Transcribed RNA is separated from a single strand of DNA it used as a template, then cut into pieces to be translated into proteins, while DNA that forms during DNA replication remains bound to the single strand to become a new, full DNA molecule with a double strand. RNA transcription and DNA replication begin in very similar ways.
In both cases, an enzyme, RNA polymerase for transcription and DNA polymerase for replication, splits the double-stranded DNA molecule down the middle, forming single strands. Nucleotides are then bound to the DNA nucleotides in a sequence determined by the DNA. All four possible nucleotides for DNA fit with only one of the other four.
The first difference arises in that, during transcription, the DNA double strand is only partially split, while during DNA replication, the double strand must be entirely split. The next difference is that RNA is not identical to DNA, despite being similar. One of the nucleotides of RNA is different, although it is still compatible with the pairing of the nucleotide it replaces. The cutting of the resulting RNA results in unused sections, called introns, being discarded, while the useful exons are stuck back together with full instructions for building a protein molecule.