Transcription is the process by which a DNA sequence is copied into a complementary RNA sequence. RNA polymerase binds to the promoter region of the DNA chain and causes the unwinding and separation of the DNA strands, melting the bonds between the strands. The RNA polymerase binds nitrogenous bases in a complementary manner to the antisense strand (uracil pairs with adenine, cytosine pairs with guanine). This new strand will be identical to the sense strand, but with uracil instead of thyamine. RNA polymerase creates phosphodiester bonds between bases, releasing two phosphates in the process as ATP is converted into ADP. RNA polymerase synthesises an RNA strand in a 5' - 3' direction until it reaches the terminator region. At the terminator, RNA polymerase and the newly formed RNA strand both detach from the antisense template, and the DNA rewinds. Post-transcriptional modification is necessary in eukaryotes in order to produce mature RNA.