The logic is that the nucleotide code must be able to specify the placement of 20 amino acids. Since there are only four nucleotides, a code of single nucleotides would only represent four amino acids, such that A, C, G and U could be translated to encode amino acids.
A doublet code could code for 16 amino acids 4 x 4. A triplet code could make a genetic code for 64 different combinations 4 X 4 X 4 genetic code and provide plenty of information in the DNA molecule to specify the placement of all 20 amino acids. When experiments were performed to crack the genetic code it was found to be a code that was triplet. The discordance between the number of nucleic acid bases and the number of amino acids immediately eliminates the possibility of a code of one base per amino acid.
Thus, the smallest combination of four bases that could encode all 20 amino acids would be a triplet code. Thus, a triplet code introduces the problem of there being more than three times the number of codons than amino acids.
Either these "extra" codons produce redundancy, with multiple codons encoding the same amino acid, or there must instead be numerous dead-end codons that are not linked to any amino acid. Preliminary evidence indicating that the genetic code was indeed a triplet code came from an experiment by Francis Crick and Sydney Brenner This experiment examined the effect of frameshift mutations on protein synthesis. Frameshift mutations are much more disruptive to the genetic code than simple base substitutions, because they involve a base insertion or deletion, thus changing the number of bases and their positions in a gene.
For example, the mutagen proflavine causes frameshift mutations by inserting itself between DNA bases. The presence of proflavine in a DNA molecule thus interferes with the molecule's replication such that the resultant DNA copy has a base inserted or deleted. Crick and Brenner showed that proflavine-mutated bacteriophages viruses that infect bacteria with single-base insertion or deletion mutations did not produce functional copies of the protein encoded by the mutated gene.
The production of defective proteins under these circumstances can be attributed to misdirected translation. Mutant proteins with two- or four-nucleotide insertions or deletions were also nonfunctional. However, some mutant strains became functional again when they accumulated a total of three extra nucleotides or when they were missing three nucleotides.
This rescue effect provided compelling evidence that the genetic code for one amino acid is indeed a three-base, or triplet, code. However, at the time when this decoding project was conducted, researchers did not yet have the benefit of modern sequencing techniques. To circumvent this challenge, Marshall W. Nirenberg and Heinrich J.
Matthaei made their own simple, artificial mRNA and identified the polypeptide product that was encoded by it. To do this, they used the enzyme polynucleotide phosphorylase, which randomly joins together any RNA nucleotides that it finds. Nirenberg and Matthaei began with the simplest codes possible. Specifically, they added polynucleotide phosphorylase to a solution of pure uracil U , such that the enzyme would generate RNA molecules consisting entirely of a sequence of U's; these molecules were known as poly U RNAs.
These poly U RNAs were added to 20 tubes containing components for protein synthesis ribosomes , activating enzymes, tRNAs, and other factors. Each tube contained one of the 20 amino acids, which were radioactively labeled.
Of the 20 tubes, 19 failed to yield a radioactive polypeptide product. Only one tube, the one that had been loaded with the labeled amino acid phenylalanine, yielded a product. Nirenberg and Matthaei had therefore found that the UUU codon could be translated into the amino acid phenylalanine. These eight random poly AC RNAs produced proteins containing only six amino acids: asparagine, glutamine, histidine, lysine, proline, and threonine.
With the random sequence approach, the decoding endeavor was almost completed, but some work remained to be done. Thus, in , H. Gobind Khorana and his colleagues used another method to further crack the genetic code.
These researchers had the insight to employ chemically synthesized RNA molecules of known repeating sequences rather than random sequences. They showed that a short mRNA sequence—even a single codon three bases —could still bind to a ribosome , even if this short sequence was incapable of directing protein synthesis. The ribosome-bound codon could then base pair with a particular tRNA that carried the amino acid specified by the codon Figure 2.
Nirenberg and Leder thus synthesized many short mRNAs with known codons. They then added the mRNAs one by one to a mix of ribosomes and aminoacyl-tRNAs with one amino acid radioactively labeled. For each, they determined whether the aminoacyl-tRNA was bound to the short mRNA-like sequence and ribosome the rest passed through the filter , providing conclusive demonstrations of the particular aminoacyl-tRNA that bound to each mRNA codon. Examination of the full table of codons enables one to immediately determine whether the "extra" codons are associated with redundancy or dead-end codes Figure 3.
Note that both possibilities occur in the code. There are only a few instances in which one codon codes for one amino acid, such as the codon for tryptophan. Moreover, the genetic code also includes stop codons, which do not code for any amino acid. The stop codons serve as termination signals for translation. When a ribosome reaches a stop codon, translation stops, and the polypeptide is released.
Crick, F. General nature of the genetic code for proteins. Nature , — link to article. Jones, D. Further syntheses, in vitro, of copolypeptides containing two amino acids in alternating sequence dependent upon DNA-like polymers containing two nucleotides in alternating sequence. Journal of Molecular Biology 16 , — Chromatin Remodeling and DNase 1 Sensitivity.
Chromatin Remodeling in Eukaryotes. RNA Functions. Citation: Ralston, A. Nature Education 1 1 How can just four nitrogenous bases--adenine, cytosine, guanine, and uracil--possibly code for all 20 amino acids?
Aa Aa Aa. Figure Detail. Table 1: Did the code have commas or not? A non-overlapping code provided scientists with predictions they could test. Ruling Out Overlaps. Determining Codon Length. Figure 2: Frameshift. The letters A, B, and C each represent a different base of the nucleic acid. For simplicity a repeating sequence of bases, ABC, is shown. This would code for a polypeptide for which every amino-acid was the same.
A triplet code is assumed. The dotted lines represent the imaginary 'reading frame,' implying that the sequence is read in sets of three starting on the left. General Nature of the Genetic Code for Proteins. Nature , All rights reserved. References and Recommended Reading Crick, F. Article History Close. Share Cancel. Revoke Cancel. Keywords Keywords for this Article.
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