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.

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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.

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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.

mRNA codon:    5' — A U G — 3'
                   | | |
tRNA anticodon: 3' — U A C — 5'
(carries methionine)

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).

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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