Genetic Code &
Codon Translation
Master how triplet codons in mRNA are decoded by the ribosome to build proteins: the universal language of life at the molecular level.
MCAT Biological Foundations
What Is the Genetic Code?
The genetic code is the set of rules by which information encoded in mRNA sequences is translated into amino acid sequences (proteins) by ribosomes. Each "word" in this code is a codon: a sequence of three consecutive nucleotides.
Essential Vocabulary
Codon
A triplet of nucleotides on mRNA that specifies one amino acid or a stop signal
Anticodon
A complementary triplet on tRNA that base-pairs with the mRNA codon
Reading Frame
The grouping of nucleotides into non-overlapping triplets, set by the start codon
From DNA to Protein
The Central Dogma flows from DNA to mRNA (transcription) and from mRNA to protein (translation). Codon translation is the decoding step where ribosomes read mRNA triplets and assemble polypeptides.
Core Properties of the Code
1
Triplet: Each codon consists of exactly 3 nucleotides.
2
Non-overlapping: Each nucleotide belongs to only one codon in a given reading frame.
3
Degenerate (redundant): Multiple codons can encode the same amino acid (e.g., Leu has 6 codons).
4
Unambiguous: Each codon specifies only one amino acid: never two.
5
Universal: Nearly all organisms use the same code (with rare exceptions in mitochondria).
Degeneracy and the Wobble Position
Start and Stop Codons
AUG
Start codon: codes for methionine (Met) in eukaryotes and fMet in prokaryotes. Sets the reading frame.
UAA, UAG, UGA
Stop codons: do not code for any amino acid. Recognized by release factors, not tRNAs, to terminate translation.
Transcription vs. Translation
| Feature | Transcription | Translation |
|---|---|---|
| Template | DNA (3' → 5' strand) | mRNA (5' → 3') |
| Product | mRNA | Polypeptide (protein) |
| Location (eukaryotes) | Nucleus | Ribosome (cytoplasm / RER) |
| Key enzyme | RNA Polymerase | Ribosome (ribozyme) |
| Complementarity | DNA → RNA (A→U, T→A) | Codon → Anticodon |
The Translation Machinery
Ribosome
Has A (aminoacyl), P (peptidyl), and E (exit) sites. Catalyzes peptide bond formation.
tRNA
Carries amino acids. Anticodon loop base-pairs with mRNA codon. Charged by aminoacyl-tRNA synthetase.
mRNA
Read 5' → 3' by the ribosome. Contains codons in the coding sequence flanked by UTRs.
Steps of Translation
1
Initiation: Small ribosomal subunit binds mRNA at 5' cap, scans to AUG. Initiator tRNA (Met) enters the P site. Large subunit joins.
2
Elongation: Charged tRNA enters the A site. Peptide bond forms (peptidyl transferase). Ribosome translocates one codon downstream. Repeat.
3
Termination: A stop codon enters the A site. Release factor binds (not a tRNA). Polypeptide is released and ribosome dissociates.
How to Read a Codon Table
1
Identify the mRNA sequence. If given DNA template, first transcribe it (complement + U for T).
2
Locate the start codon (AUG). This sets the reading frame: group into triplets from here.
3
Read the table: 1st base on the left column, 2nd base on the top row, 3rd base on the right column.
4
Continue until a stop codon (UAA, UAG, or UGA). The stop codon is not translated into an amino acid.
Example 1: mRNA to Protein
Example 2: DNA Template to Protein
Example 3: Point Mutation Effects
Common Mistakes to Avoid
✕ Reading the Wrong Strand
Codons are read from mRNA, not from the DNA template strand. Always transcribe DNA → mRNA first.
✕ Wrong Direction
mRNA is always read 5' → 3'. The template strand runs 3' → 5'. Mixing these up flips the entire sequence.
✕ Confusing Degenerate with Ambiguous
Degenerate means multiple codons → same AA. The code is never ambiguous: one codon = exactly one amino acid.
Summary at a Glance
| Concept | Key Fact | MCAT Significance |
|---|---|---|
| Codon size | 3 nucleotides | \( 4^3 = 64 \) combinations |
| Start codon | AUG (Met / fMet) | Sets reading frame |
| Stop codons | UAA, UAG, UGA | Bound by release factors |
| Degeneracy | Multiple codons → 1 AA | Wobble at 3rd position |
| Universality | Nearly all organisms | Exceptions in mitochondria |
| Frameshift | Insertion / deletion | Most severe mutation type |
Question 1 of 5
The genetic code is described as "degenerate." This means:
Degeneracy (redundancy) means that most amino acids are encoded by more than one codon. For example, Leucine has 6 codons. The code is never ambiguous: each codon specifies exactly one amino acid.
Question 2 of 5
A DNA template strand reads 3'-TAC GGA ACT-5'. What is the resulting mRNA sequence?
Transcription creates an mRNA strand that is complementary to the DNA template. The base pairing rules are: A (on DNA) pairs with U (in mRNA), T with A, C with G, and G with C. Therefore, the template 3'-TAC GGA ACT-5' is transcribed into 5'-AUG CCU UGA-3'.
Question 3 of 5
Which of the following is a stop codon?
The three stop codons are UAA, UAG, and UGA. AUG is the start codon (methionine). GUA codes for Valine, and CUG codes for Leucine.
Question 4 of 5
A single nucleotide insertion at position 4 of an mRNA coding sequence would most likely result in:
Inserting a single nucleotide shifts the reading frame for every codon downstream of the insertion. This is a frameshift mutation, which typically produces a completely different (and usually nonfunctional) protein. Frameshifts are generally more damaging than point substitutions.
Question 5 of 5
The wobble hypothesis explains why:
The wobble hypothesis states that the 3rd base of the codon (wobble position) can form non-standard base pairs with the 1st base of the anticodon. This allows one tRNA to recognize multiple codons that differ only at the 3rd position, explaining why fewer than 61 tRNAs are needed.