Lesson 9 of 13 · 8 min
tRNA and translation
NCERT § "Genetic Code" and § "Translation"
Something has to physically match a three-base codon on mRNA to the right amino acid — nothing about a codon's shape 'looks like' glutamic acid or valine. Francis Crick guessed such a go-between had to exist before anyone had seen it.
The lesson in notes
In short
Francis Crick postulated an adapter molecule that could read the code and also bind amino acids; this is tRNA (earlier called sRNA, soluble RNA).
tRNA has an anticodon loop with bases complementary to the codon and an amino-acid acceptor end. Each amino acid has its own specific tRNA, there is a special initiator tRNA, and there are no tRNAs for stop codons.
Its secondary structure is drawn as a clover-leaf; in three dimensions it is a compact inverted L shape.
Translation first needs amino acids to be activated using ATP and joined to their matching tRNA; this is charging of tRNA, or aminoacylation. Forming the peptide bond needs energy.
The ribosome, the protein factory of the cell, consists of about 80 different proteins together with structural RNAs. An inactive ribosome exists as separate large and small subunits; translation starts when the small subunit meets an mRNA.
The large subunit has two sites where amino acids carried by tRNAs are brought near enough to each other to form a peptide bond. The ribosome is also a catalyst: in bacteria, the enzyme is the 23S rRNA (a ribozyme).
A translational unit in mRNA runs from a start codon (AUG) to a stop codon. Untranslated regions (UTRs) at the 5′ end before the start codon and at the 3′ end after the stop codon are not translated but are needed for efficient translation.
Initiation: the ribosome binds the mRNA at AUG, which only the initiator tRNA recognises. Elongation: charged tRNAs bind their codons in turn by base pairing with their anticodons, and the ribosome moves codon to codon adding amino acids. Termination: a release factor binds the stop codon, ending translation and freeing the polypeptide.