The structure of DNA, or deoxyribonucleic acid, was first described in 1953 by scientists James Watson and Francis Crick. At that time, the importance of DNA itself had only recently been discovered. In the early 1950s, scientists understood that chromosomes were the physical “particles” that contained genetic material passed from one generation to the next; however, chromosomes were understood to be composed of both DNA and certain proteins. Scientists believed that those proteins—and not DNA—were the actual genetic material passed between generations, given their comparative structural complexity. Thus, research on DNA structure took a backseat to the investigation of these proteins until the late 40s and early 50s. In 1952, however, research conducted by Alfred Hershey and Martha Chase—using radioactive labeling—demonstrated the contrary. It was then that genetic study took a new turn.
After this discovery, an English researcher named Rosalind Franklin began to collect data regarding DNA structure by bouncing X-rays off of the substance, and then exposing it to photographic film. Her partner, Maurice Wilkins, secretly revealed Franklin’s results to biologist James Watson and physicist Francis Crick. From this data, Watson and Crick determined together the structure of the substance. Their description of DNA structure as a double helix, has endured throughout scientific academic research to this day. For their trouble, Wilkins, Watson, and Crick shared a Nobel Prize in 1962; having already passed away, Franklin didn’t share the prize, though the original data was her own and she is said to have been only a few calculations from the fateful discovery.
We now know, in addition to its two-stranded helical structure, that DNA is composed of basic building blocks called nucleotides. Both strands of the genetic helix are made of these nucleotides, stacked on top of each other like a ladder, and each helical turn of the DNA is comprised of ten nucleotide pairs, stuck together. How do the nucleotide pairs stick together? A nucleotide strand itself is made up of a sugar/phosphate backbone and a base, which shares a hydrogen bond with its partner’s base across the helix. There are only four types of bases (adenine, thymine, guanine, and cytosine), each of which can bond only with one kind of partner. For example, an adenine base (A) can bond only with a thymine base (T), and a guanine base (G) can bond only with a cytosine base (C). The sequential order of these base pairings is responsible for genetic variety among chromosomes. It seems nearly impossible that the variety of these sequences could account for the incredible variety among earth’s living things—but it does!
Watson and Crick recognized that their proposed (and since accepted) DNA structure suggested a possible mechanism for copying of genetic material. By a process called replication, each strand of the genetic helix copies itself, so that one DNA molecule becomes, essentially, three (one parent molecule and two daughters, each molecule made up of two polynucleotide strands). This process, dubbed semi-conservative replication, accounts for cell division and the transmission of genetic information from cell to cell.
By Nicole Zillmer