The genetic code: from DNA to protein
In protein synthesis DNA is first copied into mRNA (transcription), then the ribosome reads the mRNA in groups of three and builds the amino acid chain (translation).
Rules
- Transcription: mRNA complementary to the template strand, U instead of T
- Template strand 3′ → 5′, mRNA 5′ → 3′
- Translation starts at the start codon AUG (methionine)
- One codon = three bases = one amino acid
- Stop codons UAA, UAG, UGA end translation
Your turn
Three problems on this topic. Type probabilities as a percentage number, e.g. 18.75.
Task
A section of the template DNA strand reads 3′-TAC CTG ACG TAA TAT ATT-5′. Work out the mRNA (5′ → 3′) and the amino acid sequence of the peptide. The table shows the meaning of the codons that may occur.
| Codon (mRNA) | Amino acid |
|---|---|
| ACU | Thr |
| AUA | Ile |
| AUG | Met |
| AUU | Ile |
| CGU | Arg |
| GAC | Asp |
| GCA | Ala |
| UAA | Stop |
| UAU | Tyr |
| UGC | Cys |
| UUA | Leu |
mRNA 5′ → 3′
Amino acid sequence
Probabilities as a percentage number (25 or 18.75), rounded to 2 decimal places; allele frequencies as a decimal (0.02).
Worked example
A section of the template DNA strand reads 3′-TAC TCT CGC ACC CTG ATT-5′. Work out the mRNA (5′ → 3′) and the amino acid sequence of the peptide. The table shows the meaning of the codons that may occur.
Worked solution
- 1.Transcription: the complementary RNA base for each base of the template strand (A → U, T → A, G → C, C → G): 5′-AUG AGA GCG UGG GAC UAA-3′
- 2.Translation from the start codon AUG in groups of three: AUG = Met, AGA = Arg, GCG = Ala, UGG = Trp, GAC = Asp, UAA = Stop
- 3.Translation ends at the stop codon: Met-Arg-Ala-Trp-Asp
Typical mistake: Reading the template strand directly as mRNA (just swapping T for U). The mRNA is complementary to the template strand and matches the coding strand.
Common questions
Which DNA strand is read?
The template strand. The mRNA is complementary to it and so matches the other (coding) strand, just with U instead of T.
What does it mean that the code is degenerate?
Most amino acids have several codons. That is why a base change in the third position often changes nothing.