One strand of the DNA double helix is used as a template by the RNA polymerase to create a mRNA. RNA polymerase recognizes a sequence called a promoter. The promoter is a sequence on the DNA that tells the RNA polymerase both where to start and in which direction (on which strand) to continue synthesis.
The RNA polymerase then stretches open the double helix at the promoter in the DNA. One strand of the DNA double helix is used as a template by the RNA polymerase to synthesize a mRNA. Synthesis is single stranded and only proceeds in the 5' to 3' direction on the mRNA.
The nucleotides are paired like this:
DNA = mRNA
This mRNA moves from the nucleus to the cytoplasm. During this step, mRNA goes through splicing when the non-coding sequences (introns) are eliminated. The below diagram is a demonstration of splicing.
How does RNA polymerase know when to stop transcribing a gene? At the end of a gene, the sequence of the mRNA allows it to form a tail, which blocks the ribosome. The ribosome falls off the mRNA, and that is the termination signal recognized by the RNA polymerase. As soon as the ribosome falls off the mRNA, the RNA polymerase falls off the DNA and transcription ends. Below is a diagram of transcription.
