Translation — Building Polypeptides at the Ribosome

Translation is the second stage of gene expression: the decoding of the mRNA codon sequence into a polypeptide (chain of amino acids). It takes place on ribosomes in the cytoplasm and requires tRNA, mRNA, amino acids and energy. This lesson covers the OCR A-Level Biology A specification point 2.1.3 (g) — an overview of the process of translation, including the roles of ribosomes, tRNA and peptide bond formation.

Translation is where the genetic information finally gets "cashed in" as proteins — the molecules that actually build and run the cell.

1. The Components of Translation

Component	Role
mRNA	Carries the genetic code as a sequence of codons (triplets of bases)
Ribosome	Molecular machine made of rRNA + proteins; holds mRNA and tRNAs in place and catalyses peptide bond formation
tRNA	Carries a specific amino acid and has an anticodon complementary to a codon
Amino acids	20 types; used as building blocks to make the polypeptide
ATP / GTP	Provide energy for attaching amino acids to tRNAs and for movement along the mRNA

Exam Tip: You can remember the components as "m, t, r, a" — mRNA, tRNA, ribosome, amino acids.

2. Codons and Anticodons

Recall from Lesson 5:

A codon is a sequence of three bases on mRNA that codes for a specific amino acid.
An anticodon is a sequence of three unpaired bases on tRNA that is complementary to a codon.

Complementary base pairing between codon and anticodon ensures the correct amino acid is brought to the correct position on the mRNA.

Strand	Direction	Base 1	Base 2	Base 3
mRNA codon	5' → 3'	A	U	G
tRNA anticodon	3' → 5'	U	A	C

The tRNA above carries methionine; complementary hydrogen-bonded pairing aligns each base of the anticodon with its codon partner (A–U, U–A, G–C).

Each codon is recognised by its complementary anticodon on a tRNA molecule that carries the correct amino acid. Because the code is degenerate, several different tRNAs can carry the same amino acid (but each tRNA is specific for one amino acid).

3. The Steps of Translation

graph TD
  A[mRNA binds to small ribosomal subunit] --> B[Ribosome locates start codon AUG]
  B --> C["tRNA with anticodon UAC and methionine<br/>binds to start codon in P site"]
  C --> D["Second tRNA with complementary anticodon<br/>binds to next codon in A site"]
  D --> E["Peptide bond forms between adjacent amino acids<br/>catalysed by the ribosome rRNA"]
  E --> F["Ribosome moves one codon along mRNA<br/>translocation"]
  F --> G["Empty tRNA released;<br/>next tRNA binds A site"]
  G --> H{Stop codon reached?}
  H -- no --> D
  H -- yes --> I[Polypeptide released]

3.1 Initiation

The mRNA binds to the small ribosomal subunit.
The ribosome scans along the mRNA until it finds the start codon (AUG).
A tRNA carrying methionine with the anticodon UAC pairs with AUG by complementary base pairing, hydrogen bonds forming between codon and anticodon.
The large ribosomal subunit now joins to form a complete ribosome with two binding sites for tRNA.

3.2 Elongation

The two tRNA binding sites are conventionally called the P site (peptidyl — holds the growing chain) and the A site (aminoacyl — accepts the next tRNA). You do not have to name them for OCR but it helps to keep track of the process.
A second tRNA — carrying the amino acid specified by the next codon — enters the A site. Its anticodon pairs with the mRNA codon by complementary base pairing.
A peptide bond now forms between the two amino acids. This condensation reaction is catalysed by the rRNA of the large ribosomal subunit (peptidyl transferase activity) and releases a water molecule.
The ribosome translocates — it moves along the mRNA by exactly one codon. The first tRNA (now empty of its amino acid) leaves the ribosome. The second tRNA, which now carries the growing polypeptide, moves from the A to the P site.
A third tRNA enters the vacated A site, bringing the next amino acid. Another peptide bond forms. The cycle repeats, with the polypeptide growing by one amino acid per codon.

3.3 Termination

When the ribosome reaches a stop codon (UAA, UAG or UGA) there is no matching tRNA. Instead, release factors bind.
The completed polypeptide is released from the ribosome.
The ribosome dissociates into its two subunits, which can then be reused.
The mRNA may be translated again by other ribosomes (many can translate the same mRNA at once — a "polysome") before eventually being degraded.

4. Peptide Bond Formation — A Condensation Reaction

Amino acids are joined by peptide bonds between the carboxyl (–COOH) group of one amino acid and the amino (–NH₂) group of the next, with the elimination of water.

This is the same reaction you studied in the biological molecules topic for protein formation — but now you know that it happens at the ribosome and is catalysed by the rRNA of the large subunit.

Each peptide bond formation is a condensation reaction releasing one molecule of water.

Exam Tip: Always write "peptide bond formed by condensation, releasing water". Do not write "protein bond" or just "bond".

5. Energy Requirements

Translation requires a lot of energy:

Attaching each amino acid to its tRNA (by aminoacyl-tRNA synthetase) uses one ATP (becoming AMP + PPi). This is done before the tRNA reaches the ribosome.
Each step of translocation along the mRNA uses one GTP.
Therefore a polypeptide of n amino acids requires several n phosphate-bond hydrolyses' worth of energy — much more than it takes to join the amino acids directly.

You do not need to know the exact numbers but you should be aware that translation is energetically expensive — which is why cells tightly regulate protein synthesis and only make the proteins they need.

6. The Overall Flow of Genetic Information

Translation is the second step in the central dogma of molecular biology:

graph LR
  A[DNA] -- transcription --> B[mRNA]
  B -- translation --> C[Polypeptide]
  C --> D[Folded protein]
  D --> E[Function]

Transcription (nucleus): DNA → mRNA.
Translation (cytoplasm, at the ribosome): mRNA → polypeptide.
After translation, the polypeptide folds into its 3D tertiary structure and may join other polypeptides to form a quaternary structure. It may also be modified (e.g. glycosylation, phosphorylation) before becoming a functional protein.

7. Common Exam Mistakes

Confusing codon and anticodon. Codon = mRNA, anticodon = tRNA.
Writing that "DNA is translated". mRNA is translated, not DNA.
Forgetting that the ribosome (specifically rRNA) catalyses peptide bond formation.
Saying amino acids join by "hydrogen bonds". They join by peptide bonds (covalent) formed by condensation.
Forgetting to mention that translation requires energy (ATP/GTP).
Forgetting the start codon (AUG) and stop codons (UAA, UAG, UGA).
Saying translation happens in the nucleus. It happens at the ribosome in the cytoplasm (or on the rER).

8. Exam-Style Questions

Describe the process of translation. (6)
Explain the role of tRNA in translation. (3)
A polypeptide is 100 amino acids long. Calculate the minimum number of bases in the mRNA molecule from which it was translated. (2)
Describe how a peptide bond is formed during translation. (3)

Model answers:

(3) 100 amino acids × 3 bases per codon = 300 bases. Plus 3 bases for the stop codon = 303 bases. (The stop codon does not code for an amino acid but is part of the mRNA.)
(4) "A peptide bond forms between the carboxyl group of one amino acid in the P site and the amino group of the next amino acid in the A site. This is a condensation reaction that releases a water molecule, and is catalysed by the rRNA of the large ribosomal subunit."

Summary

Translation is the decoding of mRNA codons into a polypeptide at the ribosome.
A ribosome (made of rRNA + protein) holds the mRNA and tRNAs in place.
Each tRNA carries a specific amino acid and has an anticodon complementary to a codon.
Amino acids are joined by peptide bonds formed by condensation reactions catalysed by the rRNA.
Translation begins at the start codon (AUG), elongates one codon at a time, and ends at a stop codon (UAA, UAG, UGA).
The process requires ATP and GTP and is energetically expensive, which is why protein synthesis is tightly regulated.

A-Level Deep Dive

Spec mapping

Spec Mapping: This lesson is mapped to OCR H420 Module 2.1.3 — Nucleotides and nucleic acids, covering the synthesis of polypeptides at the ribosome from an mRNA template, including the roles of tRNA, rRNA and the start/stop codons (refer to the official OCR H420 specification document for exact wording).

Translation is the final step of the central dogma — the molecular act of decoding the codon table to assemble a polypeptide from amino acids. The mechanism, the codon–anticodon relationship and the role of the ribosome are repeatedly tested. This lesson is the conceptual bridge between molecular biology (Module 2.1.3) and the protein-structure content of Module 2.1.2 — what you read off the codon table becomes the primary structure of a protein.

Scientists and paradigms

Francis Crick's adaptor hypothesis (1958): proposed before tRNA was isolated experimentally. Crick argued that there must be an adaptor molecule that physically connects an amino acid to its corresponding codon — a molecule with two distinct chemical recognition surfaces. tRNA, isolated by Mahlon Hoagland and colleagues in 1958, was that adaptor. The school of thought to take into the exam: paraphrase as "an amino acid cannot read a codon directly — it needs an adaptor".

Robert Holley (1965) solved the first tRNA sequence (yeast alanine tRNA), revealing the clover-leaf secondary structure. He shared the 1968 Nobel Prize with Nirenberg and Khorana for this work.

Lesson 7: Translation — Building Polypeptides at the Ribosome