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Protein synthesis: transcription and translation
How the instructions in DNA become the proteins that do the work in your cells.
Protein synthesis makes proteins from the instructions in DNA. For a protein-coding gene in a eukaryotic cell, transcription starts in the nucleus and RNA processing produces mature mRNA. In translation, a ribosome reads the mRNA three bases (one codon) at a time and joins the matching amino acids into a chain.
Covers GCSE, AP and A-level. Exam boards differ slightly, so check your course specification.
The steps
- 01
Transcription
NucleusFor a nuclear protein-coding gene, RNA polymerase reads the DNA template 3′ to 5′ and builds RNA 5′ to 3′. Template A pairs with U in RNA, T with A, C with G, and G with C.
- 02
mRNA leaves
Nuclear poreIn eukaryotes, RNA processing adds a cap and usually a poly-A tail. Splicing removes introns and joins exons, which can include untranslated regions. Mature mRNA then leaves through a nuclear pore.
- 03
Translation
RibosomeThe ribosome reads the mRNA one codon at a time. A tRNA with the matching anticodon brings the right amino acid, and the amino acids are joined by peptide bonds.
- 04
Protein forms
A stop codon ends translation. The chain of amino acids folds into a protein with a specific shape.
A polypeptide, which may need further processing
Try it: read the code
This short coding example shows a DNA template read left to right, 3′ to 5′, and mRNA made 5′ to 3′. Each press reveals the corresponding codon and amino acid. In a eukaryotic cell, transcription and translation happen separately, not codon by codon together.
DNA template strand (3′ → 5′, left to right)
mRNA (5′ → 3′, left to right)
Polypeptide (Stop is a signal, not an amino acid)
Press the button to transcribe and translate.
DNA vs RNA
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Strands | Double-stranded | Single-stranded |
| Bases | A, T, C, G | A, U, C, G |
| Role | Stores the genetic code | Carries and uses a copy of it (mRNA, tRNA, rRNA) |
Key terms
- Gene
- A DNA sequence used to make a functional RNA or, through mRNA, a polypeptide. Some genes can give rise to multiple protein forms.
- Codon
- A triplet of bases on mRNA that codes for one amino acid (or a stop signal).
- Anticodon
- The three bases on a tRNA that pair with a codon.
- mRNA
- Messenger RNA: a copy of a gene that carries the code to the ribosome.
- tRNA
- Transfer RNA: carries a specific amino acid to the ribosome.
- Ribosome
- The structure that reads mRNA and assembles the protein.
Common mistakes
Transcription makes RNA, not protein. Proteins are made in translation.
RNA has uracil (U), not thymine (T). A on DNA pairs with U on mRNA.
Codon vs anticodon. A codon is on mRNA. An anticodon is on tRNA.
Nuclear DNA stays in the nucleus. Its mRNA copy travels to a ribosome. Mitochondria and chloroplasts also have DNA; bacteria have no nucleus.
A worked example
This is the kind of answer SnapBio gives you when you snap a question: the answer first, then the steps.
A DNA template reads 3′-TAC-5′. What is the mRNA codon, and which amino acid does it code for?
How to get there
- 1
mRNA is built by pairing: DNA T gives A, A gives U, C gives G.
- 2
So TAC becomes the codon AUG.
- 3
AUG codes for methionine, and it's also the start codon.
Check yourself
Where is a nuclear protein-coding gene transcribed in a human cell?
Transcription copies a gene into mRNA in the nucleus, where the DNA is.
Which base is found in RNA but not in DNA?
RNA uses uracil (U) in place of thymine (T).
What is a codon?
A codon is a triplet of bases on mRNA. The matching three bases on tRNA are the anticodon.
FAQ
What is the difference between transcription and translation?
For a nuclear protein-coding gene in a eukaryote, transcription makes RNA in the nucleus, and processing produces mature mRNA. Translation reads that mRNA at a ribosome to build a polypeptide. Bacteria have no nucleus.
What is the genetic code?
The rules that link mRNA codons to amino acids. It's a triplet code: 64 possible codons code for 20 amino acids and a stop signal. Several codons can code for the same amino acid, and the code is almost universal across life.
What happens if there is a mutation in a gene?
A change in the DNA base sequence can change the mRNA codons and so the amino acid sequence. That may alter the protein's shape and function. Some mutations make no difference because of the repeated codes.
Where does the DNA come from in the first place?
It is copied before cells divide. See DNA replication explained.
Sources and further reading
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