DNA ヘリックス（二重らせん）

DNA contains four nitrogen-containing bases: adenine (A), cytosine (C), guanine (G), and thymine (T). Each base attaches to a deoxyribose sugar as part of a nucleotide subunit. These bases pair specifically with each other—C pairs with G and A pairs with T—through different numbers of hydrogen bonds, which is fundamental to DNA's double helix structure.

Nucleotides are linked by phosphodiester bonds that covalently connect the third carbon atom of one sugar to the fifth carbon atom of the next sugar molecule. This bonding pattern creates the sugar-phosphate backbone of each DNA strand. The backbone provides structural stability while bases extend inward to form base pairs between the two antiparallel strands.

Adenine and thymine pair together because they form a specific number of hydrogen bonds between them. Similarly, cytosine and guanine form a different number of hydrogen bonds. These chemical differences in hydrogen bonding ensure that base pairing is specific and consistent, which is critical for accurate DNA replication and maintaining the integrity of genetic information across generations.

The major groove occurs where the sugar-phosphate backbones are relatively far apart, providing accessible space for DNA-binding proteins such as transcription factors to interact with the DNA. The minor groove, where backbones are close together, accommodates relatively few proteins. These grooves are essential for regulating gene expression and other cellular processes that depend on protein-DNA interactions.

Each complete turn of the DNA double helix contains ten base pairs. This regular spacing results from the antiparallel arrangement of the two polynucleotide chains winding around each other. The consistent geometry of the double helix, with its predictable base pair spacing, allows the structure to efficiently pack genetic information while remaining accessible to cellular machinery.

Chargaff's Rule, discovered in the late 1940s, established that the amount of adenine in DNA equals thymine, and guanine equals cytosine. This pattern provided crucial evidence for understanding base pairing and was instrumental in enabling Watson and Crick to construct accurate physical models of DNA's three-dimensional structure in the early 1950s.

DNA is the genetic material responsible for passing traits from generation to generation in all organisms and most viruses. Its double helix structure, with specific base pairing and antiparallel strands, ensures stable storage and accurate replication of genetic information. This universal role makes DNA fundamental to understanding life across the tree of life bacteria archaea and eukaryotes.