Protein Synthesis (Translation and the Genetic Code)
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Direct answer
Translation is the process by which the base sequence of mRNA is decoded on ribosomes to build a polypeptide, following the genetic code in which each triplet of bases specifies one amino acid. A charged tRNA, carrying an amino acid at its acceptor end and the matching anticodon in its anticodon loop, reads each codon, peptide bonds join the amino acids, and translation stops at a termination codon. The code is triplet, degenerate, nearly universal and read continuously, with AUG serving as both the start codon and the codon for methionine.
What you must remember
- The code: 64 triplets — 61 sense codons specifying amino acids and 3 nonsense or stop codons (UAA, UAG and UGA) that terminate translation.
- AUG: dual-function codon coding for methionine and acting as the initiator codon; rarely, GUG also serves as an initiator, still inserting methionine.
- Properties: the code is a triplet code, degenerate (several codons may specify one amino acid), unambiguous (one codon specifies only one amino acid), comma-less, collinear with the polypeptide and nearly universal from bacteria to humans.
- The adaptor: tRNA has an anticodon loop for base pairing with mRNA and an amino acid acceptor end to which the correct amino acid is attached — the translator of the genetic code.
- Charging: aminoacylation of tRNA, carried out by specific enzymes using ATP energy, is essential because the specificity of codon recognition alone would not guarantee the right amino acid.
- Stages: initiation (ribosome assembles at AUG with the initiator tRNA bearing methionine), elongation (charged tRNAs deliver amino acids, peptide bonds form and the ribosome advances along the mRNA) and termination (a release factor acts at a stop codon and the polypeptide is released).
- UTRs: untranslated regions at both ends of mRNA are not translated but assist efficient translation; protein synthesis is energy-expensive.
Common confusion
Degeneracy does not mean ambiguity. A single codon always specifies one amino acid (unambiguous), but most amino acids are specified by more than one codon (degenerate). Students also mix codon with anticodon — the codon lies on mRNA, the anticodon on tRNA — and forget that the stop codons do not code for any amino acid at all; they are read by release factors, not by tRNA.
Exam-focused takeaway
NEET-UG tests the salient features of the genetic code as statement questions, asks which codons are stop signals, and probes the adaptor role and two functional ends of tRNA. Sequences of the three phases of translation, the role of ATP-dependent charging, and the significance of UTRs appear in assertion-reason formats. Remember AUG's dual role and the three stop codons verbatim — they are among the most repeated single-fact questions in molecular genetics.
Frequently asked questions
What are the three stop codons?
UAA, UAG and UGA — nonsense codons that signal termination and do not specify any amino acid.
What makes the genetic code degenerate?
Most amino acids are specified by two or more codons, so some base changes, particularly at the third position, do not alter the amino acid inserted.
What is the role of tRNA in translation?
It acts as the adaptor molecule, using its anticodon loop to read the mRNA codon and its amino acid acceptor end to carry the corresponding amino acid.
Why is charging of tRNA necessary?
Aminoacylation ensures each tRNA carries its specific amino acid; since neither the codon nor the anticodon can recognise amino acids directly, this enzyme-driven step, powered by ATP, maintains the fidelity of translation.
Where does translation begin on an mRNA?
At the AUG start codon, read by the initiator tRNA carrying methionine, with the ribosome assembling guided by upstream untranslated sequences.