Before revision:  While walking like a molecular motor along a DNA track in a step-by-step manner, a RNAP simultaneously synthesizes a RNA chain , in each forward step it elongates the nascent RNA molecule by one unit, using the DNA track also as the template.
After revision:  While walking like a molecular motor along a DNA track in a step-by-step manner, a RNAP simultaneously synthesizes a RNA chain ; in each forward step it elongates the nascent RNA molecule by one unit, using the DNA track also as the template.