A Little Wobble Hypothesis Goes a Long Way: Explaining Codon-Anticodon Interactions
The Genetic Basis of Stuff and Things
25,332 views
How can a single transfer RNA read more than one codon in the genetic code? After Marshall Nirenberg cracked the code in the 1960s, testing all 64 trinucleotides against the 20 amino acids, a puzzle remained: only 61 codons specify an amino acid, and many amino acids are encoded by two, three, or four synonymous codons that differ only in their third base. This built-in redundancy, along with Robert Holley's finding that synonymous codons often bind the same transfer RNA (tRNA), suggested that pairing at the third position breaks the usual rules. Francis Crick, who helped devise those rules, explained it in 1966 with his wobble hypothesis: the first base of the tRNA anticodon can shift slightly in three-dimensional space, letting it form non-standard pairs such as guanine–uracil and inosine with uracil or adenine, on top of ordinary Watson–Crick pairing. Here I walk through how Crick reasoned out which pairings are chemically and geometrically possible, why he settled on six binding combinations, and the assumptions about #anticodon sequences that no tRNA has yet violated. I finish with what wobble means in practice, from the minimum of 32 tRNAs it predicts to real counts like 36 in Methanocaldococcus jannaschii, 44 in flies and 51 in humans, and how #codonbias shapes that variation. This is the story of the #WobbleHypothesis and the elegant logic behind it.
Creator: Tobias Massang
References
Holley RW, Apgar J, Everett GA, Madison JT, Marquisee M, Merrill SH, Penswick JR, Zamir A. 1965. Structure of a ribonucleic acid. Science 147(3664):1462–1465. https://doi.org/10.1126/science.147.3...
Iben JR, Maraia RJ. 2014. tRNA gene copy number variation in humans. Gene 536(2):376–384. https://doi.org/10.1016/j.gene.2013.1...
Lander ES, Linton LM, Birren B, Nusbaum C, Zody MC, Baldwin J, Devon K, Dewar K, Doyle M, FitzHugh W, et al. 2001. Initial sequencing and analysis of the human genome. Nature 409(6822):860–921. https://doi.org/10.1038/35057062
Nirenberg M, Leder P. 1964. RNA codewords and protein synthesis. The effect of trinucleotides upon the binding of sRNA to ribosomes. Science 145(3639):1399–1407. https://doi.org/10.1126/science.145.3...
Nirenberg MW, Matthaei JH. 1961. The dependence of cell-free protein synthesis in E. coli upon naturally occurring or synthetic polyribonucleotides. Proc Natl Acad Sci U S A 47(10):1588–1602. https://doi.org/10.1073/pnas.47.10.1588
Rocha EPC. 2004. Codon usage bias from tRNA's point of view: redundancy, specialization, and efficient decoding for translation optimization. Genome Res 14(11):2279–2286. https://doi.org/10.1101/gr.2896904
Rogers HH, Bergman CM, Griffiths-Jones S. 2010. The evolution of tRNA genes in Drosophila. Genome Biol Evol 2:467–477. https://doi.org/10.1093/gbe/evq034
A Little Wobble Hypothesis Goes a Long Way: Explaining Codon-Anticodon Interactions
The Genetic Basis of Stuff and Things
25,332 views
How can a single transfer RNA read more than one codon in the genetic code? After Marshall Nirenberg cracked the code in the 1960s, testing all 64 trinucleotides against the 20 amino acids, a puzzle remained: only 61 codons specify an amino acid, and many amino acids are encoded by two, three, or four synonymous codons that differ only in their third base. This built-in redundancy, along with Robert Holley's finding that synonymous codons often bind the same transfer RNA (tRNA), suggested that pairing at the third position breaks the usual rules. Francis Crick, who helped devise those rules, explained it in 1966 with his wobble hypothesis: the first base of the tRNA anticodon can shift slightly in three-dimensional space, letting it form non-standard pairs such as guanine–uracil and inosine with uracil or adenine, on top of ordinary Watson–Crick pairing. Here I walk through how Crick reasoned out which pairings are chemically and geometrically possible, why he settled on six binding combinations, and the assumptions about #anticodon sequences that no tRNA has yet violated. I finish with what wobble means in practice, from the minimum of 32 tRNAs it predicts to real counts like 36 in Methanocaldococcus jannaschii, 44 in flies and 51 in humans, and how #codonbias shapes that variation. This is the story of the #WobbleHypothesis and the elegant logic behind it.
Creator: Tobias Massang
References
Holley RW, Apgar J, Everett GA, Madison JT, Marquisee M, Merrill SH, Penswick JR, Zamir A. 1965. Structure of a ribonucleic acid. Science 147(3664):1462–1465. https://doi.org/10.1126/science.147.3...
Iben JR, Maraia RJ. 2014. tRNA gene copy number variation in humans. Gene 536(2):376–384. https://doi.org/10.1016/j.gene.2013.1...
Lander ES, Linton LM, Birren B, Nusbaum C, Zody MC, Baldwin J, Devon K, Dewar K, Doyle M, FitzHugh W, et al. 2001. Initial sequencing and analysis of the human genome. Nature 409(6822):860–921. https://doi.org/10.1038/35057062
Nirenberg M, Leder P. 1964. RNA codewords and protein synthesis. The effect of trinucleotides upon the binding of sRNA to ribosomes. Science 145(3639):1399–1407. https://doi.org/10.1126/science.145.3...
Nirenberg MW, Matthaei JH. 1961. The dependence of cell-free protein synthesis in E. coli upon naturally occurring or synthetic polyribonucleotides. Proc Natl Acad Sci U S A 47(10):1588–1602. https://doi.org/10.1073/pnas.47.10.1588
Rocha EPC. 2004. Codon usage bias from tRNA's point of view: redundancy, specialization, and efficient decoding for translation optimization. Genome Res 14(11):2279–2286. https://doi.org/10.1101/gr.2896904
Rogers HH, Bergman CM, Griffiths-Jones S. 2010. The evolution of tRNA genes in Drosophila. Genome Biol Evol 2:467–477. https://doi.org/10.1093/gbe/evq034