The Open complex forms during the transcription stage of protein synthesis after the less stable closed complex. In prokaryotes the closed complex is formed following the binding of the enzyme RNA polymerase and σ-factor (holoenzyme) to the DNA strand in the promoter region. The change from a closed complex to an open complex involves the unwinding of the DNA strands (forming a transcription bubble). The unwinding of the strands enables RNA polymerase to synthesize mRNA by the hydrogen bonding of the nucleotides ATP, CTP, GTP and UTP to their complementary DNA base pairs. This stage of transcription is known as elongation. Transcription produces an mRNA strand in preparation for protein synthesis.