The complementary DNA strand will be :
gac tga cga tta
ctg act gct aat
In double - stranded DNA, the hydrogen bonds between the bases of one strand and the bases of the other strand maintain the 2 strands united.
One purine on one strand necessarily binds to a pyrimidine on the other strand.
A binds to T (by 2 hydrogen bonds).
G binds to C (via 3 hydrogen bonds)
The A composition of the DNA is therefore equal to the T composition, and the G composition of the DNA is equal to the C composition.
This strict correspondence (A <-> T and G <-> C) makes the two strands complementary. One is the template, the mold, the matrix of the other, and vice versa, and vice versa: this property allows the exact replication (semi-conservative replication: one strand -the mold - is preserved, one new is synthesized ("de novo") on this mold, likewise for the other strand, another mold, preserved, allowing a strand to be synthesized de novo.