The transfer of the genetic message from DNA to mRNA is called transcription. Transcription occurs in a cell's nucleus. RNA polymerase is the enzyme that catalyzes the process of transcription.
The first part of transcription is called initiation. Transcription factors are proteins that bind to the DNA molecule to initiate transcription. RNA polymerase then binds to DNA at a region called the promoter via binding to the transcription factors. The double-stranded DNA molecule unzips to expose the region of DNA to be copied.
Nucleotides are added to form an mRNA molecule in the next step called elongation. The mRNA nucleotides match up with the nucleotides on DNA, a concept called complementary base pairing. Nucleotides are the building blocks of DNA and RNA and are made up of a phosphate group connected to a sugar that is bonded to one of four nitrogenous bases. The four nitrogenous bases making up DNA are adenine, thymine, guanine and cytosine whereas the bases making up RNA are adenine, guanine, cytosine and uracil. In DNA, adenine only binds with thymine and guanine binds with cytosine. In RNA, guanine still binds with cytosine, but adenine now binds with uracil; for example, if the DNA molecule has the adenine base, then the base uracil would bond with the DNA molecule on the mRNA strand.
Termination is the last step of transcription. Transcription stops when RNA polymerase recognizes a sequence of bases on the DNA molecule called the stop codon, which signifies the end of the gene. RNA polymerase then releases the DNA molecule.