Fix the mistake: Genes and Their Functions

The Discovery and Structure of DNA: The Chemistry of the Double Helix

DNA (deoxyribonucleic acid) is the substance that carries genetic information. Molecular biology began when its structure was discovered. Accurately understanding its chemical structure is essential.

Basic Knowledge

The basic unit of DNA is the nucleotide. One nucleotide consists of:
① Sugar: deoxyribose (a five-carbon sugar)
② Phosphate (one phosphate group)
③ Base: one of four types — adenine (A), thymine (T), guanine (G), and cytosine (C)
DNA is a long chain formed by many nucleotides joined together.
DNA consists of two strands that form a double helix, with the bases joined by hydrogen bonds inside the strands. The base pairs are strictly determined: A-T (2 hydrogen bonds) and G-C (3 hydrogen bonds). This is called Chargaff's rule (A=T, G=C) and complementarity.
In 1953, Watson and Crick announced the double-helix structure of DNA and received the Nobel Prize. Before this, Rosalind Franklin's X-ray diffraction photograph (Photo 51) provided a decisive clue.

Further Study

The chemical beauty of DNA is concentrated in complementarity and its double-helix structure. An A-T base pair has two hydrogen bonds, while a G-C base pair has three, making the latter slightly stronger. This explains why DNA with a higher G-C content has a higher thermal denaturation (melting) temperature.
The alternating chain of sugar and phosphate, called the “sugar-phosphate backbone,” forms the outside of DNA. The bases form hydrogen bonds with one another inside and fit neatly into the interior of the helix. Hydrophobic stacking between bases (the stacking of aromatic rings) is also an important chemical factor that stabilizes the double helix.
The two strands run in an antiparallel arrangement. If one strand runs 5'→3', the other runs 3'→5'. This is essential knowledge for understanding the direction of replication and transcription. Chemically, the phosphate of one nucleotide bonds to the 5' carbon and the sugar of the next nucleotide bonds to the 3' carbon (5'-3' phosphodiester bond), creating one-way directionality.
Chargaff's rule (1950) was an empirical rule stating that the amounts of A and T, and of G and C, in DNA extracts are almost equal. Its chemical meaning became clear after the discovery of the double-helix structure. It is a remarkable historical example in which “chemistry = function derived from structure” directly predicted a function: complementarity as a template for replication.

Points to Note

① Distinguish between purine bases (A・G, two-ring structures) and pyrimidine bases (T・C・U, one-ring structures). Purines are larger, and pyrimidines are smaller. ② A-T has 2 hydrogen bonds, whereas G-C has 3. Do not confuse them. ③ DNA contains deoxyribose, whereas RNA contains ribose. The difference is whether the 2' carbon has an OH group. ④ Chargaff's rule is A=T, G=C (equal amounts), not A+T=G+C.

Practice

1. Give all four types of bases that make up DNA using letters.
2. Who were the two scientists who announced the double-helix structure of DNA in 1953?
3. Which DNA base pair is joined by three hydrogen bonds?