What enzyme starts the process of transcription?

Transcription is the 1st step in the gene expression (DNA based) by which the information in a sequence of DNA is copied into a molecule of mRNA (messenger RNA). Transcription involves 3 steps:

Initiation: In which the DNA molecule sequence unwind and form a small open complex. Elongation: In which RNA polymerase moves with the template strand & synthesizing a messenger RNA. Termination: Occurs when an RNA polymerase crosses a termination sequence in the gene. In the termination step, the mRNA strand is complete & it detaches from DNA.

RNA polymerase is a multi sub-unite enzyme which is responsible for copying the DNA template into an RNA template during the process of transcription.