Make a working RNA copy
A protein-coding gene is a DNA sequence with instructions for an amino-acid sequence. During transcription, an enzyme called RNA polymerase reads one DNA strand and uses base-pairing rules to build an RNA copy. That DNA strand is called the template strand. The opposite DNA strand is called the coding strand because, when both are written 5′ to 3′, its base order matches the mRNA except that DNA uses T where mRNA uses U. Messenger RNA, or mRNA, carries the copied message to a ribosome; the original DNA does not turn into RNA or become the protein-building material.
The common DNA → RNA → protein model describes the main direction of information flow for protein-coding genes. It is a useful model, not a claim that every stretch of DNA becomes a protein. Cells transcribe only particular sequences, some genes produce functional RNAs instead of proteins, and gene expression includes additional control and processing steps that this introductory path leaves out.
Translate three letters at a time
At a ribosome, the mRNA sequence is read in groups of three nucleotides called codons. Each codon specifies an amino acid or a stop signal. Transfer RNAs help match codons with amino acids, and the ribosome joins the amino acids in the order encoded by the mRNA.
A start codon establishes where translation begins, while a stop codon signals release of the completed amino-acid chain and does not add an amino acid. More than one codon can specify the same amino acid. After translation, the chain must fold and may be changed or combined with other chains before it becomes a working protein, so a codon chart predicts sequence rather than final protein shape or activity.
Remember these ideas
Key ideas
- Transcription copies information from a DNA template into RNA; DNA itself does not become the RNA copy.
- Translation reads mRNA in three-nucleotide codons to build an amino-acid sequence.
- Start and stop signals establish the section of mRNA that a ribosome translates.
- The DNA → RNA → protein pathway describes protein-coding genes, while some genes produce functional RNAs instead.
Make the reasoning visible
Trace a short coding sequence
- 1
Begin with the coding-strand DNA sequence 5′-ATG GAA TAA-3′. The other DNA strand serves as the template during transcription.
- 2
Write the corresponding mRNA in the same direction by using U where the coding DNA has T: 5′-AUG GAA UAA-3′.
- 3
Read the mRNA as three codons: AUG is the start codon and specifies methionine, GAA specifies glutamate, and UAA is a stop signal.
- 4
The translated amino-acid sequence is methionine–glutamate. The stop codon ends translation; it does not add a third amino acid.
Try the next step
Decode one more message
A short coding-strand DNA sequence is 5′-ATG CCT TGA-3′. Write the corresponding mRNA and use these codon clues—AUG: methionine/start, CCU: proline, UGA: stop—to predict the amino-acid sequence.
Why would reading the same mRNA from a different starting nucleotide usually produce a different amino-acid sequence?