Phases of transcription:

Gene transcription through E. coli RNA polymerase takes place in three phases that are: initiation, termination and elongation.  In During initiation  RNA  polymerase  identify a specific site on the DNA upstream from the gene which will be transcribed,  known a promoter site and then unwinds  the DNA locally.  In During elongation the RNA  polymerase uses  the  antisense  (-)  strand  of  DNA  as sample and synthesizes a complementary  RNA molecule by using ribonucleoside 5'-triphosphates  as precursors. RNA produced has the similar sequence as the nontemplate strand, known as the sense (+) strand (or coding strand) except in which the RNA contains U instead of T. At different locations on the bacterial chromosome,   sometimes   one   strand   is   used   as   sample,   sometimes   the   other, depending on that strand is the coding strand for the gene in question.  The correct strand to be used as sample is identified for the RNA polymerase through the presence of the promoter site.  At last,  the  RNA  polymerase  encounters  a termination  signal  and  ceases  transcription  releasing  the RNA  dissociating and transcript from the DNA.