AP Syllabus focus:
‘Transfer RNA molecules deliver specific amino acids to the ribosome, matching their anticodons with mRNA codons to build a growing polypeptide chain.’
Transfer RNA (tRNA) is the molecular “translator” that converts nucleotide information in mRNA into an amino acid sequence.
Its specific shape and base-pairing rules ensure accurate protein construction at the ribosome.
Core role of tRNA in translation
tRNA (transfer RNA): An RNA molecule that carries a specific amino acid and uses a three-base anticodon to recognise complementary codons on mRNA at the ribosome.
tRNA links two kinds of specificity at once:
Chemical specificity: each tRNA is attached to one correct amino acid.
Information specificity: that tRNA’s anticodon base-pairs with an mRNA codon, positioning the amino acid for addition to the polypeptide.
Structure–function features that enable tRNA action
Anticodon–codon recognition
Anticodon: A three-nucleotide sequence on tRNA that base-pairs (antiparallel and complementary) with a codon on mRNA.
Anticodon binding is the key “reading” step that aligns the incoming amino acid with the next codon to be translated.
Binding depends on standard RNA base pairing (A–U, G–C).
Many amino acids have multiple codons; cells often use multiple tRNAs and flexible pairing at the third codon position (often called wobble) to match these codons.
Amino acid attachment site
Each tRNA has an acceptor stem (3′ end) where its specific amino acid is covalently attached, forming an aminoacyl-tRNA (“charged tRNA”). This attachment is what makes tRNA a true adaptor: the ribosome reads RNA, but builds a chain of amino acids.
Charging tRNA: ensuring the correct amino acid is delivered
Before tRNA can function at the ribosome, it must be “charged” by an enzyme called aminoacyl-tRNA synthetase.
Each synthetase recognises:
a particular amino acid
the corresponding tRNA(s) for that amino acid
The enzyme catalyses attachment of the amino acid to the 3′ end of the tRNA.
This figure depicts tRNA “charging” by aminoacyl-tRNA synthetase, including ATP consumption and covalent attachment of the correct amino acid to the tRNA. It makes explicit why charging is an energy-investing step and why correct charging is a major fidelity checkpoint before codon–anticodon selection at the ribosome. Source
High fidelity at this step is essential because the ribosome generally checks codon–anticodon pairing, not whether the amino acid itself is correct.
Key outcomes of charging:
Produces a pool of charged tRNAs ready to decode mRNA.
Helps maintain accurate translation because most synthetases can proofread and reject incorrect amino acids.
How tRNA builds a growing polypeptide chain at the ribosome
tRNAs interact with ribosomes in an ordered cycle that directly supports the syllabus focus: delivery, matching, and chain growth.
A charged tRNA enters the ribosome and its anticodon attempts to pair with the next mRNA codon.
Correct pairing stabilises the tRNA in place so its amino acid is positioned next to the growing chain.
The ribosome then forms a peptide bond, transferring the growing polypeptide onto the newly arrived amino acid (now at the end of the chain).
The ribosome shifts along the mRNA; tRNAs move through ribosomal positions (commonly described as A, P, and E sites), ensuring:
This diagram summarizes the ribosome’s three tRNA-binding sites (A, P, and E) and the directional flow of tRNAs during elongation. It visually links codon–anticodon pairing in the A site to peptide bond formation and translocation, helping explain how the ribosome advances one codon at a time while maintaining the correct reading frame. Source
continuous reading of codons in order
repeated delivery of the next correct amino acid
Energy use and fidelity linked to tRNA function
tRNA’s role is tightly connected to accuracy and energy investment:
Charging tRNA requires energy (from ATP) to create a high-energy bond that helps drive peptide bond formation.
Delivery and positioning of tRNA at the ribosome uses translation factors (often powered by GTP), which increases accuracy by favouring correct codon–anticodon matches.
Overall fidelity depends on two checkpoints:
correct amino acid–tRNA pairing (synthetase recognition/proofreading)
correct codon–anticodon pairing (ribosome selection)
FAQ
Not necessarily.
Because of wobble pairing at the third codon position, one tRNA can sometimes recognise multiple codons for the same amino acid.
tRNAs often contain modified bases (e.g., inosine).
These can stabilise structure and broaden or restrict codon recognition, affecting decoding efficiency and accuracy.
They read “identity elements” on tRNA (anticodon and structural features).
Some synthetases also proofread by hydrolysing incorrectly attached amino acids.
Yes.
A faulty anticodon or charging site can cause misincorporation of amino acids or slow translation, changing protein function and potentially altering traits.
Some drugs block tRNA entry or positioning on the ribosome.
Others interfere with factors that deliver tRNA, reducing accurate codon–anticodon matching and halting protein synthesis.
Practice Questions
State the role of tRNA in translation and describe how it ensures the correct amino acid is added. (2 marks)
tRNA delivers a specific amino acid to the ribosome (1)
anticodon base-pairs with complementary mRNA codon to ensure correct amino acid placement (1)
Explain how tRNA contributes to accurate polypeptide synthesis, from being charged with an amino acid to its interaction with mRNA at the ribosome. (5 marks)
aminoacyl-tRNA synthetase attaches the correct amino acid to the correct tRNA (1)
forms a charged aminoacyl-tRNA at the 3′ end/acceptor stem (1)
anticodon on tRNA pairs complementarily with codon on mRNA (1)
correct pairing positions the amino acid for peptide bond formation/growth of the polypeptide (1)
proofreading/checking increases fidelity (e.g., synthetase proofreading or ribosomal selection) (1)
