The genetic code is the set of rules by which information encoded in genetic material (DNA or mRNA sequences) is translated into proteins (amino acid sequences) by living cells.
The code defines how sequences of three nucleotides, called codons, specify which amino acid will be added next during protein synthesis. With some exceptions,[1] a three-nucleotide codon in a nucleic acid sequence specifies a single amino acid. Because the vast majority of genes are encoded with exactly the same code (see the RNA codon table), this particular code is often referred to as the canonical or standard genetic code, or simply the genetic code, though in fact some variant codes have evolved. For example, protein synthesis in human mitochondria relies on a genetic code that differs from the standard genetic code.
Not all genetic information is stored using the genetic code. All organisms' DNA contains regulatory sequences, intergenic segments, chromosomal structural areas, and other non-coding DNA that can contribute greatly to phenotype. Those elements operate under sets of rules that are distinct from the codon-to-amino acid paradigm underlying the genetic code.
Contents |
Serious efforts to understand how proteins are encoded began after the structure of DNA was discovered by James Watson and Francis Crick, who used the experimental evidence of Maurice Wilkins and Rosalind Franklin (among others). George Gamow postulated that a three-letter code must be employed to encode the 20 standard amino acids used by living cells to encode proteins. With four different nucleotides, a code of 2 nucleotides could only code for a maximum of 42 or 16 amino acids. A code of 3 nucleotides could code for a maximum of 43 or 64 amino acids.[2]
The Crick, Brenner et al. experiment was the first to demonstrate that codons consist of three DNA bases. Marshall Nirenberg and Heinrich J. Matthaei were the first to elucidate the nature of a codon in 1961 at the National Institutes of Health. They used a cell-free system to translate a poly-uracil RNA sequence (i.e., UUUUU...) and discovered that the polypeptide that they had synthesized consisted of only the amino acid phenylalanine. They thereby deduced that the codon UUU specified the amino acid phenylalanine. This was followed by experiments in Severo Ochoa's laboratory that demonstrated that the poly-adenine RNA sequence (AAAAA...) coded for the polypeptide poly-lysine[3] and that the poly-cytosine RNA sequence (CCCCC...) coded for the polypeptide poly-proline.[4] Therefore the codon AAA specified the amino acid lysine, and the codon CCC specified the amino acid proline. Using different copolymers most of the remaining codons were then determined. Subsequent work by Har Gobind Khorana identified the rest of the genetic code. Shortly thereafter, Robert W. Holley determined the structure of transfer RNA (tRNA), the adapter molecule that facilitates the process of translating RNA into protein. This work was based upon earlier studies by Severo Ochoa, who received the Nobel prize in 1959 for his work on the enzymology of RNA synthesis.[5]
Extending this work, Nirenberg and Philip Leder revealed the triplet nature of the genetic code and deciphered the codons of the standard genetic code. In these experiments, various combinations of mRNA were passed through a filter that contained ribosomes, the components of cells that translate RNA into protein. Unique triplets promoted the binding of specific tRNAs to the ribosome. Leder and Nirenberg were able to determine the sequences of 54 out of 64 codons in their experiments.[6] In 1968, Khorana, Holley and Nirenberg received the Nobel Prize in Physiology or Medicine for their work.[7]
A codon is defined by the initial nucleotide from which translation starts. For example, the string GGGAAACCC, if read from the first position, contains the codons GGG, AAA, and CCC; and, if read from the second position, it contains the codons GGA and AAC; if read starting from the third position, GAA and ACC. Every sequence can, thus, be read in three reading frames, each of which will produce a different amino acid sequence (in the given example, Gly-Lys-Pro, Gly-Asn, or Glu-Thr, respectively). With double-stranded DNA, there are six possible reading frames, three in the forward orientation on one strand and three reverse on the opposite strand.[8]:330 The actual frame in which a protein sequence is translated is defined by a start codon, usually the first AUG codon in the mRNA sequence.
Translation starts with a chain initiation codon (start codon). Unlike stop codons, the codon alone is not sufficient to begin the process. Nearby sequences (such as the Shine-Dalgarno sequence in E. coli) and initiation factors are also required to start translation. The most common start codon is AUG, which is read as methionine or, in bacteria, as formylmethionine. Alternative start codons (depending on the organism), include "GUG" or "UUG"; these codons normally represent valine and leucine, respectively, but, as a start codon, they are translated as methionine or formylmethionine.[9]
The three stop codons have been given names: UAG is amber, UGA is opal (sometimes also called umber), and UAA is ochre. "Amber" was named by discoverers Richard Epstein and Charles Steinberg after their friend Harris Bernstein, whose last name means "amber" in German. The other two stop codons were named "ochre" and "opal" in order to keep the "color names" theme. Stop codons are also called "termination" or "nonsense" codons. They signal release of the nascent polypeptide from the ribosome because there is no cognate tRNA that has anticodons complementary to these stop signals, and so a release factor binds to the ribosome instead.[10]
During the process of DNA replication, errors occasionally occur in the polymerization of the second strand. These errors, called mutations, can have an impact on the phenotype of an organism, especially if they occur within the protein coding sequence of a gene. Error rates are usually very low—1 error in every 10–100 million bases—due to the "proofreading" ability of DNA polymerases.[12][13]
Missense mutations and nonsense mutations are examples of point mutations, which can cause genetic diseases such as sickle-cell disease and thalassemia respectively.[14][15][16] Clinically important missense mutations generally change the properties of the coded amino acid residue between being basic, acidic polar or non-polar, whereas nonsense mutations result in a stop codon.[8]:266
Mutations that disrupt the reading frame sequence by indels (insertions or deletions) of a non-multiple of 3 nucleotide bases are known as frameshift mutations. These mutations usually result in a completely different translation from the original, and are also very likely to cause a stop codon to be read, which truncates the creation of the protein.[17] These mutations may impair the function of the resulting protein, and are thus rare in in vivo protein-coding sequences. One reason inheritance of frameshift mutations is rare is that, if the protein being translated is essential for growth under the selective pressures the organism faces, absence of a functional protein may cause death before the organism is viable.[18] Frameshift mutations may result in severe genetic diseases such as Tay-Sachs disease.[19]
Although most mutations that change protein sequences are harmful or neutral, some mutations have a positive effect on an organism.[20] These mutations may enable the mutant organism to withstand particular environmental stresses better than wild-type organisms, or reproduce more quickly. In these cases a mutation will tend to become more common in a population through natural selection.[21] Viruses that use RNA as their genetic material have rapid mutation rates,[22] which can be an advantage, since these viruses will evolve constantly and rapidly, and thus evade the defensive responses of e.g. the human immune system.[23] In large populations of asexually reproducing organisms, for example, E. coli, multiple beneficial mutations may co-occur. This phenomenon is called clonal interference and causes competition among the mutations.[24]
Degeneracy is the redundancy of the genetic code. The genetic code has redundancy but no ambiguity (see the codon tables above for the full correlation). For example, although codons GAA and GAG both specify glutamic acid (redundancy), neither of them specifies any other amino acid (no ambiguity). The codons encoding one amino acid may differ in any of their three positions. For example the amino acid glutamic acid is specified by GAA and GAG codons (difference in the third position), the amino acid leucine is specified by UUA, UUG, CUU, CUC, CUA, CUG codons (difference in the first or third position), while the amino acid serine is specified by UCA, UCG, UCC, UCU, AGU, AGC (difference in the first, second, or third position).[25]:521–522
A position of a codon is said to be a fourfold degenerate site if any nucleotide at this position specifies the same amino acid. For example, the third position of the glycine codons (GGA, GGG, GGC, GGU) is a fourfold degenerate site, because all nucleotide substitutions at this site are synonymous; i.e., they do not change the amino acid. Only the third positions of some codons may be fourfold degenerate.[25]:521–522 A position of a codon is said to be a twofold degenerate site if only two of four possible nucleotides at this position specify the same amino acid. For example, the third position of the glutamic acid codons (GAA, GAG) is a twofold degenerate site. In twofold degenerate sites, the equivalent nucleotides are always either two purines (A/G) or two pyrimidines (C/U), so only transversional substitutions (purine to pyrimidine or pyrimidine to purine) in twofold degenerate sites are nonsynonymous.[25]:521–522 A position of a codon is said to be a non-degenerate site if any mutation at this position results in amino acid substitution. There is only one threefold degenerate site where changing to three of the four nucleotides may have no effect on the amino acid (depending on what it is changed to), while changing to the fourth possible nucleotide always results in an amino acid substitution. This is the third position of an isoleucine codon: AUU, AUC, or AUA all encode isoleucine, but AUG encodes methionine. In computation this position is often treated as a twofold degenerate site.[25]:521–522
There are three amino acids encoded by six different codons: serine, leucine, and arginine. Only two amino acids are specified by a single codon. One of these is the amino-acid methionine, specified by the codon AUG, which also specifies the start of translation; the other is tryptophan, specified by the codon UGG. The degeneracy of the genetic code is what accounts for the existence of synonymous mutations.[25]:Chp 15
Degeneracy results because there are more codons than encodable amino acids. For example, if there were two bases per codon, then only 16 amino acids could be coded for (4²=16). Because at least 21 codes are required (20 amino acids plus stop) and the next largest number of bases is three, then 4³ gives 64 possible codons, meaning that some degeneracy must exist.[25]:521–522
These properties of the genetic code make it more fault-tolerant for point mutations. For example, in theory, fourfold degenerate codons can tolerate any point mutation at the third position, although codon usage bias restricts this in practice in many organisms; twofold degenerate codons can tolerate one out of the three possible point mutations at the third position. Since transition mutations (purine to purine or pyrimidine to pyrimidine mutations) are more likely than transversion (purine to pyrimidine or vice-versa) mutations, the equivalence of purines or that of pyrimidines at twofold degenerate sites adds a further fault-tolerance.[25]:531–532
A practical consequence of redundancy is that some errors in the genetic code cause only a silent mutation or an error that would not affect the protein because the hydrophilicity or hydrophobicity is maintained by equivalent substitution of amino acids; for example, a codon of NUN (where N = any nucleotide) tends to code for hydrophobic amino acids. NCN yields amino acid residues that are small in size and moderate in hydropathy; NAN encodes average size hydrophilic residues.[26][27] These tendencies may result from the shared ancestry of the aminoacyl tRNA synthetases related to these codons.
Despite the redundancy of the genetic code, single-point mutations can still cause dysfunctional proteins. For example, a mutated hemoglobin gene causes sickle-cell disease. In the mutant hemoglobin, a hydrophilic glutamate (Glu) is substituted by the hydrophobic valine (Val); that is, GAA or GAG becomes GUA or GUG. The substitution of glutamate by valine reduces the solubility of β-globin, which causes hemoglobin to form linear polymers linked by the hydrophobic interaction between the valine groups, causing sickle-cell deformation of erythrocytes. In general, sickle-cell disease is not caused by a de novo mutation. It is, rather, selected for in geographic regions where malaria is common (in a way similar to thalassemia), as heterozygous people have some resistance to the malarial Plasmodium parasite (heterozygote advantage).[28]
These variable codes for amino acids are allowed because of modified bases in the first base of the anticodon of the tRNA, and the base-pair formed is called a wobble base pair. The modified bases include inosine and the Non-Watson-Crick U-G basepair.[29]
The genome of an organism is inscribed in DNA, or, in the case of some viruses, RNA. The portion of the genome that codes for a protein or an RNA is called a gene. Those genes that code for proteins are composed of tri-nucleotide units called codons, each coding for a single amino acid. Each nucleotide sub-unit consists of a phosphate, a deoxyribose sugar, and one of the four nitrogenous nucleobases. The purine bases adenine (A) and guanine (G) are larger and consist of two aromatic rings. The pyrimidine bases cytosine (C) and thymine (T) are smaller and consist of only one aromatic ring. In the double-helix configuration, two strands of DNA are joined to each other by hydrogen bonds in an arrangement known as base pairing. These bonds almost always form between an adenine base on one strand and a thymine base on the other strand, or between a cytosine base on one strand and a guanine base on the other. This means that the number of A and T bases will be the same in a given double helix, as will the number of G and C bases.[25]:102–117 In RNA, thymine (T) is replaced by uracil (U), and the deoxyribose is substituted by ribose.[25]:127
Each protein-coding gene is transcribed into a molecule of the related RNA polymer. In prokaryotes, this RNA functions as messenger RNA or mRNA; in eukaryotes, the transcript needs to be processed to produce a mature mRNA. The mRNA is, in turn, translated on a ribosome into a chain of amino acids otherwise known as a polypeptide.[25]:Chp 12 The process of translation requires transfer RNAs which are covalently attached to a specific amino acid, guanosine triphosphate as an energy source, and a number of translation factors. tRNAs have anticodons complementary to the codons in an mRNA and can be covalently "charged" with specific amino acids at their 3' terminal CCA ends by enzymes known as aminoacyl tRNA synthetases, which have high specificity for both their cognate amino acid and tRNA. The high specificity of these enzymes is a major reason why the fidelity of protein translation is maintained.[25]:464–469
There are 4³ = 64 different codon combinations possible with a triplet codon of three nucleotides; all 64 codons are assigned to either an amino acid or a stop signal. If, for example, an RNA sequence UUUAAACCC is considered and the reading frame starts with the first U (by convention, 5' to 3'), there are three codons, namely, UUU, AAA, and CCC, each of which specifies one amino acid. Therefore, this 9 base RNA sequence will be translated into an amino acid sequence that is three amino acids long.[25]:521–539 A given amino acid may be encoded by between one and six different codon sequences. A comparison may be made using bioinformatics tools wherein the codon is similar to a word, which is the standard data "chunk" and a nucleotide is similar to a bit, in that it is the smallest unit. This allows for powerful comparisons across species as well as within organisms.
The standard genetic code is shown in the following tables. Table 1 shows which amino acid each of the 64 codons specifies. Table 2 shows which codons specify each of the 20 standard amino acids involved in translation. These are called forward and reverse codon tables, respectively. For example, the codon "AAU" represents the amino acid asparagine, and "UGU" and "UGC" represent cysteine (standard three-letter designations, Asn and Cys, respectively).[25]:522
| nonpolar | polar | basic | acidic | (stop codon) |
| 2nd base | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| U | C | A | G | ||||||
| 1st base | U | UUU | (Phe/F) Phenylalanine | UCU | (Ser/S) Serine | UAU | (Tyr/Y) Tyrosine | UGU | (Cys/C) Cysteine |
| UUC | (Phe/F) Phenylalanine | UCC | (Ser/S) Serine | UAC | (Tyr/Y) Tyrosine | UGC | (Cys/C) Cysteine | ||
| UUA | (Leu/L) Leucine | UCA | (Ser/S) Serine | UAA | Stop (Ochre) | UGA | Stop (Opal) | ||
| UUG | (Leu/L) Leucine | UCG | (Ser/S) Serine | UAG | Stop (Amber) | UGG | (Trp/W) Tryptophan | ||
| C | CUU | (Leu/L) Leucine | CCU | (Pro/P) Proline | CAU | (His/H) Histidine | CGU | (Arg/R) Arginine | |
| CUC | (Leu/L) Leucine | CCC | (Pro/P) Proline | CAC | (His/H) Histidine | CGC | (Arg/R) Arginine | ||
| CUA | (Leu/L) Leucine | CCA | (Pro/P) Proline | CAA | (Gln/Q) Glutamine | CGA | (Arg/R) Arginine | ||
| CUG | (Leu/L) Leucine | CCG | (Pro/P) Proline | CAG | (Gln/Q) Glutamine | CGG | (Arg/R) Arginine | ||
| A | AUU | (Ile/I) Isoleucine | ACU | (Thr/T) Threonine | AAU | (Asn/N) Asparagine | AGU | (Ser/S) Serine | |
| AUC | (Ile/I) Isoleucine | ACC | (Thr/T) Threonine | AAC | (Asn/N) Asparagine | AGC | (Ser/S) Serine | ||
| AUA | (Ile/I) Isoleucine | ACA | (Thr/T) Threonine | AAA | (Lys/K) Lysine | AGA | (Arg/R) Arginine | ||
| AUG[A] | (Met/M) Methionine | ACG | (Thr/T) Threonine | AAG | (Lys/K) Lysine | AGG | (Arg/R) Arginine | ||
| G | GUU | (Val/V) Valine | GCU | (Ala/A) Alanine | GAU | (Asp/D) Aspartic acid | GGU | (Gly/G) Glycine | |
| GUC | (Val/V) Valine | GCC | (Ala/A) Alanine | GAC | (Asp/D) Aspartic acid | GGC | (Gly/G) Glycine | ||
| GUA | (Val/V) Valine | GCA | (Ala/A) Alanine | GAA | (Glu/E) Glutamic acid | GGA | (Gly/G) Glycine | ||
| GUG | (Val/V) Valine | GCG | (Ala/A) Alanine | GAG | (Glu/E) Glutamic acid | GGG | (Gly/G) Glycine | ||
| Ala/A | GCU, GCC, GCA, GCG | Leu/L | UUA, UUG, CUU, CUC, CUA, CUG |
|---|---|---|---|
| Arg/R | CGU, CGC, CGA, CGG, AGA, AGG | Lys/K | AAA, AAG |
| Asn/N | AAU, AAC | Met/M | AUG |
| Asp/D | GAU, GAC | Phe/F | UUU, UUC |
| Cys/C | UGU, UGC | Pro/P | CCU, CCC, CCA, CCG |
| Gln/Q | CAA, CAG | Ser/S | UCU, UCC, UCA, UCG, AGU, AGC |
| Glu/E | GAA, GAG | Thr/T | ACU, ACC, ACA, ACG |
| Gly/G | GGU, GGC, GGA, GGG | Trp/W | UGG |
| His/H | CAU, CAC | Tyr/Y | UAU, UAC |
| Ile/I | AUU, AUC, AUA | Val/V | GUU, GUC, GUA, GUG |
| START | AUG | STOP | UAA, UGA, UAG |
The DNA codon table is essentially identical to that for RNA, but with U replaced by T.
While slight variations on the standard code had been predicted earlier,[31] none were discovered until 1979, when researchers studying human mitochondrial genes discovered they used an alternative code. Many slight variants have been discovered since then,[32] including various alternative mitochondrial codes,[33] and small variants such as translation of the codon UGA as tryptophan in the species Mycoplasma and translation of CUG as a serine rather than a leucine in some members of the genus Candida (see the article on Candida albicans).[34][35] In bacteria and archaea, GUG and UUG are common start codons, but in rare cases, certain proteins may use alternative start codons not normally used by that species.[32]
In certain proteins, non-standard amino acids are substituted for standard stop codons, depending on associated signal sequences in the messenger RNA. For example, UGA can code for selenocysteine, and UAG can code for pyrrolysine. Selenocysteine is now viewed as the 21st amino acid, and pyrrolysine is viewed as the 22nd.[32]
Despite these differences, all known naturally-occurring codes are very similar to each other, and the coding mechanism is the same for all organisms: three-base codons, tRNA, ribosomes, reading the code in the same direction and translating the code three letters at a time into sequences of amino acids.
Since 2001, 40 non-natural amino acids have been added into protein by creating a unique codon (recoding) and a corresponding transfer-RNA:aminoacyl – tRNA-synthetase pair to encode it with diverse physicochemical and biological properties in order to be used as a tool to exploring protein structure and function or to create novel or enhanced proteins.[36][37]
H. Murakami and M. Sisido have extended some codons to have four and five bases. Steven A. Benner constructed a functional 65th (in vivo) codon.[38]
If amino acids were randomly assigned to triplet codons, then there would be 1.5 x 1084 possible genetic codes to choose from[39]:163. However, the genetic code used by all known forms of life is nearly universal with few minor variations. This suggests that a single evolutionary history underlies the origin of the genetic code. Many hypotheses on the evolutionary origins of the universal genetic code have been proposed.
Four themes run through the many hypotheses about the evolution of the genetic code:[40]
Transfer RNA molecules appear to have evolved prior to modern aminoacyl-tRNA synthetases, so the latter cannot be part of the explanation of its patterns.[54]
There are enough data to refute the possibility that the genetic code was randomly constructed ("a frozen accident").[55] For example, the genetic code clusters certain amino acid assignments. Amino acids that share the same biosynthetic pathway tend to have the same first base in their codons.[56] Amino acids with similar physical properties tend to have similar codons.[57][58] A robust hypothesis for the origin of genetic code should also address or predict the following gross features of the codon table[43]:
|
||||||||||||||