How does a gene become a protein?
A gene becomes a protein in two steps: transcription, where RNA polymerase copies the gene's DNA into messenger RNA, and translation, where a ribosome reads the mRNA three bases at a time and links the matching amino acids into a chain.
Transcription: DNA to mRNA
RNA polymerase binds a promoter at the start of the gene, separates the two DNA strands and builds RNA complementary to the template strand, 5' to 3', using uracil (U) in place of thymine (T). So the mRNA has the same sequence as the gene's other strand, the coding strand, with U for T.
In eukaryotes, transcription happens in the nucleus and the RNA is processed before it leaves: a 5' cap and a poly-A tail are added, and introns are spliced out so only exons remain. Bacteria have no nucleus, so their ribosomes can start translating while the mRNA is still being made.
Translation: mRNA to protein
- A ribosome assembles on the mRNA at the start codon, AUG, which codes for methionine.
- Each transfer RNA carries one amino acid and has an anticodon that pairs with a matching codon on the mRNA.
- The ribosome joins each new amino acid to the growing chain with a peptide bond, then shifts one codon along.
- At a stop codon (UAA, UAG or UGA), a release factor binds instead of a tRNA and the finished chain is released to fold.
Of the 64 possible codons, 61 code for amino acids and 3 are stops. Since there are only 20 amino acids, most have more than one codon; the code is redundant, but each codon has only one meaning.
Worked example
Template strand: 3'-TAC AAA CCG ATT-5'. Transcribe by pairing each base (T to A, A to U, C to G, G to C): mRNA 5'-AUG UUU GGC UAA-3'. Translate with a codon table: AUG is methionine, UUU is phenylalanine, GGC is glycine, and UAA is stop. The protein is Met-Phe-Gly.
Now change one base. If GGC became GGA, it still codes for glycine (a silent mutation). If UUU became UAU, phenylalanine becomes tyrosine (missense). If an extra base were inserted, every codon after it would shift (a frameshift), usually ruining the protein.
Seeing the ribosome work
Codons and anticodons are easier to follow when you watch tRNAs arrive one by one. Type "How does a gene become a protein?" into Isotoon for a short animated walk from DNA to folded chain. See also DNA replication, the biology page and AP Biology.
Questions
- Is the mRNA the same as the template strand or the coding strand?
- It matches the coding strand, with U in place of T. It is complementary to the template strand, which is the one RNA polymerase actually reads.
- Do all genes code for proteins?
- No. Some genes are transcribed into RNAs that do their job as RNA, such as the transfer RNAs and ribosomal RNAs used in translation itself.
- Where does translation happen?
- On ribosomes, either free in the cytoplasm or attached to the rough endoplasmic reticulum, which handles many proteins headed for membranes or for secretion.