 To transcribe a gene, RNA polymerase proceeds through a series of well-defined steps and these all steps are grouped into three phases and these three phases are initiation, elongation and termination. So first of all let's see what is initiation, that is initiation of transcription. A promoter is the DNA sequence that initially binds to the RNA polymerase together with any other transcription factor. So one requirement is RNA polymerase and some more transcription factors are also required. So the first step is the binding of these promoter and RNA polymerase in addition to any other transcription factor. Once formed the promoter polymerase complex undergoes structural changes which are required for the initiation to proceed. Then the DNA around the point where transcription will start unwinds that is there is some point on the DNA. And that is the site of initiation of transcription. So once this complex is formed the DNA unwinds from this site, the base pairs are disrupted and they produce a transcription bubble of single-stranded DNA. So if DNA is like helically coiled and it is double-stranded from the site where initiation will start it begins opening up and after opening it forms a transcription bubble. So this is a transcription bubble and in this transcription bubble the two DNA strands are open and they are single-stranded DNA. In like DNA replication transcription always occur in a 5 prime to 3 prime direction that is the new ribonucleotide is added to the 3 prime end of the growing chain. So you know in DNA the polarity is 5 prime and 3 prime end. So transcription proceeds from 5 prime to 3 prime end. So it means the new nucleotides will be added up at the 3 prime end. However unlike replication only one of the DNA strands acts as a template on which the RNA strand is built. As you know in case of replication when it is replication fork both these DNA strands they act as templates and two new strands are synthesized along these two single strands. But in case of transcription if this is transcription bubble only one strand will act as a template and the synthesis of new RNA will only take place along this strand the other strand will not transcribe will not be transcribed. The initiation can itself be broken down into a series of defined steps. This initiation phase this is further divided into different steps. So the first step in the initiation is the initial binding of polymerase to a promoter to form what is called a closed complex. So a polymerase binds to the DNA and it makes a closed complex. In this form the DNA remains double stranded and the enzyme is bound to the one phase of the helix. So this is helix and enzyme is bound to the one phase of this helix. In the second step of initiation the closed complex undergoes a transition to the open complex in which the DNA strand separate over a distance of about 13 base pair around the start site to form the transcription bubble. So because the DNA is opened up in this step so that's why it is called open complex. And in this step the DNA double strand opens about 13 base pair long strand will open up and will form a transcription bubble. In the next stage of initiation polymerase enters the phase of initial transcription followed by promoter escape. So actually transcription starts here but this is initial transcription and it is different from actual transcription. So this initial transcription is followed by the escape of promoter. So promoter region is escaped. The opening up of the DNA frees the template strand. So the opening up of DNA it frees the template strand. Now the template strand is free to be transcribed. The first two ribonucleotides are brought into the active site of the RNA polymerase which you know active site is the cleft in this claw in the center of this claw. So these two ribonucleotides then aligned on the template strand and then are joined together. So if this is the template strand two ribonucleotides come here they align across this template they join together and they then stayed here. In the same way subsequent ribonucleotides are incorporated into the growing RNA chain. So after the joining of these two first nucleotides new nucleotides then keep on entering and they keep on joining with the existing RNA chain. So incorporation of the first 10 or so ribonucleotides is a rather inefficient process and this is actually called initial transcription and at that stage the enzyme often releases short transcripts each of less than 10 or so nucleotides and then begins synthesis again and this synthesis is actual transcription. So the first synthesis of about 10 nucleotide long transcript that is inefficient. In this phase the polymerase promoter complex is called initial transcribing complex. So this complex is now called initial transcribing complex. Once an enzyme makes a transcript longer than 10 nucleotides it is said to have escaped the promoter. So on this template strand about first 10 bases are considered as promoter region and after if these 10 bases are synthesized then it is actually escaped promoter. At this point it has formed a stable ternary complex and this ternary complex includes enzyme, DNA and RNA. This is the transition to the elongation phase. So actually the formation of this ternary complex is the end of initiation phase and then as the actual transcription will start from here. So then it will be entered into the elongation phase.