THE SECONDARY STRUCTURE OF DNA AND CHARGAFF’S RULES

Watson-Crick Model

The secondary structure of DNA, as proposed by James Watson and Francis Crick in 1953, is a double helix. This model consists of two polynucleotide chains that are coiled around the same axis. The backbone of the DNA molecule is composed of sugar-phosphate groups, while the nitrogenous bases extend into the interior of the helix. The nitrogenous bases form hydrogen bonds with each other, holding the two strands together.

Antiparallel Polynucleotide Chains

In the Watson-Crick model, the two polynucleotide chains run in opposite directions, a configuration known as antiparallel. This means that one strand runs in the 5’ to 3’ direction, while the other runs in the 3’ to 5’ direction. The antiparallel arrangement is crucial for the complementary base pairing that occurs between the nitrogenous bases of the two strands.

Chargaff’s Rules and the Principle of Complementarity

Chargaff’s rules state that in a DNA molecule, the amount of adenine (A) equals the amount of thymine (T), and the amount of guanine (G) equals the amount of cytosine (C). This discovery was pivotal in understanding the structure of DNA and its replication. The principle of complementarity arises from Chargaff’s rules and dictates that adenine pairs with thymine through two hydrogen bonds, while guanine pairs with cytosine through three hydrogen bonds. This complementary base pairing is essential for maintaining the structure and integrity of DNA during replication and transcription processes.

In summary, the Watson-Crick model describes DNA as a double helix with antiparallel polynucleotide chains held together by complementary base pairing, following Chargaff’s rules and the principle of complementarity.