Both the strands of DNA are not copied during transcription because if both strands act as a template, they would code for RNA with different sequence the two RNA molecules, if produced simultaneously would be complementary to each other, hence would form a double stranded RNA. Transcription is when RNA is made from DNA. During transcription, RNA polymerase makes a copy of a gene from the DNA to mRNA as needed. This process is similar in eukaryotes and prokaryotes. One difference, however, is that eukaryotic RNA polymerase associates with mRNA processing enzymes during transcription so that processing can proceed quickly after the start of transcription.