The process by which mRNA is produced from DNA is called transcription. Reverse transcription occurs in HIV. The length of DNA that an RNA transcribes is called a transcription unit. Transcription unit consists of promoter, start point and termination point. Transcription is a very fast process. It takes one second to transcribe an mRNA of 1000 nucleotides from a gene in E. coli bacteria. In actual cells, an mRNA is transcribed from a gene and a protein is translated from it. This is called monocistronic transcript. Multiple proteins are translated to transcribe a recipe in progenitor cells. This is called a polycistronic transcript.