Final answer:
The first RNA processing event during elongation is the addition of the 5' cap to the nascent RNA, which protects it from degradation and aids in initiation of translation.
Step-by-step explanation:
The first RNA processing event to occur during elongation is the addition of the 5' cap to the growing transcript. This special nucleotide, a 7-methylguanosine, is added to the 5' end of the transcript by a processing enzyme. The 5' cap serves several critical functions; it protects the RNA from degradation, is recognized by factors involved in protein synthesis, and helps initiate translation by ribosomes. Elongation itself includes the RNA polymerase tracking along the DNA template, synthesizing mRNA in the 5' to 3' direction, and continuously unwinding and rewinding the DNA as it is read.