Deoxyribonucleic acid, or DNA, is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms. The main role of DNA molecules is the long-term storage of information and DNA is often compared to a set of blueprints, since it contains the instructions needed to construct other components of cells, such as proteins and RNA molecules. The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in regulating the use of this genetic information.Chemically, DNA is a long polymer of simple units called nucleotides, with a backbone made of sugars and phosphate atoms joined by ester bonds. Attached to each sugar is one of four types of molecules called bases. It is the sequence of these four bases along the backbone that encodes information. This information is read using the genetic code, which specifies the sequence of the amino acids within proteins. The code is read by copying stretches of DNA into the related nucleic acid RNA, in a process called transcription. Most of these RNA molecules are used to synthesize proteins, but others are used directly in structures such as ribosomes and spliceosomes.Within cells, DNA is organized into structures called chromosomes and the set of chromosomes within a cell make up a genome. These chromosomes are duplicated before cells divide, in a process called DNA replication. Eukaryotic organisms such as animals, plants, and fungi store their DNA inside the cell nucleus, while in prokaryotes such as bacteria it is found in the cell’s cytoplasm. Within the chromosomes, chromatin proteins such as histones compact and organize DNA, which helps control its interactions with other proteins and thereby control which genes are transcribed.
DNA was first isolated by the Swiss physician Friedrich Miescher who, in 1869, discovered a microscopic substance in the pus of discarded surgical bandages. As it resided in the nuclei of cells, he called it “nuclein”. In 1919 this discovery was followed by Phoebus Levene‘s identification of the base, sugar and phosphate nucleotide unit. Levene suggested that DNA consisted of a string of nucleotide units linked together through the phosphate groups. However, Levene thought the chain was short and the bases repeated in a fixed order. In 1937 William Astbury produced the first X-ray diffraction patterns that showed that DNA had a regular structure. In 1943, Oswald Theodore Avery discovered that traits of the “smooth” form of the Pneumococcus could be transferred to the “rough” form of the same bacteria by mixing killed “smooth” bacteria with the live “rough” form. Avery, along with coworkers Colin MacLeod and Maclyn McCarty, identified DNA as this transforming principle. DNA’s role in heredity was confirmed in 1953, when Alfred Hershey and Martha Chase in the Hershey-Chase experiment showed that DNA is the genetic material of the T2 phage. In 1953, based on X-ray diffraction images taken by Rosalind Franklin and the information that the bases were paired, James D. Watson and Francis Crick suggestedwhat is now accepted as the first accurate model of DNA structure in the journal Nature. Experimental evidence for Watson and Crick’s model were published in a series of five articles in the same issue of Nature. Of these, Franklin and Raymond Gosling‘s paper was the first publication of X-ray diffraction data that supported the Watson and Crick model, this issue also contained an article on DNA structure by Maurice Wilkins and his colleagues. In 1962, after Franklin’s death, Watson, Crick, and Wilkins jointly received the Nobel Prize in Physiology or Medicine. However, speculation continues on who should have received credit for the discovery, as it was based on Franklin’s data.In an influential presentation in 1957, Crick laid out the “Central Dogma” of molecular biology, which foretold the relationship between DNA, RNA, and proteins, and articulated the “adaptor hypothesis”. Final confirmation of the replication mechanism that was implied by the double-helical structure followed in 1958 through the Meselson-Stahl experiment. Further work by Crick and coworkers showed that the genetic code was based on non-overlapping triplets of bases, called codons, allowing Har Gobind Khorana, Robert W. Holley and Marshall Warren Nirenberg to decipher the genetic code. These findings represent the birth of molecular biology.