During which phase does RNA polymerase synthesize a new mRNA molecule?

During the elongation phase, RNA polymerase is actively synthesizing a new mRNA molecule. This process occurs after the initiation phase, where RNA polymerase binds to the promoter region of the DNA and unwinds the DNA strands, allowing access to the template strand. Once initiation is successfully completed, RNA polymerase moves along the DNA strand, reading the template and adding complementary RNA nucleotides one by one to the growing mRNA chain. This synthesis proceeds in the 5' to 3' direction, resulting in the production of a single-stranded RNA transcript that corresponds to the gene being expressed. The elongation phase continues until RNA polymerase reaches a termination signal on the DNA, which triggers the end of transcription.