Transcription is the process cells use to copy information from DNA into RNA. It is the first major step in gene expression, allowing the instructions stored in a gene to be used without changing the original DNA. In eukaryotes, transcription happens in the nucleus, while translation happens later in the cytoplasm.
Understanding transcription helps explain how cells control which proteins they make and when they make them.
During transcription, RNA polymerase binds near a gene at a promoter, opens a small region of DNA, and builds an RNA strand using one DNA strand as a template. The RNA grows by base pairing, with A pairing with U in RNA and C pairing with G. After termination, the RNA transcript is released, and in eukaryotic cells it is processed by adding a 5 prime cap, adding a poly-A tail, and removing introns by splicing.
These processing steps help protect the RNA and prepare it to be translated into protein.
Key Facts
- Transcription copies DNA into RNA, not protein.
- RNA polymerase builds RNA in the 5 prime to 3 prime direction.
- RNA base pairing rules during transcription are A to U, T to A, C to G, and G to C.
- The DNA template strand is read by RNA polymerase in the 3 prime to 5 prime direction.
- Initiation begins when RNA polymerase binds to a promoter near the start of a gene.
- In eukaryotes, pre-mRNA processing includes a 5 prime cap, a poly-A tail, and splicing to remove introns.
Vocabulary
- Transcription
- Transcription is the process of making an RNA copy of a gene from a DNA template.
- RNA polymerase
- RNA polymerase is the enzyme that opens DNA and links RNA nucleotides together during transcription.
- Promoter
- A promoter is a DNA sequence where RNA polymerase and other proteins bind to begin transcription.
- Transcription bubble
- A transcription bubble is the small opened region of DNA where RNA polymerase reads the template strand and makes RNA.
- Splicing
- Splicing is the process that removes introns from pre-mRNA and joins exons together.
Common Mistakes to Avoid
- Confusing transcription with translation is wrong because transcription makes RNA from DNA, while translation makes a protein from mRNA.
- Writing thymine in the RNA strand is wrong because RNA uses uracil instead of thymine.
- Using the coding DNA strand as the direct template is wrong because RNA polymerase reads the template strand, while the RNA sequence usually matches the coding strand except U replaces T.
- Forgetting RNA processing in eukaryotes is wrong because pre-mRNA usually must receive a 5 prime cap, a poly-A tail, and splicing before it becomes mature mRNA.
Practice Questions
- 1 A DNA template strand has the sequence 3 prime TAC GGA CTT 5 prime. What RNA sequence is produced from it?
- 2 An mRNA contains 900 nucleotides after splicing. If every 3 nucleotides form one codon, how many codons are present?
- 3 Explain why a mutation in a promoter can reduce the amount of mRNA made from a gene even if the protein-coding sequence is unchanged.