Translation and the Genetic Code
How the ribosome reads a messenger RNA three bases at a time and builds the corresponding protein, and how the genetic code maps codons to amino acids.
Definition
Translation is the ribosome-catalysed synthesis of a polypeptide from the codon sequence of a messenger RNA; the genetic code is the set of rules by which nucleotide triplets (codons) specify the amino acids and stop signals that define a protein.
Scope
This area covers the decoding of mRNA into protein and the code that underlies it. It spans the genetic code and its properties, the structure and catalytic role of the ribosome, the transfer RNAs and the aminoacyl-tRNA synthetases that charge them, and the initiation, elongation, and termination phases of translation. Post-translational modification and folding are noted as neighbours rather than developed here.
Sub-topics
Core questions
- How are nucleotide triplets matched to specific amino acids?
- What is the structure of the ribosome and how does it catalyse peptide bond formation?
- How do transfer RNAs carry the right amino acid to the right codon?
- How does translation start, elongate, and stop accurately?
Key theories
- Triplet, nearly universal genetic code
- Each amino acid is specified by one or more three-nucleotide codons, a code that is degenerate and largely shared across life, established by cell-free synthesis experiments that decoded the first codons.
- Central dogma — RNA to protein
- Translation realises the protein-directing step of the central dogma, converting the sequence information carried by mRNA into the amino-acid sequence of a protein.
Mechanisms
Aminoacyl-tRNA synthetases attach each amino acid to its cognate tRNA, whose anticodon matches the corresponding mRNA codon. The small ribosomal subunit, with initiation factors, locates the start codon; the large subunit then joins, and the ribosome moves codon by codon, catalysing peptide bond formation between the growing chain and each incoming aminoacyl-tRNA at its catalytic centre. Elongation factors deliver tRNAs and drive translocation, and release factors recognise stop codons to free the completed protein.
Clinical relevance
The translation apparatus is the target of many antibiotics that exploit differences between bacterial and human ribosomes, and code-reading errors and tRNA defects contribute to disease; given as significance, not clinical guidance.
History
The genetic code was deciphered in the early-to-mid 1960s through cell-free synthesis with synthetic RNAs by Nirenberg and Matthaei and codon assignment work by Khorana and others; subsequent structural studies of the ribosome revealed it to be a ribozyme, completing the modern account of translation.
Key figures
- Marshall Nirenberg
- Francis Crick
- Har Gobind Khorana
- Ada Yonath
Related topics
Seminal works
- nirenberg1961
- crick1970
- watson2013
Frequently asked questions
- Why is the genetic code called degenerate?
- Because most amino acids are specified by more than one codon, so several different triplets can encode the same amino acid.
- Is the genetic code the same in all organisms?
- It is nearly universal, with the same codon assignments across most life, though a few organelles and organisms use minor variations.