Transcription

Science

Definition

The process by which the information in a gene's DNA sequence is copied into a complementary strand of messenger RNA (mRNA). This occurs in the nucleus and is the first step in gene expression, carried out by the enzyme RNA polymerase.

How It Works

  1. RNA polymerase binds to the promoter region of the gene on the DNA template strand.
  2. The DNA double helix unwinds, exposing the template strand.
  3. RNA polymerase reads the template strand 3'→5' and synthesizes mRNA 5'→3' using complementary base pairing (A→U, T→A, C→G, G→C).
  4. The mRNA strand elongates as RNA polymerase moves along the gene.
  5. RNA polymerase reaches a terminator sequence and releases the completed pre-mRNA.
  6. In eukaryotes, the pre-mRNA is processed (5' cap, 3' poly-A tail, intron splicing) to form mature mRNA.

Examples

  • A skin cell transcribing the keratin gene to produce keratin protein for hair and nails
  • Cells increasing transcription of heat shock protein genes when exposed to high temperatures
  • Lac operon genes being transcribed in E. coli only when lactose is present
Key Fact

RNA polymerase reads the template strand 3'→5' and builds mRNA 5'→3'. In RNA, uracil (U) replaces thymine (T).

Related Terms

Transcription and translation Translation Gene regulation Dna structure

Study This Concept

Practice transcription with free review games in these units:

AP Biology Unit 6: Gene Expression and Regulation