The synthesis of messenger RNA is called transcription. Transcription begins when RNA polymerase recognizes and binds to the promoter region on the double-stranded DNA molecule. A particular subunit of the messenger RNA, called the sigma factor, participates in recognition of the promoter region. Soon after transcription is initiated, the sigma factor dissociates from the RNA polymerase. RNA polymerase moves along the template strand of the DNA, synthesizing the complementary single-stranded messenger RNA molecule. Synthesis is in the 5-3 direction, with new nucleotides being added to the 3-end of the growing messenger RNA molecule. As the RNA polymerase advances along the DNA, it melts a new stretch of DNA and allows the previous stretch to close. When RNA polymerase reaches a specific sequence of nucleotides on the DNA, called the transcription terminator, a hairpin loop structure forms in the messenger RNA, causing the RNA polymerase and the messenger RNA to dissociate from the DNA.