Final answer:
If the DNA sequence is TACCCGAGG, the complementary mRNA sequence is AUGGGCUCC. During transcription, A pairs with U and C pairs with G, while T in DNA is replaced by A in mRNA.
Step-by-step explanation:
In mRNA synthesis, if the sequence of DNA bases is TACCCGAGG, the complementary bases in the developing mRNA are AUGGGCUCC. This is because during transcription, adenine (A) in DNA pairs with uracil (U) in RNA, and cytosine (C) pairs with guanine (G), which is the same pairing as in DNA. Thymine (T) in DNA is replaced by adenine (A) when transcribing to mRNA. This process is part of the translation of the genetic code from DNA to mRNA, and it is fundamental for protein synthesis.