Transcription is the synthesis of an RNA chain by copying a complementary DNA strand of a particular gene (or cluster of genes in bacterial operons). Both in prokaryotic and eukaryotic cells, a DNA template is required for transcription as well as the four ribonucleoside triphosphates (ATP, GTP, CTP, and UTP) acting as substrates and the enzyme RNA polymerase. As a result, a molecule of RNA (mRNA, tRNA, or rRNA) is produced. The initiation of transcription usually requires the presence of an upstream promoter sequence – i.e., a specific sequence to which RNA polymerase binds tightly and that indicates where to start the transcription and which strand of DNA to transcribe. Some RNA viruses (e.g., human immunodeficiency virus, HIV) make a DNA copy of their genome, i.e., synthesize a DNA molecule using RNA as template in a process called reverse transcription (these viruses are known as retrovirus).