Chargaff's Rules

Erwin Chargaff was a biochemist who analyzed the base compositions of DNA. His findings became known as Chargaff's rules, which will be explained later.

To understand his rules, it is important to first understand DNA structure. DNA consists of units called nucleotides, which are made of three components: a nitrogen-containing (nitrogenous) base, a phosphate group, and the deoxyribose sugar. There are four types of bases: adenine (A), thymine (T), cytosine (C) and guanine (G). Erwin Chargaff made an interesting discovery about the ratio of nitrogenous bases; the number of adenines approximately equaled the number of thymines and the number of cytosines equaled the number of guanines. This discovery constitutes his first rule:

1) For each species, the percentages of A and T bases are equal, as are those of C and G bases.

Chargaff also conducted research regarding the base concentrations among different species. His findings led to his second rule:

2) DNA base compositions vary between species.

For example, he found that 32.8% of sea urchin DNA have the base A while 30.4% of human DNA have the base A and only 24.7% of DNA from E. coli have the base A.

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