Chargaff's rules

Erwin chargaff, an Australian Chemist migrated from America discovered Chargaff’s rule in the year 1952. This was the first rule proposed on behalf of genetic material present in the organism. These rules helped in the identification of genetic material i.e. DNA.

Chargaff’s first rule

Chargaff’s rule states the number of pyrimidine bases should be equal to number of purine bases. DNA bases are found in 1:1 ratio.

• number of guanine is equal to number of cytosine
• number of adenine is equal to number of thymine

Human DNA has four bases in different percentages. There are 30.9 % of adenine, 29.4 % of thymine, 19.9 % of guanine and 19.8 % of thymine. This helped in challenging and disproving of tetra nucleotide hypothesis. Earlier it is believed that base ratio of nucleotide base is equimolar, which are due to experimental error. That concept is also ruled out by Chargaff’s rule proposal. By paper chromatography and ultraviolet spectrophotometer he proved it was equimolar base ratio calculation is experimental error. These hints on genetic material helped Watson and Crick to discover DNA.

Chargaff’s second rule

Percentage of nucleotide base like A, T, G and C are different for different species. This helped in molecular diversity study. In other words percentages of base pairs are same only for both strands of DNA.

Especially in higher organism like eukaryotes, bacteria, viral genome and archeal chromosomes adhere to second parity rule of Chargaff’s. However organellar genomes like mitochondria and plastids, single stranded viral genome and RNA genome is devoid of this rule. This rule is feasible if the plastids and genomes of organelles are longer than 20 to 30 kilo base pair.

Szybalski rule associated with Chargaff’s rule

Nucleotide bases are equally distributed in both the strands. Exception is found in bacteriophage has more purines i.e. A and G than pyrimidines i.e. C and T. Waclaw Szybalski has confirmed in 1960 and hence referred as Szybalski rule. However basis for Szybalski rule is not much transparent as Chargaff’s rule. Bacterial genome has a combined effect of Chargaff’s second rule and Szybalski rule. It is because improper distribution of coding sequences and replication mechanism.