The process of copying genetic information from one strand of the DNA into RNA is termed as transcription. Here also, the principle of complementarity governs the process of transcription, except the adenosine complements now forms base pair with uracil instead of thymine. However, unlike in the process of replication, which once set in, the total DNA of an organism gets duplicated, in transcription only a segment of DNA and only one of the strands is copied into RNA. This necessitates defining the boundaries that would demarcate the region and the strand of DNA that would be transcribed. Why both the strands are not copied during transcription has the simple answer. First, if both strands act as a template, they would code for RNA molecule with different sequences (Remember complementarity does not mean identical), and in turn, if they code for proteins, the sequence of amino acids in the proteins would be different. Hence, one segment of the DNA would be coding for two different proteins, and this would complicate the genetic information transfer machinery. Second, the two RNA molecules if produced simultaneously would be complementary to each other, hence would form a double stranded RNA. This would prevent RNA from being translated into protein and the exercise of transcription would become a futile one.

In transcription ,which step is catalysed by RNA polymerase ?

Elongation

RNA polymerase catalyses elongation during transcription. During elongation RNA polymerase moves  along one strand of DNA known as the template strand in the 3' → 5' direction. For each nucleotide in the template RNA polymerase adds a matching (complementary) RNA nucleotide to the 3' end of the RNA strand. It associates transiently with initiation factor (σ) and termination factor (ρ) to initiate and terminate the transcription, respectively.