DNA has 4 nitrogen bases which are Adenine(A) , Guanine(G) , Thymine(T) , Cytosine(C). RNA also has 4 bases but in RNA Uracil (U) is present instead thymine. Adenine always pair with thymine by twi hydrogen bonds in DNA and to Uracil in RNA. Guanine pair with cytosine by three hydrogen bonds. In a double stranded DNA base pairing exists. Thus, adenine is present opposite to thymine and guanine present opposite to cytosine in double strand. This is called as complementary base pairing.
If a strand has the sequence 5′ ATTGCCC 3′ then the complementary strand means other strand of double strand would have the sequence 3′ TAACGGG 5′ .
The original sequence is from 5′to 3′ sequence that pair Adenine with Thymine and cytosine with Guanine. The complementary strand read from 3′ to 5′direction. Thus, complementary strand start with 3’ and ends with 5’.