Biotechnology and Bioinformatics (1+1)

1.4.8.1.Initiation

RNA polymerase recognizes a specific base sequence in the DNA called a promoter and binds to it. The promoter identifies the start of a gene, which strand is to be copied, and the direction that it is to be copied. RNA polymerase unwinds the DNA, and the base pairs are disrupted, producing a “bubble” of single-stranded DNA. Like DNA replication, transcription always occurs in a 5’ to 3’ direction. That is, the new ribo nucleotide is added to the 3’end of the growing chain. Unlike replication , however, only one of the strands acts as template on which the RNA strand is built. As RNA polymerase binds promoters in a defined orientation, the same strand is always transcribed from a given promoter.

The choice of promoter determines which stretch of DNA is transcribed and is the main step at which regulation is imposed. That is, the decision of whether or not to initiate transcription of a given gene is chiefly how a cell regulates which protein s it will make at any given time.