During which stage of transcription does RNA polymerase synthesize a complementary RNA strand?

During transcription, the stage when RNA polymerase synthesizes a complementary RNA strand is known as elongation. This phase follows initiation, where RNA polymerase binds to the promoter region of the DNA and unwinds the helix to begin transcription. During elongation, RNA polymerase moves along the DNA template strand, reading the nucleotide sequence and adding complementary RNA nucleotides to build the growing RNA molecule. This process continues until the polymerase reaches a termination signal, at which point RNA synthesis stops.

The key aspect of elongation is that it involves the actual synthesis of RNA, where the complementary base pairing occurs between the DNA template and the RNA nucleotides. This leads to the formation of a single-stranded RNA transcript that carries the genetic information from DNA, ready for further processing such as splicing and eventually translation into a protein.