Protein Synthesis
From the four-letter alphabet of DNA to a functional protein in the cell — the central dogma of molecular biology, step by step.
Primary sources: NCBI Bookshelf: Protein Synthesis · Khan Academy AP Biology
01 · The central dogma
DNA → RNA → Protein.
Francis Crick coined the term central dogma in 1958 to describe the flow of genetic information in cells: DNA is transcribed into RNA, which is translated into protein. The direction is normally one-way — information flows from nucleic acid to protein, not back from protein to nucleic acid.
Exceptions exist: retroviruses use reverse transcriptase to copy RNA back into DNA. CRISPR-Cas9 exploits this type of molecular machinery — read our CRISPR news article. The genetic sequence is stored in the nucleus; see our DNA glossary entry for structure.
02 · Six steps
From gene to finished protein.
Transcription
RNA polymerase unwinds DNA and synthesises a complementary pre-mRNA strand using the template strand (3′→5′ direction).
RNA processing
A 5′ cap and poly-A tail are added. Introns are spliced out by the spliceosome; exons are joined. Mature mRNA exits through nuclear pores.
Initiation
The small ribosomal subunit binds the mRNA at the 5′ cap and scans for the start codon (AUG). The initiator tRNA (Met-tRNAi) binds AUG.
Elongation
tRNAs deliver amino acids; peptide bonds form between adjacent amino acids. The ribosome moves 3 bases (one codon) along the mRNA each cycle.
Termination
A stop codon (UAA, UAG or UGA) is reached. A release factor binds; the polypeptide chain is hydrolysed from the last tRNA and released.
Post-translational modification
The polypeptide folds (aided by chaperones), may be cleaved or chemically modified (glycosylation, phosphorylation), and is trafficked to its destination.
03 · The genetic code
64 codons, 20 amino acids.
A codon is a sequence of three consecutive mRNA bases that specifies one amino acid (or a start/stop signal). With four bases (A, U, G, C) and three positions, there are 4³ = 64 possible codons — more than enough to encode 20 amino acids. The code is therefore degenerate: most amino acids are specified by 2–6 synonymous codons.
- AUG — the start codon; codes for methionine (the first amino acid of every protein).
- UAA, UAG, UGA — the three stop codons; no amino acid is added; translation terminates.
- The remaining 60 codons specify the 19 other amino acids. Use our codon translator tool to explore the full table.
The genetic code is nearly universal — the same codons specify the same amino acids in bacteria, fungi, plants and animals. This is powerful evidence for common ancestry of all life.
04 · Regulation and application
Cells control which genes are expressed.
Not all genes are transcribed at all times. Gene expression is regulated at multiple levels: transcription factors control whether RNA polymerase binds to a gene; epigenetic marks (methylation, histone modification) can silence whole regions of the genome; microRNAs can target mRNA for degradation.
Understanding this regulation has enormous practical value. mRNA vaccines deliver synthetic mRNA that cells translate into a harmless antigen, triggering immunity without viral DNA — see our mRNA vaccines article. CRISPR-Cas9 edits the DNA template itself — see the CRISPR article.
Questions about protein synthesis
- Transcription occurs in the nucleus: RNA polymerase reads the template strand of DNA and synthesises a complementary mRNA strand (using U instead of T). Translation occurs at ribosomes: the mRNA sequence is read in triplets (codons), and transfer RNA (tRNA) molecules bring the matching amino acids. See our DNA glossary entry for the central dogma context.
- There are 4³ = 64 possible codons but only 20 amino acids plus stop signals, so most amino acids are coded by 2–6 synonymous codons. For example, leucine has 6 codons. This redundancy provides some protection against point mutations. The code is nearly universal across all life, with minor exceptions in mitochondria and some protists. See the NCBI codon table.
- mRNA is single-stranded and contains the base uracil (U) instead of thymine. It uses ribose sugar (not deoxyribose). After transcription in the nucleus, pre-mRNA is processed: a 5′ cap and poly-A tail are added, and introns are spliced out, leaving only exons. The mature mRNA then exits through nuclear pores. See our mRNA vaccines article for how this is exploited therapeutically.
- Each tRNA is an adaptor molecule: it has an anticodon loop (3 bases complementary to the mRNA codon) and an amino acid attachment site at the 3′ end. Aminoacyl-tRNA synthetase enzymes attach the correct amino acid to each tRNA. At the ribosome, the A site holds the incoming tRNA, the P site holds the growing chain, and the E site releases the empty tRNA. See NCBI Bookshelf on translation.
Try the codon translator
Enter a DNA or mRNA sequence and see which amino acids it encodes, with each codon colour-coded.