The RNA molecule is a molecule copied a from a segment of DNA by a process called Transcription. In order to start this transcription process, an enzyme called RNA polymerase is needed. This RNA polymerase will bind to a certain length of amino acids called the Promoter sequences. The transcription process will now start at a nucleotide near the promoter site. The name give for this site is called the "Transcription Start Site".
Along the process, a Phosphodiester bond is formed between the 3'- hydroxyl group of the first mucleotide and 5'- triphosphate on the subsequence nucleotide, thus the direction of synthesis is from 5' to 3'. RNA sequence is complementery to the DNA sequence just noted that the the RNA lacks T, Thymine, instead they have U, Uracil. So the pairing between DNA and RNA now is A to U or T to A and C to G or G or C. Transcription will stop after it read a stop codon and the RNA will bud off.
1. Daniel L. Hartl, Elizabeth W. Jones ( 2005 ) Genetics-Analysis of genes and genomes, sixth edition, page 403 and 404