Final answer:
The +1 site in DNA transcription is the starting point for mRNA synthesis, with upstream nucleotides numbered negatively and downstream nucleotides positively.
Step-by-step explanation:
The nucleotide pair in the DNA double helix that serves as the template for the first nucleotide of mRNA during transcription is known as the +1 site or the initiation site. Nucleotides that are located before this site on the DNA strand are numbered negatively and are referred to as upstream nucleotides. In contrast, nucleotides that are positioned after the initiation site are numbered positively and are known as downstream nucleotides. This system of numbering is crucial for understanding the arrangement of sequences within a gene, as well as the transcription process which occurs in three key stages: initiation, elongation, and termination. The initiation process begins at the promoter region, usually found upstream of the gene it regulates, which contains specific sequences such as the -10 and -35 consensus sequences in prokaryotes.