DNA – New World Encyclopedia

From New World Encyclopedia

Deoxyribonucleic acid (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. 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 (deoxyribose) and phosphate groups 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. RNA also serves as a a genetic blueprint for certain viruses.

Within cells, DNA is organized into structures called chromosomes. 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, which lack a cell nucleus, 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. Some eukaryotic cell organelles, mitochondria and chloroplasts, also contain DNA, giving rise to the endosymbionic theory that these organelles may have arisen from prokaryotes in a symbionic relationship.

The identification of DNA, combined with human creativity, has been of tremendous importance not only for understanding life but for practical applications in medicine, agriculture, and other areas. Technologies have been developed using recombinant DNA to mass produce medically important proteins, such as insulin, and have found application in agriculture to make plants with desirable qualities. Through understanding the alleles that one is carrying for particular genes, one can gain an understanding of the probability that one's offspring may inherent certain genetic disorders, or one's own predisposition for a particular disease. DNA technology is used in forensics, anthropology, and many other areas as well.

DNA and the biological processes centered on its activities (translation, transcription, replication, genetic recombination, and so forth) are amazing in their complexity and coordination. The presence of DNA also reflects on the unity of life, since organisms share nucleic acids as genetic blueprints and share a nearly universal genetic code. On the other hand, the discovery of DNA has at times led to an overemphasis on DNA to the point of believing that life can be totally explained by physico-chemical processes alone.

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."[1] In 1919, this discovery was followed by Phoebus Levene's identification of the base, sugar, and phosphate nucleotide unit.[2] 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.[3]

In 1928, Frederick Griffith discovered that traits of the "smooth" form of the Pneumococcus bacteria could be transferred to the "rough" form of the same bacteria by mixing killed "smooth" bacteria with the live "rough" form.[4] This system provided the first clear suggestion that DNA carried genetic information, when Oswald Theodore Avery, along with coworkers Colin MacLeod and Maclyn McCarty, identified DNA as the transforming principle in 1943.[5] 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.[6]

In 1953, based on X-ray diffraction images[7] taken by Rosalind Franklin and the information that the bases were paired, James D. Watson and Francis Crick suggested[7] what is now accepted as the first accurate model of DNA structure in the journal Nature.[8] Experimental evidence for Watson and Crick's model were published in a series of five articles in the same issue of Nature.[9] Of these, Franklin and Raymond Gosling's paper was the first publication of X-ray diffraction data that supported the Watson and Crick model,[10][11] This issue also contained an article on DNA structure by Maurice Wilkins and his colleagues.[12] 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".[13] Final confirmation of the replication mechanism that was implied by the double-helical structure followed in 1958 through the Meselson-Stahl experiment.[14] 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.[15] These findings represent the birth of molecular biology.

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DNA - New World Encyclopedia