What is a Double Helix?

The double helix structure is a common basic unit in biological structures. In 1953, it was discovered by James Watson, 25, and Francis Crick, 37.

Presenter of double helix

In February 1953, Watson and Crick saw a very beautiful X-ray diffraction picture of DNA crystals taken by Rosalind Franklin in November 1951 through Wilkins. Got their inspiration. They not only confirmed that the DNA must be a helix structure, but also analyzed the helix parameters. They used the judgments of Franklin and Wilkins, and added that the phosphate radicals form the backbone of two polynucleotide chains on the outside of the helix, and the directions are opposite; the bases are on the inside of the helix, which correspond to each other. For days, Watson and Crick happily built models of iron sheet and wire in their offices. On February 28, 1953, the first molecular model of the DNA double helix structure was finally born. ^[1]

The significance of the double helix model not only means that the structure of the DNA molecule is ascertained, but more importantly, it also suggests the mechanism of DNA replication: because adenine (A) is always paired with thymine (T), guanine (G) is always paired with cytosine (C), which indicates that the base sequence of the two chains is complementary to each other. As long as the base sequence of one chain is determined, the base sequence of the other chain is also determined. Therefore, only one of the chains is used as a template to synthesize and reproduce the other chain. From the beginning, Crick insisted on adding the phrase "the principle of specific pairing of DNA to immediately remind people of the possible replication mechanism of genetic material" in published papers. He believes that without this sentence, it would mean that he and Watson "lacked insight to see this." Shortly after the publication of the DNA double-helix structure paper, Nature published another Creek paper shortly thereafter, illustrating the semi-reserved replication mechanism of DNA. ^[2]

Backbone

It is formed by deoxyribose and phosphate group alternately connected through an ester bond. There are two main chains. They are twisted and circle in a right-hand direction around a common axis, parallel to each other and running in opposite directions to form a double helix configuration. The main chain is outside the helix, which explains the hydrophilicity of the main chain composed of sugar and phosphoric acid. On the outside of DNA is a backbone of alternately linked deoxyribose and phosphate. The so-called double helix is for the shape of the two main chains.

Base pair

The bases are located inside the helix, and they are connected to the main chain glycosyl group through glycosidic bonds in an orientation perpendicular to the helix axis. Bases in the same plane form base pairs between the two main chains. The paired bases are always A and T and G and C. Base pairs are held by hydrogen bonds. Two hydrogen bonds are formed between A and T, and three hydrogen bonds are formed between G and C. The base pairs in the DNA structure coincide exactly with Chatgaff's findings. From the perspective of stereochemistry, only the pairing of purine and pyrimidine can meet the requirements of the base pair space of the helix, and the geometric sizes of these two base pairs are very similar, and they have a suitable bond length and angle to form a hydrogen bond. condition. Each pair of bases is on its own plane, but the orientation of each base-pair plane in the helix cycle is different. The base pair has the characteristic of secondary rotation symmetry, that is, the base rotation of 180 ° does not affect the symmetry of the double helix. In other words, on the premise that the two-strand bases are complementary, the primary structure of DNA is not restricted. This feature can well illustrate the universal significance of DNA as a carrier of genetic information in the biological world.

Big and small ditch

Large grooves and small grooves refer to larger grooves and smaller grooves with a double-spiral surface recessed, respectively. Small grooves are located between the complementary strands of the double helix, while large grooves are located between adjacent double strands. This is because the paired bases connected to the two main chain glycosyl groups are not directly opposed, so that large grooves and small grooves with unequal spaces are formed along the spiral between the main chains. The N and O atoms in the base pairs in the major and minor grooves face the molecular surface.

Structural parameters

The diameter of the helix is 2nm; the period of the helix contains 10 pairs of bases; the pitch of the helix is 3.4nm; the spacing between adjacent base-pair planes is 0.34nm.