTA  GGG CCC ATA CTC

therefore after replication the base sequence of copies 1 (1a)(1b) and copy 2 (2a)(2b) are identical to each other. Importantly they are also identical in base sequence to the original base sequence of (1a)(2a).