What is Chargaff’s Rule?

Do you know what is Chargaff’s rule? According to Levene’s tetranucleotide theory in the 1920s, DNA was considered as a monotonous polymer with four DNA bases, in approximately equal molar proportions. Hotchkiss and Chargaff in 1948 analysed isolated DNA samples using paper chromatography, etc.,and found that the four nucleotide bases are not necessarily present in DNA in equal proportions. 

Chargaff reported that the DNA in calf-thymus nuclei contains four bases in the following molar proportions:

• 28% adenine
• 24% guanine
• 20% cytosine
• 28% thymine

The analyzed DNA samples of varied animals also showed that the exact base composition of DNA differs according to its biological source. This suggests that DNA is not a monotonous polymer, but are specific nucleotide base sequences that carry genetic information.

What is Chargaff’s Rule or Equivalence Rule

Chargaff’s Law, named after Erwin Chargaff, explains the relationship between the four nitrogenous bases in DNA. In 1950, Chargaff discovered that, despite the wider compositional variations by the existence of different types of DNA, the total amount of purines (Adenine and Guanine) is equal to the total amount of pyrimidines (Thymine and Cytosine). 

• Chargaff’s Law specifies a fundamental principle in molecular biology that every organism should have a base pair rule of a 1:1 ratio for pyrimidine and purine bases.
• The amount of adenine (A) in a DNA molecule is equal to the amount of thymine (T), and the amount of guanine (G) is equal to the amount of cytosine (C).
• Thus, a DNA molecule will always have the ratio of A to T and the ratio of G to C at 1:1.
• This means that the total amount of purines (A and G) will be equal to the total amount of pyrimidines (T and C), so that the DNA molecule can maintain a balance regarding the composition of bases.

Chargaff’s Rule, or the equivalence rule, applies almost universally to different organisms (viruses, bacteria, plants, and animals). At the same time, DNA isolated from higher plants and animals shows that they are rich in adenine and thymine (A: T) and show a relatively poorer amount of guanine and cytosine (G: C). 

For example, the AT/GC ratio in human DNA is 1.40:1); while that is microbes is, in general, rich in guanine and cytosine and relatively poor in adenine and thymine. E.g..  Mycobacterium tuberculosis was 0.60: 1

Such differences indicate differences in genetic information and the phylogenetic, evolutionary, and taxonomical significance. 

Importance of Chargaff’s Rule

• Chargaff’s rule has played an important role in the understanding of the structure and replication of DNA.
• The complementary base pairing between A and T, and G and C explains how one strand serves as a template for the newly forming strand during DNA replication.
• It has also helped with the discovery of the DNA helix model explained by Watson and Crick through the arrangement of the bases within the molecule.
• Chargaff’s rule applies only to the DNA molecule and not to RNA, since the latter does not have a complementary pair in its molecule.

Chargaff’s rule can be summarised as

• Adenine = Thymine
• Guanine = Cytosine
• Purines = Pyramidines

References

• Agarwal, P. V. |. V. (2004). Cell Biology, Genetics, Molecular Biology, Evolution, and Ecology: Evolution and Ecology. S. Chand Publishing.
• https://flexbooks.ck12.org/cbook/ck-12-advanced-biology/section/8.3/primary/lesson/chargaffs-base-pairing-rules-advanced-bio-adv/
• Rosandić, M., Vlahović, I., Pilaš, I., Glunčić, M., & Paar, V. (2022). An Explanation of Exceptions from Chargaff’s Second Parity Rule/Strand Symmetry of DNA Molecules. Genes, 13(11), 1929. https://doi.org/10.3390/genes13111929
• https://epgp.inflibnet.ac.in/epgpdata/uploads/epgp_content/S001174BS/P001199/M010849/ET/1479280871P4M14eTextSept22.pdf

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