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Translation & the
Genetic Code

Discover how ribosomes read mRNA codons to assemble amino acid chains, turning nucleotide sequences into functional proteins.

Biochemistry

What Is Translation?

Translation is the process by which ribosomes decode a messenger RNA (mRNA) sequence into a polypeptide chain: a specific sequence of amino acids. It is the second major step of gene expression, following transcription.

The Key Players

mRNA
Carries the genetic message from DNA as a sequence of codons (three-nucleotide units).
tRNA
Adapter molecule with an anticodon on one end and an amino acid on the other.
Ribosome
Molecular machine (rRNA + protein) that catalyzes peptide bond formation.

From Gene to Protein

DNA Transcription mRNA Translation Protein Function tRNA carries amino acids

Translation converts the nucleotide language of mRNA (four letters: A, U, G, C) into the amino acid language of proteins (twenty letters). This is why we call it "translation": it is a change of chemical alphabet.

The Genetic Code: Codons

The genetic code is a set of rules mapping each three-nucleotide codon on mRNA to one amino acid (or a stop signal). With 4 bases taken 3 at a time, there are \( 4^3 = 64 \) possible codons encoding only 20 amino acids plus stop signals.

Degenerate
Multiple codons can specify the same amino acid (e.g., Leu has 6 codons).
Unambiguous
Each codon specifies only one amino acid: never two.
Universal
Nearly all organisms use the same code, with very rare exceptions.

Start and Stop Codons

5' 3' AUG START (Met) ...coding codons... UAA STOP Open Reading Frame (ORF)

The Reading Frame Matters

Codons are read in a non-overlapping, continuous fashion starting from the AUG start codon. Shifting the reading frame by even one nucleotide completely changes every downstream amino acid.

mRNA sequence: A U G C A U G C U A A Frame 1: Met His Ala Frame 2: Cys Met Leu Shifting by 1 base = entirely different protein!

Transcription vs Translation

Feature Transcription Translation
Template DNA (template strand) mRNA
Product mRNA Polypeptide (protein)
Location Nucleus (eukaryotes) Ribosome (cytoplasm / rER)
Enzyme RNA polymerase Ribosome (ribozyme)
Monomers Used Ribonucleotides (A, U, G, C) Amino acids (20 types)
Direction 5' → 3' (mRNA) N-terminus → C-terminus

Inside the Ribosome

Large Subunit (60S/50S) Small Subunit (40S/30S) 5' 3' mRNA A P E Exit Peptidyl Aminoacyl Growing polypeptide

Three Stages of Translation

1
Initiation: The small ribosomal subunit binds mRNA at the 5' cap, scans to the AUG start codon, initiator tRNA (carrying Met) binds, then the large subunit joins.
2
Elongation: A charged tRNA enters the A site, a peptide bond forms (catalyzed by rRNA), the ribosome translocates one codon forward, shifting tRNAs from A→P→E.
3
Termination: A stop codon enters the A site, a release factor binds (no tRNA matches), the polypeptide is freed, and the ribosome disassembles.

The Wobble Hypothesis

The third position of a codon (3' end) allows non-standard base pairing with the first position of the anticodon (5' end of tRNA). This "wobble" explains why multiple codons can be read by a single tRNA.

G C U mRNA codon (5' → 3') wobble C G I tRNA anticodon (3' → 5') I = Inosine Can pair with U, C, or A

How to Decode an mRNA Sequence

1
Find AUG: Locate the first AUG codon in the 5'→3' direction. This sets the reading frame.
2
Read in triplets: Divide the sequence into groups of three nucleotides starting from AUG. Do not skip or overlap.
3
Look up each codon: Use the codon table to match each triplet to its amino acid.
4
Stop at a stop codon: When you hit UAA, UAG, or UGA, the sequence ends. Write the polypeptide from N-terminus to C-terminus.

Example 1: Simple Decoding

Given mRNA: 5'-AUGUUUGCAGACUAA-3'

AUG UUU GCA GAC UAA Met Phe Ala Asp STOP Polypeptide: Met-Phe-Ala-Asp

Example 2: Finding the ORF

Given mRNA: 5'-GCCUCCAUGAAACGUUGACUUA-3'

1
Scan for AUG: Skip 5'-GCCUCC (not in frame). First AUG is at position 7.
2
Read triplets from AUG: AUG | AAA | CGU | UGA
3
Translate: Met - Lys - Arg - STOP

Example 3: Point Mutation Effects

Original mRNA codon: GAG (Glu). A single base change:

Silent
GAG → GAA: still codes for Glu. No change in protein (degeneracy).
Missense
GUG → Val: different amino acid. This exact change causes sickle cell disease.
Nonsense
GAG → UAG: creates a premature stop codon. Protein is truncated and usually nonfunctional.

Common Mistakes

✕ Reading from DNA directly
Translation reads mRNA, not DNA. You must first transcribe DNA→mRNA, then translate. Remember: use U (not T) in mRNA.
✕ Wrong reading frame
Always start reading from the AUG start codon. Grouping nucleotides without finding AUG first gives the wrong amino acid sequence.
✕ Confusing anticodon direction
Anticodons are written 3'→5', which is antiparallel to the mRNA codon (5'→3'). Reversing the direction gives incorrect base pairing.

Summary: Key Concepts

Concept Details
Codon 3-nucleotide unit on mRNA encoding one amino acid
Anticodon Complementary triplet on tRNA, reads mRNA codon
Start codon AUG: Met, initiates translation
Stop codons UAA, UAG, UGA: release factor binds
Degeneracy 64 codons → 20 amino acids + stops (redundancy)
Wobble Flexible 3rd codon position allows fewer tRNAs
Ribosome sites A (entry), P (peptidyl), E (exit)

Question 1 of 5

How many possible three-nucleotide codons can be formed from the four RNA bases (A, U, G, C)?

Question 2 of 5

Which of the following is a stop codon in the standard genetic code?

Question 3 of 5

Given the mRNA sequence 5'-AUGCCCUUUGAAUAA-3', what is the resulting polypeptide?

Question 4 of 5

A charged tRNA first enters which ribosome site during elongation?

Question 5 of 5

The genetic code is described as "degenerate." This means: