Final answer:
During transcription, the mRNA strand complementary to the DNA strand GCA TAGA would be CGU AUCA, following the base pairing rules and replacing thymine with uracil.
Step-by-step explanation:
The strand of mRNA that would be formed during transcription using the given DNA strand (GCA TAGA) will be complementary to the DNA template strand. Transcription involves converting a DNA sequence into mRNA by following base pairing rules, where adenine (A) pairs with uracil (U) and cytosine (C) with guanine (G) in the mRNA. Therefore, the complementary mRNA sequence to the DNA strand GCA TAGA would be CGU AUCA. It is important to note that thymine (T) in DNA is replaced by uracil (U) in mRNA.