The DNA double helix model was proposed by James Watson and Francis Crick in 1953. According to their model, DNA is the genetic material that carries hereditary information in living organisms and has a unique double helical structure. This structure explains how genetic information is stored and transmitted from one generation to the next.
DNA is composed of two long chains of nucleotides that coil around each other to form a double helix. Each nucleotide consists of three components: a nitrogenous base, a deoxyribose sugar, and a phosphate group. The nitrogenous bases present in DNA are adenine (A), thymine (T), guanine (G), and cytosine (C).
In the double helix structure, the two DNA strands run in opposite directions and are described as antiparallel strands. The sugar and phosphate groups form the outer backbone of the DNA molecule, while the nitrogenous bases project inward toward the center of the helix.
The nitrogenous bases of the two strands pair with each other through hydrogen bonds according to specific base pairing rules. Adenine always pairs with thymine through two hydrogen bonds, while guanine pairs with cytosine through three hydrogen bonds. This specific pairing is known as complementary base pairing and ensures the accurate copying of genetic information.
The DNA molecule has a regular and stable structure. The helix makes one complete turn after approximately 10 base pairs, and the distance between two successive base pairs is about 0.34 nm. The diameter of the DNA molecule is approximately 2 nm.
Thus, the Watson and Crick model describes DNA as a double helical structure composed of two antiparallel strands with complementary base pairing, which provides a stable mechanism for storing and transmitting genetic information in living organisms.