Transcription (Genetics)

Transcription is the process of making an RNA copy of a gene's DNA sequence. In transcription, an enzyme (RNA polymerase) reads one strand of DNA in the nucleus and synthesizes a complementary RNA strand (messenger RNA, mRNA) that carries the genetic message out for protein synthesis.

• Transcription occurs in the nucleus of eukaryotic cells (since DNA is there). The DNA double helix unwinds locally, and RNA polymerase builds the RNA strand by matching RNA nucleotides to the DNA template. Notably, in RNA, uracil (U) is inserted where DNA has adenine (A) - RNA contains U instead of thymine (T).
• Only one strand of DNA is transcribed for a given gene (the template strand). The produced mRNA is complementary to the DNA template and essentially a copy of the DNA coding strand (except U replaces T).
• After transcription in eukaryotes, the initial mRNA (pre-mRNA) undergoes processing: introns (non-coding sequences) are removed and exons spliced together, and a 5' cap and 3' poly-A tail are added to stabilize the mRNA before it leaves the nucleus. This RNA processing does not occur in prokaryotes (they lack introns).

• If a question asks "Which process produces mRNA from DNA?" -> the answer is transcription (DNA -> RNA).
• If you see mention of RNA polymerase building a strand or the presence of uracil in a newly made nucleic acid, that indicates transcription (since RNA is being made and RNA uses U in place of T).
• Be careful not to confuse transcription with translation: Transcription is in the nucleus and makes RNA; for example, an item asking where messenger RNA comes from (nucleus via copying DNA) is about transcription, not the cytoplasmic protein synthesis step.