Final answer:
Reversing the promoter's orientation relative to gene A and transcribing across it would result in mRNA that does not produce a functional protein A, due to incorrect transcription directionality, which impacts both amino acid coding and RNA splicing.
Step-by-step explanation:
When a genetic engineer excises the promoter sequence for a gene and reinserts it at the other end of the gene, oriented in such a way that RNA polymerase would transcribe across the gene, the mRNA synthesized will not result in a functional protein. This is due to the directionality of transcription and translation processes, which are crucial for the assembly of a correct amino acid sequence.
In the DNA, each amino acid is coded by a sequence of three nucleotides, and this coding is unidirectional. If the promoter is placed at the opposite end and in reverse orientation relative to the original direction of transcription, the RNA polymerase will synthesize RNA in the wrong direction, leading to a sequence that does not correspond to the necessary mRNA for a functional protein.
Furthermore, eukaryotic genes are composed of exons and introns. During the processing of the pre-mRNA, introns are removed in a process called splicing, and exons are joined to form a continuous coding sequence. This process also requires the correct orientation of the genetic sequences to produce a functional mRNA which, when translated, will result in a functional protein. If the entire gene including exons and introns are transcribed in the reverse direction, the resulting RNA will not be properly spliced, thus again failing to produce functional protein.
Therefore, the reorientation of the promoter will disrupt the gene expression process and will not lead to the production of a functional protein A.