Although RNA polymerase traverses the DNA template strand from 3' → 5', the coding (non-template) strand is usually used as the reference point. Hence, the process proceeds in the 5' → 3' direction, like in DNA replication. However, unlike DNA replication, transcription does not need a primer to start and it uses base pairing to create an RNA copy containing uracil instead of thymine.
In prokaryotes transcription occurs in the cytoplasm whereas in eukaryotes it takes place primarily in the nucleus before the mRNA is transported into the cytoplasm for translation or for protein synthesis.
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Transcription is the first step of gene expression, in which a particular segment of DNA is copied into RNA by the enzyme RNA polymerase. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes. During transcription, a DNA sequence is read by an RNA polymerase, which produces a complementary, antiparallel RNA strand called a primary transcript. As opposed to DNA replication, transcription results in an RNA complement that includes the nucleotide uracil (U) in all instances where thymine (T) would have occurred in a DNA complement. Also unlike DNA replication where DNA is synthesized, transcription does not involve an RNA primer to initiate RNA synthesis.