A gene has the base sequence that starts with gac tga cga tta. what would be the complementary dna strand formed from this dna?

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.