Final answer:
Being processive during elongation means that RNA polymerase continuously synthesizes mRNA from the DNA template without detaching, unwinding, and rewinding DNA in the process.
Step-by-step explanation:
For RNA polymerase to be processive during elongation means that once the enzyme begins synthesizing mRNA, it continues without dissociating from the DNA template. RNA polymerase is an enzyme that adds new nucleotides to a growing strand of RNA, and during this phase, it will proceed along the DNA template at a rate of approximately 40 nucleotides per second. The enzyme unwinds the DNA ahead of itself and rewinds it afterward while continuously synthesizing the mRNA in the 5' to 3' direction until the entire gene is transcribed.
In eukaryotes, particularly with RNA polymerase II, elongation continues 1,000 to 2,000 nucleotides beyond the gene being transcribed. This overhang is later processed and removed to form the mature mRNA. The processivity of RNA polymerase ensures that the entire mRNA strand is synthesized efficiently and accurately.