1.1: The DNA Helix
Deoxyribonucleic acid, or DNA, is the genetic material responsible for passing traits from generation to generation in all organisms and most viruses. DNA is composed of two strands of nucleotides that wind around each other to form a double helix. The discovery of the structure of DNA occurred incrementally over nearly a century, representing one of the most famous and captivating stories in the history of science.
DNA Structure in Detail
Each strand of DNA consists of subunits called nucleotides that contain the sugar deoxyribose, a phosphate group, and one of four nitrogen-containing bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Adenine and guanine are members of a larger class of chemicals called purines that all contain two-ringed structures. Cytosine and thymine belong to a group of single-ringed structures called pyrimidines.
Adjacent nucleotides in the same strand are covalently linked by phosphodiester bonds. The two strands of nucleotides are held together by hydrogen bonds, in which the adenines in one strand pair with thymines at the same position in the other strand, and the cytosines in one strand pair with guanines in the same position in the other strand. This hydrogen bonding is made possible by the antiparallel arrangement of the two DNA strands, in which the 5’ and 3’ ends of the strands are oriented in opposite directions. Without this arrangement, the nucleotides would be in the wrong position to form hydrogen bonds between strands.
The two strands of the DNA molecule are tightly wound into a spring-like structure called a double helix. However, the double helix is not perfectly symmetrical. Instead, there are regularly occurring grooves in the structure. The major groove occurs where the sugar-phosphate backbones are relatively far apart. This space grants access to DNA-binding proteins, such as transcription factors. The minor groove, by contrast, occurs where the sugar-phosphate backbones are close together. Relatively few proteins bind to DNA via the minor groove.
The Discovery of DNA Structure: A Brief History
The story of the discovery of DNA structure begins in 1869 when Swiss scientist Friedrich Miescher discovered a substance he called “nuclein”. In the process of extracting protein from white blood cells, Miescher found an unexpected substance that had relatively high phosphorus content. He did not know what it was, but he suspected that it could be biologically important. Miescher was right, but it took decades for the scientific community to fully appreciate his insights.
The next critical discovery was made by Russian biochemist Phoebus Levene. In 1919, Levene proposed that nuclein, by then known as a nucleic acid, was composed of chains of molecules that he called polynucleotides. Levene’s proposal stemmed from his research on yeast, in which he found that individual nucleotides were composed of a phosphate group, a sugar, and a nitrogen-containing base. Although Levene’s polynucleotide model was correct in many respects, it was still unclear how the bases were arranged in the DNA molecule.
The Austrian biochemist Erwin Chargaff expanded on the work of Levene. Working in the late 1940s, Chargaff made a key finding: the amount of adenine in DNA is always roughly equal to the amount of thymine, and the amount of guanine is always roughly equal to the amount of cytosine. This pattern became known as Chargaff’s Rule and was a key piece of evidence that enabled the final elucidation of DNA structure.
In the early 1950s, American biologist James Watson and English physicist Francis Crick were racing their chief rival, American Linus Pauling, to discover the three-dimensional structure of DNA. Building on the work of Chargaff, they used knowledge of physics, mathematics, and chemistry to construct physical models of DNA. But they were unsuccessful until they received a critical piece of data: an X-ray ‘photograph’ of DNA that indicated in precise detail its double-helical structure. This photograph was the unpublished data of physicist Rosalind Franklin and was given to Watson and Crick without Franklin’s knowledge. Watson and Crick published their description of DNA structure in 1953, and together with Maurice Wilkins (a co-worker of Franklin’s), they won the 1962 Nobel Prize in Physiology or Medicine for this discovery. Sadly, Franklin died in 1958 and was therefore ineligible for a Nobel Prize.