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Chargaff's rules state that DNA from any cell of all organisms should have a 1:1 ratio of pyrimidine and purine bases and, more specifically, that the amount of guanine is equal to cytosine and the amount of adenine is equal to thymine. This pattern is found in both strands of the DNA. They were discovered by Austrian chemist Erwin Chargaff.[1][2][3][4][5][6]


Chargaff Parity Rule 1

The first rule holds that a double-stranded DNA molecule globally has percentage base pair equality: %A = %T and %G = %C.[6] The rigorous validation of the rule constitutes the basis of Watson-Crick pairs in the DNA double helix.

Chargaff Parity Rule 2

The second rule holds that both %A ~ %T and %G ~ %C are valid for each of the two DNA strands.[7] This describes only a global feature of the base composition in a single DNA strand.[8]


The second of Chargaff's rules (or "Chargaff's second parity rule") is that the composition of DNA varies from one species to another; in particular in the relative amounts of A, G, T, and C bases. Such evidence of molecular diversity, which had been presumed absent from DNA, made DNA a more credible candidate for the genetic material than protein.

In 2006 it was shown that this rule applies to four of the five types of double stranded genomes; specifically it applies to the eukaryotic chromosomes, the bacterial chromosomes, the double stranded DNA viral genomes, and the archeal chromosomes.[9] It does not apply to the organellar genomes (mitochondria and plastids) nor does it apply to the single stranded DNA (viral) genomes or any type of RNA genome. The basis for this rule is still under investigation.

The rule itself has consequences. In most bacterial genomes (which are generally 80-90% coding) genes are arranged in such a fashion that approximately 50% of the coding sequence lies on either strand. Wacław Szybalski, in the 1960s, showed that in bacteriophage coding sequences purines (A and G) exceed pyrimidines (C and T).[10] This rule has since been confirmed in other organisms and should probably be now termed "Szybalski's rule". While Szybalski's rule generally holds, exceptions are known to exist.[11][12][13] The biological basis for Szybalski's rule, like Chargaff's, is not yet known.

The combined effect of Chargaff's second rule and Szybalski's rule can be seen in bacterial genomes where the coding sequences are not equally distributed. The genetic code has 64 codons of which 3 function as termination codons: there are only 20 amino acids normally present in proteins. (There are two uncommon amino acids - selenocysteine and pyrrolysine - found in a limited number of proteins and encoded by the 'stop' codons - TGA and TAG respectively.) The mismatch between the number of codons and amino acids allows several codons to code for a single amino acid. These codons normally differ in the third codon base position.

Multivariate statistical analysis of codon use within genomes with unequal quantities of coding sequences on the two strands has shown that codon use in the third position depends on the strand on which the gene is located. This seems likely to be the result of Szybalski's and Chargaff's rules. Because of the asymmetry in pyrimidine and purine use in coding sequences, the strand with the greater coding content will tend to have the greater number of purine bases (Szybalski's rule). Because the number of purine bases will to a very good approximation equal the number of their complementary pyrimidines within the same strand and because the coding sequences occupy 80-90% of the strand, there appears to be (1) a selective pressure on the third base to minimize the number of purine bases in the strand with the greater coding content and (2) that this pressure is proportional to the mismatch in the length of the coding sequences between the two strands.

The origin of the deviation from Chargaff's rule in the organelles has been suggested to be a consequence of the mechanism of replication.[14] During replication the DNA strands separate. In single stranded DNA, cytosine spontaneously slowly deaminates to adenosine (a C to A transversion). The longer the strands are separated the greater the quantity of deamination. For reasons that are not yet clear the strands tend to exist longer in single form in mitochondria than in chromsomal DNA. This process tends to yield one strand that is enriched in guanine (G) and thymine (T) with its complement enriched in cytosine (C) and adenosine (A) and this process may have given rise to the deviations found in the mitochondria.

Chargaff's second rule appears to be the consequence of a more complex parity rule: within a single strand of DNA any oligonucleotide is present in equal numbers to its reverse complementary nucleotide. Because of the computational requirements this has not been verified in all genomes for all oligonucleotides. It has been verified for triplet oligonucleotides for a large data set.[15] Albrecht-Buehler has suggested that this rule is the consequence of genomes evolving by a process of inversion and transposition. This process does not appear to have acted on the mitochondrial genomes.

Relative proportions (%) of bases in DNA

The following table is a representative sample of Erwin Chargaff's 1952 data, listing the base composition of DNA from various organisms a support both of Chargaff's rules.[16]

Organism %A %G %C %T A/T G/C %GC
φX174 24.0 23.3 21.5 31.2 0.77 1.08 44.8
Maize 26.8 22.8 23.2 27.2 0.99 0.98 46.1
Octopus 33.2 17.6 17.6 31.6 1.05 1.00 35.2
Chicken 28.0 22.0 21.6 28.4 0.99 1.02 43.7
Rat 28.6 21.4 20.5 28.4 1.01 1.00 42.9
Human 29.3 20.7 20.0 30.0 0.98 1.04 40.7
Grasshopper 29.3 20.5 20.7 29.3
Sea Urchin 32.8 17.7 17.3 32.1
Wheat 27.3 22.7 22.8 27.1
Yeast 31.3 18.7 17.1 32.9
E. Coli 24.7 26.0 25.7 23.6

See also


  1. ^ Elson D, Chargaff E (1952). "On the deoxyribonucleic acid content of sea urchin gametes". Experientia 8 (4): 143–145.  
  2. ^ Chargaff E, Lipshitz R, Green C (1952). "Composition of the deoxypentose nucleic acids of four genera of sea-urchin". J Biol Chem 195 (1): 155–160. PMID 14938364.  
  3. ^ Chargaff E, Lipshitz R, Green C, Hodes ME (1951). "The composition of the deoxyribonucleic acid of salmon sperm". J Biol Chem 192 (1): 223–230. PMID 14917668.  
  4. ^ Chargaff E (1951). "Some recent studies on the composition and structure of nucleic acids". J Cell Physiol Suppl 38 (Suppl).  
  5. ^ Magasanik B, Vischer E, Doniger R, Elson D, Chargaff E (1950). "The separation and estimation of ribonucleotides in minute quantities". J Biol Chem 186 (1): 37–50. PMID 14778802.  
  6. ^ a b Chargaff E (1950). "Chemical specificity of nucleic acids and mechanism of their enzymatic degradation". Experientia 6 (6): 201–209.  
  7. ^ Rudner R, Karkas JD, Chargaff E (1968). Separation of B. subtilis DNA into complementary strands. III. Direct Analysis. Proc Natl Acad Sci USA, 60:921-922. PubMed
  8. ^ a b Zhang CT, Zhang R, Ou HY (2003). "The Z curve database: a graphic representation of genome sequences". Bioinformatics 19 (5): 593–599. doi:10.1093/bioinformatics/btg041.  
  9. ^ Mitchell D, Bridge R (2006). A test of Chargaff's second rule. Biochem Biophys Res Commun, 340(1):90-94.
  10. ^ Szybalski W, Kubinski H, Sheldrick O (1966). Pyrimidine clusters on the transcribing strand of DNA and their possible role in the initiation of RNA synthesis. Cold Spring Harbor Symp Quant Biol, 31:123-127.
  11. ^ Cristillo AD (1998). "Characterization of G0/G1 switch genes in cultured T lymphocytes". PhD thesis. Queen's University, Kingston, Ontario Canada.
  12. ^ Bell SJ, Forsdyke DR (1999). Deviations from Chargaff's second parity rule correlate with direction of transcription. J Theor Biol, 197:63-76.
  13. ^ Lao PJ, Forsdyke DR (2000). Thermophilic Bacteria Strictly Obey Szybalski's Transcription Direction Rule and Politely Purine-Load RNAs with Both Adenine and Guanine. Genome, 10(2):228-236.
  14. ^ Nikolaou C, Almirantis Y (2006). Deviations from Chargaff's second parity rule in organellar DNA. Insights into the evolution of organellar genomes. Gene, 381:34-41.
  15. ^ Albrecht-Buehler G (2006). Asymptotically increasing compliance of genomes with Chargaff's second parity rules through inversions and inverted transpositions. Proc Natl Acad Sci USA 103 (47) 17828-17833
  16. ^ Bansal M (2003). "DNA structure: Revisiting the Watson-Crick double helix". Current Science 85 (11): 1556–1563.  
  17. ^ Hallin PF, David Ussery D (2004). "CBS Genome Atlas Database: A dynamic storage for bioinformatic results and sequence data". Bioinformatics 20: 3682–3686.  

Further reading

  • Szybalski W, Kubinski H, Sheldrick P (1966). "Pyrimidine clusters on the transcribing strands of DNA and their possible role in the initiation of RNA synthesis". Cold Spring Harbor NY Symp. Quant Biol 31: 123–127.  
  • Lobry JR (1996). "Asymmetric substitution patterns in the two DNA strands of bacteria". Mol. Biol. Evol. 13: 660–665.  
  • Lafay B, Lloyd AT, McLean MJ, Devine KM, Sharp PM, Wolfe KH (1999). "Proteome composition and codon usage in spirochaetes: species-specific and DNA strand-specific mutational biases". Nucleic Acids Res 27: 1642–1649.  
  • McLean MJ, Wolfe KH, Devine KM (1998). "Base composition skews, replication orientation, and gene orientation in 12 prokaryote genomes". J Mol Evol 47 (6): 691–696. PMID 9847411.  
  • McInerney JO (1998). "Replicational and transcriptional selection on codon usage in Borrelia burgdorferi". Proc Natl Acad Sci USA 95: 10698–10703.  


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