During transcription the DNA double helix is separated into two individual strands. Each strand may serve as a template for RNA polymerase, which travels along the DNA structure in a 3' to 5' direction. As it progresses down the strand, RNA polymerase synthesizes a pre-messenger RNA strand that is complementary to the sequence on the DNA template. For example if the DNA sequence on the template was 5' ATACA 3', then the pre mRNA sequence synthesized would be 3' UAUGU 5'. (Remember, RNA synthesis utilizes the nucleotide uracil instead of thyamine).