Transcription is the first step in gene expression and initiates the central dogma of molecular biology. Transcription is the process in which DNA sequences are transcribed to make RNA molecules over 3 steps; initiation, elongation, and termination. The process is initiated by RNA polymerase binding to a promotor; a specified sequence of DNA nucleotides. Once bound to a promotor RNA polymerase separates the two DNA strands. Once the two strands of DNA are separated elongation occurs, one strand acts as a template for RNA polymerase to build complimentary RNA nucleotides one base at a time. As RNA polymerase reads the DNA template, forming the RNA molecule, the chain grows from 5’ to 3’. RNA polymerase will continue transcribing until it reaches a specific group of nucleotides forming a terminator, this causes the entire process to end and is known as termination. The RNA molecule is then processed to make messenger RNA or mRNA, this molecule will go on to be translated into a protein.

References:

https://www.khanacademy.org/science/biology/gene-expression-central-dogma/transcription-of-dna-into-rna/a/stages-of-transcription