Course: SAT 170 Introductory Biotechnology

Unit 3 Reading and Multimedia Assignment

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Evidence for DNA as the Carrier of Genetic Information

A DNA molecule consists of three components: a nitrogen-containing base, a pentose sugar (deoxyribose), and a phosphate group.

DNA consists of two long polymers of simple units called nucleotides. Each nucleotide has three components: a nitrogenous (nitrogen-containing) base, a pentose sugar, and a phosphate group. The nucleotides are arranged so that the DNA strands have backbones made of sugars and phosphate groups joined by ester bonds (see above figure). The backbone of DNA forms the outside of each of the two helices, with the nucleotide bases bound together and stacked in the center of the helix.
The nitrogenous bases found in DNA are adenine (A), thymine (T), guanine (G), and cytosine (C). Through careful experimentation, a biochemist named Erwin Chargaff discovered two trends about base sequences that helped lead to the discovery of DNA's double-helix structure and helped to reinforce the evidence that DNA is the basic molecule of inheritance.

Chargaff proposed two main rules that appropriately became know as Chargaff's rules. One rule is that the composition of DNA varies from one species to another, particularly 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.

The other and best known of Chargaff's rules is that, in a single species, the number of A and T bases are equal and the number of G and C bases are equal. In human DNA, for example, the four bases are present in these percentages: A=30.9% and T=29.4%; G=19.9% and C=19.8%. This strongly hinted towards the base pair makeup of the DNA (in which As bind with Ts, and Cs bind with Gs), although Chargaff did not explicitly state this connection himself.