In biology, transcription is the first step in the process of gene expression, in which the information stored in DNA is copied into a complementary RNA molecule. The process of transcription occurs in the nucleus of eukaryotic cells and in the cytoplasm of prokaryotic cells. The transcription process starts when the enzyme RNA polymerase recognizes a specific DNA sequence, called a promoter, which indicates the starting point of transcription. RNA polymerase then unwinds the double helix of the DNA molecule and uses one strand as a template to synthesize a complementary RNA molecule. During transcription, RNA polymerase adds nucleotides, the building blocks of RNA, to the 3' end of the RNA molecule, following the base-pairing rules of A-U and C-G. The newly synthesized RNA molecule is called a primary transcript, which may undergo further processing, including splicing and capping, to form a mature RNA molecule. In eukaryotes, the mature RNA molecule is usually transported out of the nucleus into the cytoplasm, where it can be translated into a protein. In prokaryotes, the RNA molecule can be translated immediately into a protein without leaving the cytoplasm. Transcription is a complex process that is regulated by various factors, including DNA-binding proteins and small regulatory RNA molecules, to ensure that the correct genes are expressed at the appropriate times and in the appropriate cells.