Nutrition

Vitamin

A vitamin is an organic compound required as a nutrient in tiny amounts by an organism.[1] A compound is called a vitamin when it cannot be synthesized in sufficient quantities by an organism, and must be obtained from the diet. Thus, the term is conditional both on the circumstances and the particular organism. For example, ascorbic acid functions as vitamin C for some animals but not others, and vitamins D and K are required in the human diet only in certain circumstances.[2] Vitamins are defined by their biological activity, not their structure. Thus, each "vitamin" actually refers to a number of vitamer compounds, which form a set of distinct chemical compounds that show the biological activity of a particular vitamin. Such a set of chemicals are grouped under an alphabetized vitamin "generic descriptor" title, such as "vitamin A," which (for example) includes retinal, retinol, and many carotenoids. [3] Vitamers are often inter-convertible in the body. The term vitamin does not include other essential nutrients such as dietary minerals, essential fatty acids, or essential amino acids, nor does it encompass the large number of other nutrients that promote health but that are not essential for life.
Vitamins have diverse biochemical functions, including function as hormones (e.g. vitamin D), antioxidants (e.g. vitamin E), and mediators of cell signaling and regulators of cell and tissue growth and differentiation (e.g. vitamin A) [4]. The largest number of vitamins (e.g. B complex vitamins) function as precursors for enzyme cofactor bio-molecules (coenzymes), that help act as catalysts and substrates in metabolism. When acting as part of a catalyst, vitamins are bound to enzymes and are called prosthetic groups. For example, biotin is part of enzymes involved in making fatty acids. Vitamins also act as coenzymes to carry chemical groups between enzymes. For example, folic acid carries various forms of carbon group – methyl, formyl and methylene - in the cell. Although these roles in assisting enzyme reactions are vitamins' best-known function, the other vitamin functions are equally important.[5]
Until the 1800s, vitamins were obtained solely through food intake, and changes in diet (which, for example, could occur during a particular growing season) can alter the types and amounts of vitamins ingested. Vitamins have been produced as commodity chemicals and made widely available as inexpensive pills for several decades,[6] allowing supplementation of the dietary intake.

History

The value of eating certain foods to maintain health was recognized long before vitamins were identified. The ancient Egyptians knew that feeding a patient liver would help cure night blindness, an illness now known to be caused by a vitamin A deficiency. The advancement of ocean voyage during the Renaissance resulted in prolonged periods without access to fresh fruits and vegetables, and made illnesses from vitamin deficiency common among ship's crew.
In 1749, the Scottish surgeon James Lind discovered that citrus foods helped prevent scurvy, a particularly deadly disease in which collagen is not properly formed, causing poor wound healing, bleeding of the gums, severe pain, and death.[7] In 1753, Lind published his Treatise on the Scurvy, which recommended using lemons and limes to avoid scurvy, which was adopted by the British Royal Navy. This led to the nickname Limey for sailors of that organization. Lind's discovery, however, was not widely accepted by individuals in the Royal Navy's Arctic expeditions in the 19th century, where it was widely believed that scurvy could be prevented by practicing good hygiene, regular exercise, and by maintaining the morale of the crew while on board, rather than by a diet of fresh food.[7] As a result, Arctic expeditions continued to be plagued by scurvy and other deficiency diseases. In the early 20th century, when Robert Falcon Scott made his two expeditions to the Antarctic, the prevailing medical theory was that scurvy was caused by "tainted" canned food.[7]
In 1881, Russian surgeon Nikolai Lunin studied the effects of scurvy while at the University of Tartu in present-day Estonia.[8] He fed mice an artificial mixture of all the separate constituents of milk known at that time, namely the proteins, fats, carbohydrates, and salts. The mice that received only the individual constituents died, while the mice fed by milk itself developed normally. He made a conclusion that "a natural food such as milk must therefore contain, besides these known principal ingredients, small quantities of unknown substances essential to life".[8] However, his conclusions were rejected by other researchers when they were unable to reproduce his results. One difference was that he had used table sugar (sucrose), while other researchers had used milk sugar (lactose) that still contained small amounts of vitamin B.
In the Orient where polished white rice was the common staple food of the middle class, beriberi resulting from lack of vitamin B was endemic. In 1884, Takaki Kanehiro, a British trained medical doctor of the Japanese Navy observed that beriberi was endemic among low ranking crew who often ate nothing but rice but not among crews of Western navies and officers who were entitled to a Western-style diet. Kanehiro initially believed that lack of protein was the chief cause of beriberi. With the support of Japanese navy, he experimented using crews of two battleships, one crew was fed only white rice, while the other was fed a diet of meat, fish, barley, rice, and beans. The group that ate only white rice documented 161 crew with beriberi and 25 deaths, while the latter group had only 14 cases of beriberi and no deaths. This convinced Kanehiro and the Japanese Navy that diet was the cause of beriberi. This was confirmed in 1897, when Christiaan Eijkman discovered that feeding unpolished rice instead of the polished variety to chickens helped to prevent beriberi in the chickens. The following year, Frederick Hopkins postulated that some foods contained "accessory factors"—in addition to proteins, carbohydrates, fats, et cetera—that were necessary for the functions of the human body.[7] Hopkins was awarded the 1929 Nobel Prize for Physiology or Medicine with Christiaan Eijkman for their discovery of several vitamins.
In 1910, Japanese scientist Umetaro Suzuki succeeded in extracting a water-soluble complex of micronutrients from rice bran and named it aberic acid. He published this discovery in a Japanese scientific journal.[9] When the article was translated into German, the translation failed to state that it was a newly discovered nutrient, a claim made in the original Japanese article, and hence his discovery failed to gain publicity. Polish biochemist Kazimierz Funk isolated the same complex of micronutrients and proposed the complex be named "Vitamine" (a portmanteau of "vital amine") in 1912.[10] The name soon became synonymous with Hopkins' "accessory factors", and by the time it was shown that not all vitamins were amines, the word was already ubiquitous. In 1920, Jack Cecil Drummond proposed that the final "e" be dropped to deemphasize the "amine" reference after the discovery that vitamin C had no amine component.

Throughout the early 1900s, the use of deprivation studies allowed scientists to isolate and identify a number of vitamins. Initially, lipid from fish oil was used to cure rickets in rats, and the fat-soluble nutrient was called "antirachitic A". The irony here is that the first "vitamin" bioactivity ever isolated, which cured rickets, was initially called "vitamin A", the bioactivity of which is now called vitamin D.[11] What we now call "vitamin A" was identified in fish oil because it was inactivated by ultraviolet light. In 1931, Albert Szent-Györgyi and a fellow researcher Joseph Svirbely determined that "hexuronic acid" was actually vitamin C and noted its anti-scorbutic activity. In 1937, Szent-Györgyi was awarded the Nobel Prize for his discovery. In 1943 Edward Adelbert Doisy and Henrik Dam were awarded the Nobel Prize for their discovery of vitamin K and its chemical structure.

In humans

Vitamins are classified as either water-soluble, meaning that they dissolve easily in water, or fat-soluble vitamins, which are absorbed through the intestinal tract with the help of lipids (fats).

In general, water-soluble vitamins are readily excreted from the body. Each vitamin is typically

used in multiple reactions and, therefore, most have multiple functions.[12]
In humans there are 13 vitamins: 4 fat-soluble (A, D, E and K) and 9 water-soluble (8 B vitamins and vitamin C).

Vitamin generic descriptor name : Vitamin A

Vitamer chemical name(s) : Retinoids(retinol, retinoidsand carotenoids)

Solubility : Fat

Recommended dietary allowances(male, age 19–70)[13] : 900 µg

Deficiency disease Upper Intake Level(UL/day)[13] : Night-blindness andKeratomalacia[14]

Upper Intake Level(UL/day)[13] : 3,000 µg

Overdose disease : Hypervitaminosis A

Vitamin generic descriptor name : Vitamin B1

Vitamer chemical name(s) : Thiamine

Solubility : Water

Recommended dietary allowances(male, age 19–70)[13] : 1.2 mg

Deficiency disease : Beriberi

NUpper Intake Level(UL/day)[13] : N/D[15]

Vitamin generic descriptor name : Vitamin B2

Vitamer chemical name(s) : Riboflavin

Solubility : Water

Recommended dietary allowances(male, age 19–70)[13] : 1.3 mg

Deficiency disease : Ariboflavinosis

Upper Intake Level(UL/day)[13] : N/D

Vitamin generic descriptor name : Vitamin B3

Vitamer chemical name(s) : Niacin, niacinamide

Solubility : Water

Recommended dietary allowances(male, age 19–70)[13] : 16.0 mg

Deficiency disease : Pellagra

Upper Intake Level(UL/day)[13] : 35.0 mg

Vitamin generic descriptor name : Vitamin B5

Vitamer chemical name(s) : Pantothenic acid

Solubility : Water

Recommended dietary allowances(male, age 19–70)[13] : 5.0 mg[16]

Deficiency disease : Paresthesia

Upper Intake Level(UL/day)[13] : N/D

Vitamin generic descriptor name : Vitamin B6

Vitamer chemical name(s) : Pyridoxine, pyridoxamine, pyridoxal

Solubility : Water

Recommended dietary allowances(male, age 19–70)[13] : 1.3-1.7 mg

Deficiency disease : Anaemia[17]

Upper Intake Level(UL/day)[13] : 100 mg

Overdose disease : Impairment of proprioception, nerve damage

Vitamin generic descriptor name : Vitamin B7

Vitamer chemical name(s) : Biotin

Solubility : Water

Recommended dietary allowances(male, age 19–70)[13] : 30.0 µg

Deficiency disease : Dermatitis, enteritis

Upper Intake Level(UL/day)[13] : N/D

Vitamin generic descriptor name : Vitamin B9

Vitamer chemical name(s) : Folic acid, folinic acid

Solubility : Water

Recommended dietary allowances(male, age 19–70)[13] : 400 µg

Deficiency disease : Deficiency during pregnancy is associated with birth defects, such as

neural tube defects

Upper Intake Level(UL/day)[13] : 1,000 µg

Overdose disease : Refer to deficiency of Vitamin B6

Vitamin generic descriptor name : Vitamin B12

Vitamer chemical name(s) : Cyanocobalamin, hydroxycobalamin,

methylcobalamin

Solubility : Water

Recommended dietary allowances(male, age 19–70)[13] : 2.4 µg

Deficiency disease : Megaloblastic anaemia[18]

Upper Intake Level(UL/day)[13] : N/D

Vitamin generic descriptor name : Vitamin C

Vitamer chemical name(s) : Ascorbic acid

Solubility : Water

Recommended dietary allowances(male, age 19–70)[13] : 90.0 mg

Deficiency disease : Scurvy

Upper Intake Level(UL/day)[13] : 2,000 mg

Overdose disease : Refer to Vitamin C megadosage

Vitamin generic descriptor name : Vitamin D

Vitamer chemical name(s) : Ergocalciferol, cholecalciferol

Solubility : Fat

Recommended dietary allowances(male, age 19–70)[13] : 5.0 µg-10 µg[19]

Deficiency disease : Rickets and Osteomalacia

Upper Intake Level(UL/day)[13] : 50 µg

Overdose disease : Hypervitaminosis D

Vitamin generic descriptor name : Vitamin E

Vitamer chemical name(s) : Tocopherols, tocotrienols

Solubility : Fat

Recommended dietary allowances(male, age 19–70)[13] : 15.0 mg

Deficiency disease : Deficiency is very rare; mild hemolytic anemia in newborn infants.[20]

Upper Intake Level(UL/day)[13] : 1,000 mg

Vitamin generic descriptor name : Vitamin K

Vitamer chemical name(s) : phylloquinone, menaquinones

Solubility : Fat

Recommended dietary allowances(male, age 19–70)[13] : 120 µg

Deficiency disease : Bleeding diathesis

Upper Intake Level(UL/day)[13] : N/D

In nutrition and diseasesVitamins are essential for the normal growth and development of a multicellular organism. Using the genetic blueprint inherited from its parents, a fetus begins to develop, at the moment of conception, from the nutrients it absorbs. It requires certain vitamins and minerals to be present at certain times. These nutrients facilitate the chemical reactions that produce among other things, skin, bone, and muscle. If there is serious deficiency in one or more of these nutrients, a child may develop a deficiency disease. Even minor deficiencies may cause permanent damage.[21]
For the most part, vitamins are obtained with food, but a few are obtained by other means. For example, microorganisms in the intestine—commonly known as "gut flora"—produce vitamin K and biotin, while one form of vitamin D is synthesized in the skin with the help of natural ultraviolet in sunlight. Humans can produce some vitamins from precursors they consume. Examples include vitamin A, produced from beta carotene, and niacin, from the amino acid tryptophan.[13]
Once growth and development are completed, vitamins remain essential nutrients for the healthy maintenance of the cells, tissues, and organs that make up a multicellular organism; they also enable a multicellular life form to efficiently use chemical energy provided by food it eats, and to help process the proteins, carbohydrates, and fats required for respiration.

DeficienciesDeficiencies of vitamins are classified as either primary or secondary. A primary deficiency occurs when an organism does not get enough of the vitamin in its food. A secondary deficiency may be due to an underlying disorder that prevents or limits the absorption or use of the vitamin, due to a “lifestyle factor”, such as smoking, excessive alcohol consumption, or the use of medications that interfere with the absorption or use of the vitamin.[20] People who eat a varied diet are unlikely to develop a severe primary vitamin deficiency. In contrast, restrictive diets have the potential to cause prolonged vitamin deficits, which may result in often painful and potentially deadly diseases.
Because human bodies do not store most vitamins, humans must consume them regularly to avoid deficiency. Human bodily stores for different vitamins vary widely; vitamins A, D, and B12 are stored in significant amounts in the human body, mainly in the liver,[20] and an adult human's diet may be deficient in vitamins A and B12 for many months before developing a deficiency condition. Vitamin B3 is not stored in the human body in significant amounts, so stores may only last a couple of weeks.[14][20]
Well-known human vitamin deficiencies involve thiamine (beriberi), niacin (pellagra), vitamin C (scurvy) and vitamin D (rickets). In much of the developed world, such deficiencies are rare; this is due to (1) an adequate supply of food; and (2) the addition of vitamins and minerals to common foods, often called fortification.[13][20]
Recent lines of evidence also suggest a link between nutrition and mental disorders, as evidenced by Lakhan & Vieira (2008)[22].

Side effects and overdoseIn large doses, some vitamins have documented side effects that tend to be more severe with a larger dosage. The likelihood of consuming too much of any vitamin from food is remote, but overdosing from vitamin supplementation does occur. At high enough dosages some vitamins cause side effects such as nausea, diarrhea, and vomiting.[23][14] When side effects emerge, recovery is often accomplished by reducing the dosage. The concentrations of vitamins an individual can tolerate vary widely, and appear to be related to age and state of health.[24] In the United States, overdose exposure to all formulations of vitamins was reported by 62,562 individuals in 2004 (nearly 80% of these exposures were in children under the age of 6), leading to 53 "major" life-threatening outcomes and 3 deaths[25]—a small number in comparison to the 19,250 people who died of unintentional poisoning of all kinds in the U.S. in the same year (2004).[26]

SupplementsDietary supplements, often containing vitamins, are used to ensure that adequate amounts of nutrients are obtained on a daily basis, if optimal amounts of the nutrients cannot be obtained through a varied diet. Scientific evidence supporting the benefits of some dietary supplements is well established for certain health conditions, but others need further study.[27]
In the United States, advertising for dietary supplements is required to include a disclaimer that the product is not intended to treat, diagnose, mitigate, prevent, or cure disease, and that any health claims have not been evaluated by the Food and Drug Administration.[27] In some cases, dietary supplements may have unwanted effects, especially if taken before surgery, with other dietary supplements or medicines, or if the person taking them has certain health conditions.[27] Vitamin supplements may also contain levels of vitamins many times higher, and in different forms, than one may ingest through food.[28]
Intake of excessive quantities can cause vitamin poisoning, most commonly for Vitamin A and Vitamin D. For this reason, most common vitamins have recommended upper daily intake amounts.

Governmental regulation of vitamin supplementsMost countries place dietary supplements in a special category under the general umbrella of foods, not drugs. This necessitates that the manufacturer, and not the government, be responsible for ensuring that its dietary supplement products are safe before they are marketed. Unlike drug products, that must explicitly be proven safe and effective for their intended use before marketing, there are often no provisions to "approve" dietary supplements for safety or effectiveness before they reach the consumer. Also unlike drug products, manufacturers and distributors of dietary supplements are not generally required to report any claims of injuries or illnesses that may be related to the use of their products.[29] [30]

Names in current and previous nomenclaturesThe reason the set of vitamins seems to skip directly from E to K is that the vitamins corresponding to "letters" F-J were either reclassified over time, discarded as false leads, or renamed because of their relationship to "vitamin B", which became a "complex" of vitamins. The German-speaking scientists who isolated and described vitamin K (in addition to naming it as such) did so because the vitamin is intimately involved in the Koagulation of blood following wounding. At the time, most (but not all) of the letters from F through J were already designated, so the use of the letter K was considered quite reasonable.
The following table lists chemicals that had previously been classified as vitamins, as well as the earlier names of vitamins that later became part of the B-complex:

Previous name[31][32] Chemical name[31][32] Reason for name change[31] Vitamin B4 Adenine DNA metabolite Vitamin B8 Adenylic acid DNA metabolite Vitamin F Essential fatty acids Needed in large quantities (doesnot fit the definition of a vitamin). Vitamin G Riboflavin Reclassified as Vitamin B2 Vitamin H Biotin Reclassified as Vitamin B7 Vitamin J Catechol, Flavin Protein metabolite Vitamin L1[33] Anthranilic acid Protein metabolite Vitamin L2[33] Adenylthiomethylpentose RNA metabolite Vitamin M Folic acid Reclassified as Vitamin B9 Vitamin O Carnitine Protein metabolite Vitamin P Flavonoids No longer classified as a vitamin Vitamin PP Niacin Reclassified as Vitamin B3 Vitamin U S-Methylmethionine Protein metabolite

See also

Nutrition Vitamin deficiency Dietary minerals Essential amino acids Essential nutrients Nootropics Nutrients Antioxidant Dietary supplement Dietetics Fat soluble vitamins Health freedom movement Illnesses related to poor nutrition Megavitamin therapy Orthomolecular medicine Pharmacology Vitamin poisoning (overdose) Whole food supplements

References

^ Lieberman, S, Bruning, N (1990). The Real Vitamin & Mineral Book. NY: Avery Group, 3.

^ http://www.thefreedictionary.com/vitamin

^ http://www.answers.com/topic/vitamer?cat=health#top See Oxford entry, accessed Jan 4, 2008.

^ Accessed Jan 4, 2008

^ Bolander FF (2006). "Vitamins: not just for enzymes". Curr Opin Investig Drugs 7 (10): 912–5. PMID 17086936. 

^ Kirk-Othmer (1984). Encyclopedia of Chemical Technology Third Edition. NY: John Wiley and Sons, Vol. 24:104.

^ a b c d Jack Challem (1997). "The Past, Present and Future of Vitamins"

^ a b 1929 Nobel lecture

^ [1]

^ Funk, C. and H. E. Dubin. The Vitamines. Baltimore: Williams and Wilkins Company, 1922.

^ Bellis, Mary. Vitamins - Production Methods The History of the Vitamins. Retrieved 1 Feb 2005.

^ Kutsky, R.J. (1973). Handbook of Vitamins and Hormones. New York:Van Nostrand Reinhold. ^ a b c d Dietary Reference Intakes: Vitamins The National Academies, 2001.

^ a b c Vitamin and Mineral Supplement Fact Sheets Vitamin A

^ N/D= "Amount not determinable due to lack of data of adverse effects. Source of intake should be from food only to prevent high levels of intake"(see Dietary Reference Intakes: Vitamins).

^ Plain type indicates Adequate Intakes (A/I). "The AI is believed to cover the needs of all individuals, but a lack of data prevent being able to specify with confidence the percentage of individuals covered by this intake" (see Dietary Reference Intakes: Vitamins).

^ Vitamin and Mineral Supplement Fact Sheets Vitamin B6

^ Vitamin and Mineral Supplement Fact Sheets Vitamin B12

^ Value represents suggested intake without adequate sunlight exposure (see Dietary Reference Intakes: Vitamins).

^ a b c d e The Merck Manual: Nutritional Disorders: Vitamin Introduction Please select specific vitamins from the list at the top of the page.

^ Dr. Leonid A. Gavrilov, Pieces of the Puzzle: Aging Research Today and Tomorrow

^ Lakhan SE; Vieira KF. Nutritional therapies for mental disorders. Nutrition Journal 2008;7(2). ^ Institute of Medicine. Food and Nutrition Board. Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. National Academy Press, Washington, DC, 2001.

^ Healthier Kids Section: What to take and how to take it.

^ 2004 Annual Report of the American Association of Poison Control Centers Toxic Exposure Surveillance System.

^ National Center for Health Statistics

^a b c Use and Safety of Dietary Supplements NIH office of Dietary Supplements.

^ Jane Higdon Vitamin E recommendations at Linus Pauling Institute's Micronutrient Information Center

^ Overview of Dietary Supplements

^ Illnesses and Injuries Associated with the Use of Selected Dietary Supplements U. S. FDA Center for Food Safety and Applied Nutrition

^ a b c Every Vitamin Page All Vitamins and Pseudo-Vitamins. Compiled by David Bennett.

^ a b Vitamins and minerals - names and facts

^ a b Michael W. Davidson (2004) Anthranilic Acid (Vitamin L) Florida State University. Accessed 20-02-07 General References Include:
Stedman's Medical Dictionary. Ed. Maureen Barlow Pugh et.al. 27th ed. Baltimore: Lippincott Williams & Wilkins, 2000. Donatelle, Rebecca J. Health: The Basics. 6th ed. San Francisco: Pearson Education, Inc. 2005.

External links

http://en.wikipedia.org/wiki/Vitamin, from Wikipedia, the free encyclopedia

USDA RDA chart in PDF format

Health Canada Dietary Reference Intakes Reference Chart for Vitamins

NIH Office of Dietary Supplements: Fact Sheets

NIH Office of Dietary Supplements. Dietary Supplements: Background Information

Psysiology

DNA

For other uses, see DNA (disambiguation).

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 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.
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 it is found in the cell's cytoplasm. Within the chromosomes, chromatin proteins such as histones compact and organize DNA. These compact structures guide the interactions between DNA and other proteins, helping control which parts of the DNA are transcribed.

Physical and chemical properties

DNA is a long polymer made from repeating units called nucleotides.[1][2] The DNA chain is 22 to 26 Ångströms wide (2.2 to 2.6 nanometres), and one nucleotide unit is 3.3 Å (0.33 nm) long.[3] Although each individual repeating unit is very small, DNA polymers can be enormous molecules containing millions of nucleotides. For instance, the largest human chromosome, chromosome number 1, is approximately 220 million base pairs long.[4]
In living organisms, DNA does not usually exist as a single molecule, but instead as a tightly-associated pair of molecules.[5][6] These two long strands entwine like vines, in the shape of a double helix. The nucleotide repeats contain both the segment of the backbone of the molecule, which holds the chain together, and a base, which interacts with the other DNA strand in the helix. In general, a base linked to a sugar is called a nucleoside and a base linked to a sugar and one or more phosphate groups is called a nucleotide. If multiple nucleotides are linked together, as in DNA, this polymer is called a polynucleotide.[7]
The backbone of the DNA strand is made from alternating phosphate and sugar residues.[8] The sugar in DNA is 2-deoxyribose, which is a pentose (five-carbon) sugar. The sugars are joined together by phosphate groups that form phosphodiester bonds between the third and fifth carbon atoms of adjacent sugar rings. These asymmetric bonds mean a strand of DNA has a direction. In a double helix the direction of the nucleotides in one strand is opposite to their direction in the other strand. This arrangement of DNA strands is called antiparallel. The asymmetric ends of DNA strands are referred to as the 5′ (five prime) and 3′ (three prime) ends. One of the major differences between DNA and RNA is the sugar, with 2-deoxyribose being replaced by the alternative pentose sugar ribose in RNA.[6]
The DNA double helix is stabilized by hydrogen bonds between the bases attached to the two strands. The four bases found in DNA are adenine (abbreviated A), cytosine (C), guanine (G) and thymine (T). These four bases are attached to the sugar/phosphate to form the complete nucleotide, as shown for adenosine monophosphate.
These bases are classified into two types; adenine and guanine are fused five- and six-membered heterocyclic compounds called purines, while cytosine and thymine are six-membered rings called pyrimidines.[6] A fifth pyrimidine base, called uracil (U), usually takes the place of thymine in RNA and differs from thymine by lacking a methyl group on its ring. Uracil is not usually found in DNA, occurring only as a breakdown product of cytosine.
Major and minor grooves

The double helix is a right-handed spiral. As the DNA strands wind around each other, they leave gaps between each set of phosphate backbones, revealing the sides of the bases inside (see animation). There are two of these grooves twisting around the surface of the double helix: one groove, the major groove, is 22 Å wide and the other, the minor groove, is 12 Å wide.[10] The narrowness of the minor groove means that the edges of the bases are more accessible in the major groove. As a result, proteins like transcription factors that can bind to specific sequences in double-stranded DNA usually make contacts to the sides of the bases exposed in the major groove.[11]
Base pairingFurther information: Base pair Each type of base on one strand forms a bond with just one type of base on the other strand. This is called complementary base pairing. Here, purines form hydrogen bonds to pyrimidines, with A bonding only to T, and C bonding only to G. This arrangement of two nucleotides binding together across the double helix is called a base pair. The double helix is also stabilized by the hydrophobic effect and pi stacking, which are not influenced by the sequence of the DNA.[12] As hydrogen bonds are not covalent, they can be broken and rejoined relatively easily. The two strands of DNA in a double helix can therefore be pulled apart like a zipper, either by a mechanical force or high temperature.[13] As a result of this complementarity, all the information in the double-stranded sequence of a DNA helix is duplicated on each strand, which is vital in DNA replication. Indeed, this reversible and specific interaction between complementary base pairs is critical for all the functions of DNA in living organisms.[1]

The two types of base pairs form different numbers of hydrogen bonds, AT forming two hydrogen bonds, and GC forming three hydrogen bonds (see figures, left). The GC base pair is therefore stronger than the AT base pair. As a result, it is both the percentage of GC base pairs and the overall length of a DNA double helix that determine the strength of the association between the two strands of DNA. Long DNA helices with a high GC content have stronger-interacting strands, while short helices with high AT content have weaker-interacting strands.[14] In biology, parts of the DNA double helix that need to separate easily, such as the TATAAT Pribnow box in some promoters, tend to have a high AT content, making the strands easier to pull apart.[15] In the laboratory, the strength of this interaction can be measured by finding the temperature required to break the hydrogen bonds, their melting temperature (also called Tm value). When all the base pairs in a DNA double helix melt, the strands separate and exist in solution as two entirely independent molecules. These single-stranded DNA molecules have no single common shape, but some conformations are more stable than others.[16]
Sense and antisenseFurther information: Sense (molecular biology) A DNA sequence is called "sense" if its sequence is the same as that of a messenger RNA copy that is translated into protein. The sequence on the opposite strand is called the "antisense" sequence. Both sense and antisense sequences can exist on different parts of the same strand of DNA (i.e. both strands contain both sense and antisense sequences). In both prokaryotes and eukaryotes, antisense RNA sequences are produced, but the functions of these RNAs are not entirely clear.[17] One proposal is that antisense RNAs are involved in regulating gene expression through RNA-RNA base pairing.[18]
A few DNA sequences in prokaryotes and eukaryotes, and more in plasmids and viruses, blur the distinction made above between sense and antisense strands by having overlapping genes.[19] In these cases, some DNA sequences do double duty, encoding one protein when read 5′ to 3′ along one strand, and a second protein when read in the opposite direction (still 5′ to 3′) along the other strand. In bacteria, this overlap may be involved in the regulation of gene transcription,[20] while in viruses, overlapping genes increase the amount of information that can be encoded within the small viral genome.[21]
SupercoilingFurther information: DNA supercoil DNA can be twisted like a rope in a process called DNA supercoiling. With DNA in its "relaxed" state, a strand usually circles the axis of the double helix once every 10.4 base pairs, but if the DNA is twisted the strands become more tightly or more loosely wound.[22] If the DNA is twisted in the direction of the helix, this is positive supercoiling, and the bases are held more tightly together. If they are twisted in the opposite direction, this is negative supercoiling, and the bases come apart more easily. In nature, most DNA has slight negative supercoiling that is introduced by enzymes called topoisomerases.[23] These enzymes are also needed to relieve the twisting stresses introduced into DNA strands during processes such as transcription and DNA replication.[24]

Alternative double-helical structuresFurther information: Mechanical properties of DNA DNA exists in many possible conformations.[8] However, only A-DNA, B-DNA, and Z-DNA have been observed in organisms. Which conformation DNA adopts depends on the sequence of the DNA, the amount and direction of supercoiling, chemical modifications of the bases and also solution conditions, such as the concentration of metal ions and polyamines.[25] Of these three conformations, the "B" form described above is most common under the conditions found in cells.[26] The two alternative double-helical forms of DNA differ in their geometry and dimensions.
The A form is a wider right-handed spiral, with a shallow, wide minor groove and a narrower, deeper major groove. The A form occurs under non-physiological conditions in dehydrated samples of DNA, while in the cell it may be produced in hybrid pairings of DNA and RNA strands, as well as in enzyme-DNA complexes.[27][28] Segments of DNA where the bases have been chemically-modified by methylation may undergo a larger change in conformation and adopt the Z form. Here, the strands turn about the helical axis in a left-handed spiral, the opposite of the more common B form.[29] These unusual structures can be recognized by specific Z-DNA binding proteins and may be involved in the regulation of transcription.[30]

Quadruplex structuresFurther information: G-quadruplex At the ends of the linear chromosomes are specialized regions of DNA called telomeres. The main function of these regions is to allow the cell to replicate chromosome ends using the enzyme telomerase, as the enzymes that normally replicate DNA cannot copy the extreme 3′ ends of chromosomes.[32] These specialized chromosome caps also help protect the DNA ends, and stop the DNA repair systems in the cell from treating them as damage to be corrected.[33] In human cells, telomeres are usually lengths of single-stranded DNA containing several thousand repeats of a simple TTAGGG sequence.[34]
These guanine-rich sequences may stabilize chromosome ends by forming structures of stacked sets of four-base units, rather than the usual base pairs found in other DNA molecules. Here, four guanine bases form a flat plate and these flat four-base units then stack on top of each other, to form a stable G-quadruplex structure.[35] These structures are stabilized by hydrogen bonding between the edges of the bases and chelation of a metal ion in the centre of each four-base unit.[36] Other structures can also be formed, with the central set of four bases coming from either a single strand folded around the bases, or several different parallel strands, each contributing one base to the central structure.
In addition to these stacked structures, telomeres also form large loop structures called telomere loops, or T-loops. Here, the single-stranded DNA curls around in a long circle stabilized by telomere-binding proteins.[37] At the very end of the T-loop, the single-stranded telomere DNA is held onto a region of double-stranded DNA by the telomere strand disrupting the double-helical DNA and base pairing to one of the two strands. This triple-stranded structure is called a displacement loop or D-loop.[35]
Chemical modifications

Base modificationsFurther information: DNA methylation The expression of genes is influenced by the chromatin structure of a chromosome and regions of that have low or no gene expression usually contain high levels of methylation of cytosine bases. For example, cytosine methylation, producing 5-methylcytosine, is important for X-chromosome inactivation.[38] The average level of methylation varies between organisms, with Caenorhabditis elegans lacking cytosine methylation, while vertebrates show higher levels, with up to 1% of their DNA containing 5-methylcytosine.[39] Despite the biological role of 5-methylcytosine it can deaminate to leave a thymine base, methylated cytosines are therefore particularly prone to mutations.[40] Other base modifications include adenine methylation in bacteria and the glycosylation of uracil to produce the "J-base" in kinetoplastids.[41][42]
DNA damageFurther information: Mutation

DNA can be damaged by many different sorts of mutagens, which are agents that change the DNA sequence. These agents include oxidizing agents, alkylating agents and also high-energy electromagnetic radiation such as ultraviolet light and X-rays. The type of DNA damage produced depends on the type of mutagen. For example, UV light mostly damages DNA by producing thymine dimers, which are cross-links between adjacent pyrimidine bases in a DNA strand.[44] On the other hand, oxidants such as free radicals or hydrogen peroxide produce multiple forms of damage, including base modifications, particularly of guanosine, as well as double-strand breaks.[45] It has been estimated that in each human cell, about 500 bases suffer oxidative damage per day.[46][47] Of these oxidative lesions, the most dangerous are double-strand breaks, as these are difficult to repair and can produce point mutations, insertions and deletions from the DNA sequence, as well as chromosomal translocations.[48]
Many mutagens intercalate into the space between two adjacent base pairs. Intercalators are mostly aromatic and planar molecules, and include ethidium, daunomycin, doxorubicin and thalidomide. In order for an intercalator to fit between base pairs, the bases must separate, distorting the DNA strands by unwinding of the double helix. These structural changes inhibit both transcription and DNA replication, causing toxicity and mutations. As a result, DNA intercalators are often carcinogens, with benzopyrene diol epoxide, acridines, aflatoxin and ethidium bromide being well-known examples.[49][50][51] Nevertheless, due to their properties of inhibiting DNA transcription and replication, they are also used in chemotherapy to inhibit rapidly-growing cancer cells.[52]
Overview of biological functionsDNA usually occurs as linear chromosomes in eukaryotes, and circular chromosomes in prokaryotes. The set of chromosomes in a cell makes up its genome; the human genome has approximately 3 billion base pairs of DNA arranged into 46 chromosomes.[53] The information carried by DNA is held in the sequence of pieces of DNA called genes. Transmission of genetic information in genes is achieved via complementary base pairing. For example, in transcription, when a cell uses the information in a gene, the DNA sequence is copied into a complementary RNA sequence through the attraction between the DNA and the correct RNA nucleotides. Usually, this RNA copy is then used to make a matching protein sequence in a process called translation which depends on the same interaction between RNA nucleotides. Alternatively, a cell may simply copy its genetic information in a process called DNA replication. The details of these functions are covered in other articles; here we focus on the interactions between DNA and other molecules that mediate the function of the genome.
Genes and genomesFurther information: Cell nucleus, Chromatin, Chromosome, Gene, Noncoding DNA Genomic DNA is located in the cell nucleus of eukaryotes, as well as small amounts in mitochondria and chloroplasts. In prokaryotes, the DNA is held within an irregularly shaped body in the cytoplasm called the nucleoid.[54] The genetic information in a genome is held within genes, and the complete set of this information in an organism is called its genotype. A gene is a unit of heredity and is a region of DNA that influences a particular characteristic in an organism. Genes contain an open reading frame that can be transcribed, as well as regulatory sequences such as promoters and enhancers, which control the transcription of the open reading frame.
In many species, only a small fraction of the total sequence of the genome encodes protein. For example, only about 1.5% of the human genome consists of protein-coding exons, with over 50% of human DNA consisting of non-coding repetitive sequences.[55] The reasons for the presence of so much non-coding DNA in eukaryotic genomes and the extraordinary differences in genome size, or C-value, among species represent a long-standing puzzle known as the "C-value enigma."[56] However, DNA sequences that do not code protein may still encode functional non-coding RNA molecules, which are involved in the regulation of gene expression.[57]

Some non-coding DNA sequences play structural roles in chromosomes. Telomeres and centromeres typically contain few genes, but are important for the function and stability of chromosomes.[33][59] An abundant form of non-coding DNA in humans are pseudogenes, which are copies of genes that have been disabled by mutation.[60] These sequences are usually just molecular fossils, although they can occasionally serve as raw genetic material for the creation of new genes through the process of gene duplication and divergence.[61]
Transcription and translationFurther information: Genetic code, Transcription (genetics), Protein biosynthesis A gene is a sequence of DNA that contains genetic information and can influence the phenotype of an organism. Within a gene, the sequence of bases along a DNA strand defines a messenger RNA sequence, which then defines one or more protein sequences. The relationship between the nucleotide sequences of genes and the amino-acid sequences of proteins is determined by the rules of translation, known collectively as the genetic code. The genetic code consists of three-letter 'words' called codons formed from a sequence of three nucleotides (e.g. ACT, CAG, TTT).
In transcription, the codons of a gene are copied into messenger RNA by RNA polymerase. This RNA copy is then decoded by a ribosome that reads the RNA sequence by base-pairing the messenger RNA to transfer RNA, which carries amino acids. Since there are 4 bases in 3-letter combinations, there are 64 possible codons (43 combinations). These encode the twenty standard amino acids, giving most amino acids more than one possible codon. There are also three 'stop' or 'nonsense' codons signifying the end of the coding region; these are the TAA, TGA and TAG codons.

ReplicationFurther information: DNA replication Cell division is essential for an organism to grow, but when a cell divides it must replicate the DNA in its genome so that the two daughter cells have the same genetic information as their parent. The double-stranded structure of DNA provides a simple mechanism for DNA replication. Here, the two strands are separated and then each strand's complementary DNA sequence is recreated by an enzyme called DNA polymerase. This enzyme makes the complementary strand by finding the correct base through complementary base pairing, and bonding it onto the original strand. As DNA polymerases can only extend a DNA strand in a 5′ to 3′ direction, different mechanisms are used to copy the antiparallel strands of the double helix.[62] In this way, the base on the old strand dictates which base appears on the new strand, and the cell ends up with a perfect copy of its DNA.
Interactions with proteinsAll the functions of DNA depend on interactions with proteins. These protein interactions can be non-specific, or the protein can bind specifically to a single DNA sequence. Enzymes can also bind to DNA and of these, the polymerases that copy the DNA base sequence in transcription and DNA replication are particularly important.
DNA-binding proteins

Structural proteins that bind DNA are well-understood examples of non-specific DNA-protein interactions. Within chromosomes, DNA is held in complexes with structural proteins. These proteins organize the DNA into a compact structure called chromatin. In eukaryotes this structure involves DNA binding to a complex of small basic proteins called histones, while in prokaryotes multiple types of proteins are involved.[63][64] The histones form a disk-shaped complex called a nucleosome, which contains two complete turns of double-stranded DNA wrapped around its surface. These non-specific interactions are formed through basic residues in the histones making ionic bonds to the acidic sugar-phosphate backbone of the DNA, and are therefore largely independent of the base sequence.[65] Chemical modifications of these basic amino acid residues include methylation, phosphorylation and acetylation.[66] These chemical changes alter the strength of the interaction between the DNA and the histones, making the DNA more or less accessible to transcription factors and changing the rate of transcription.[67] Other non-specific DNA-binding proteins found in chromatin include the high-mobility group proteins, which bind preferentially to bent or distorted DNA.[68] These proteins are important in bending arrays of nucleosomes and arranging them into more complex chromatin structures.[69]
A distinct group of DNA-binding proteins are the single-stranded-DNA-binding proteins that specifically bind single-stranded DNA. In humans, replication protein A is the best-characterised member of this family and is essential for most processes where the double helix is separated, including DNA replication, recombination and DNA repair.[70] These binding proteins seem to stabilize single-stranded DNA and protect it from forming stem-loops or being degraded by nucleases.

In contrast, other proteins have evolved to specifically bind particular DNA sequences. The most intensively studied of these are the various classes of transcription factors, which are proteins that regulate transcription. Each one of these proteins bind to one particular set of DNA sequences and thereby activates or inhibits the transcription of genes with these sequences close to their promoters. The transcription factors do this in two ways. Firstly, they can bind the RNA polymerase responsible for transcription, either directly or through other mediator proteins; this locates the polymerase at the promoter and allows it to begin transcription.[72] Alternatively, transcription factors can bind enzymes that modify the histones at the promoter; this will change the accessibility of the DNA template to the polymerase.[73]
As these DNA targets can occur throughout an organism's genome, changes in the activity of one type of transcription factor can affect thousands of genes.[74] Consequently, these proteins are often the targets of the signal transduction processes that mediate responses to environmental changes or cellular differentiation and development. The specificity of these transcription factors' interactions with DNA come from the proteins making multiple contacts to the edges of the DNA bases, allowing them to "read" the DNA sequence. Most of these base-interactions are made in the major groove, where the bases are most accessible.[75]

DNA-modifying enzymes
Nucleases and ligasesNucleases are enzymes that cut DNA strands by catalyzing the hydrolysis of the phosphodiester bonds. Nucleases that hydrolyse nucleotides from the ends of DNA strands are called exonucleases, while endonucleases cut within strands. The most frequently-used nucleases in molecular biology are the restriction endonucleases, which cut DNA at specific sequences. For instance, the EcoRV enzyme shown to the left recognizes the 6-base sequence 5′-GATATC-3′ and makes a cut at the vertical line. In nature, these enzymes protect bacteria against phage infection by digesting the phage DNA when it enters the bacterial cell, acting as part of the restriction modification system.[77] In technology, these sequence-specific nucleases are used in molecular cloning and DNA fingerprinting.
Enzymes called DNA ligases can rejoin cut or broken DNA strands.[78] Ligases are particularly important in lagging strand DNA replication, as they join together the short segments of DNA produced at the replication fork into a complete copy of the DNA template. They are also used in DNA repair and genetic recombination.[78]
Topoisomerases and helicasesTopoisomerases are enzymes with both nuclease and ligase activity. These proteins change the amount of supercoiling in DNA. Some of these enzyme work by cutting the DNA helix and allowing one section to rotate, thereby reducing its level of supercoiling; the enzyme then seals the DNA break.[23] Other types of these enzymes are capable of cutting one DNA helix and then passing a second strand of DNA through this break, before rejoining the helix.[79] Topoisomerases are required for many processes involving DNA, such as DNA replication and transcription.[24]
Helicases are proteins that are a type of molecular motor. They use the chemical energy in nucleoside triphosphates, predominantly ATP, to break hydrogen bonds between bases and unwind the DNA double helix into single strands.[80] These enzymes are essential for most processes where enzymes need to access the DNA bases.
PolymerasesPolymerases are enzymes that synthesize polynucleotide chains from nucleoside triphosphates. The sequence of their products are copies of existing polynucleotide chains - which are called templates. These enzymes function by adding nucleotides onto the 3′ hydroxyl group of the previous nucleotide in a DNA strand. Consequently, all polymerases work in a 5′ to 3′ direction.[81] In the active site of these enzymes, the incoming nucleoside triphosphate base-pairs to the template: this allows polymerases to accurately synthesize the complementary strand of their template. Polymerases are classified according to the type of template that they use.
In DNA replication, a DNA-dependent DNA polymerase makes a DNA copy of a DNA sequence. Accuracy is vital in this process, so many of these polymerases have a proofreading activity. Here, the polymerase recognizes the occasional mistakes in the synthesis reaction by the lack of base pairing between the mismatched nucleotides. If a mismatch is detected, a 3′ to 5′ exonuclease activity is activated and the incorrect base removed.[82] In most organisms DNA polymerases function in a large complex called the replisome that contains multiple accessory subunits, such as the DNA clamp or helicases.[83]
RNA-dependent DNA polymerases are a specialized class of polymerases that copy the sequence of an RNA strand into DNA. They include reverse transcriptase, which is a viral enzyme involved in the infection of cells by retroviruses, and telomerase, which is required for the replication of telomeres.[84][32] Telomerase is an unusual polymerase because it contains its own RNA template as part of its structure.[33]
Transcription is carried out by a DNA-dependent RNA polymerase that copies the sequence of a DNA strand into RNA. To begin transcribing a gene, the RNA polymerase binds to a sequence of DNA called a promoter and separates the DNA strands. It then copies the gene sequence into a messenger RNA transcript until it reaches a region of DNA called the terminator, where it halts and detaches from the DNA. As with human DNA-dependent DNA polymerases, RNA polymerase II, the enzyme that transcribes most of the genes in the human genome, operates as part of a large protein complex with multiple regulatory and accessory subunits.[85]
Genetic recombination

Further information: Genetic recombination

A DNA helix usually does not interact with other segments of DNA, and in human cells the different chromosomes even occupy separate areas in the nucleus called "chromosome territories".[87] This physical separation of different chromosomes is important for the ability of DNA to function as a stable repository for information, as one of the few times chromosomes interact is during chromosomal crossover when they recombine. Chromosomal crossover is when two DNA helices break, swap a section and then rejoin.
Recombination allows chromosomes to exchange genetic information and produces new combinations of genes, which increases the efficiency of natural selection and can be important in the rapid evolution of new proteins.[88] Genetic recombination can also be involved in DNA repair, particularly in the cell's response to double-strand breaks.[89]
The most common form of chromosomal crossover is homologous recombination, where the two chromosomes involved share very similar sequences. Non-homologous recombination can be damaging to cells, as it can produce chromosomal translocations and genetic abnormalities. The recombination reaction is catalyzed by enzymes known as recombinases, such as RAD51.[90] The first step in recombination is a double-stranded break either caused by an endonuclease or damage to the DNA.[91] A series of steps catalyzed in part by the recombinase then leads to joining of the two helices by at least one Holliday junction, in which a segment of a single strand in each helix is annealed to the complementary strand in the other helix. The Holliday junction is a tetrahedral junction structure that can be moved along the pair of chromosomes, swapping one strand for another. The recombination reaction is then halted by cleavage of the junction and re-ligation of the released DNA.[92]
Evolution of DNA metabolismFurther information: RNA world hypothesis DNA contains the genetic information that allows all modern living things to function, grow and reproduce. However, it is unclear how long in the 4-billion-year history of life DNA has performed this function, as it has been proposed that the earliest forms of life may have used RNA as their genetic material.[81][93] RNA may have acted as the central part of early cell metabolism as it can both transmit genetic information and carry out catalysis as part of ribozymes.[94] This ancient RNA world where nucleic acid would have been used for both catalysis and genetics may have influenced the evolution of the current genetic code based on four nucleotide bases. This would occur since the number of unique bases in such an organism is a trade-off between a small number of bases increasing replication accuracy and a large number of bases increasing the catalytic efficiency of ribozymes.[95]
Unfortunately, there is no direct evidence of ancient genetic systems, as recovery of DNA from most fossils is impossible. This is because DNA will survive in the environment for less than one million years and slowly degrades into short fragments in solution.[96] Claims for older DNA have been made, most notably a report of the isolation of a viable bacterium from a salt crystal 250-million years old,[97] but these claims are controversial.[98][99]
Uses in technology
Genetic engineeringFurther information: Molecular biology and genetic engineering Modern biology and biochemistry make intensive use of recombinant DNA technology. Recombinant DNA is a man-made DNA sequence that has been assembled from other DNA sequences. They can be transformed into organisms in the form of plasmids or in the appropriate format, by using a viral vector.[100] The genetically modified organisms produced can be used to produce products such as recombinant proteins, used in medical research,[101] or be grown in agriculture.[102][103]
ForensicsFurther information: Genetic fingerprinting Forensic scientists can use DNA in blood, semen, skin, saliva or hair at a crime scene to identify a perpetrator. This process is called genetic fingerprinting, or more accurately, DNA profiling. In DNA profiling, the lengths of variable sections of repetitive DNA, such as short tandem repeats and minisatellites, are compared between people. This method is usually an extremely reliable technique for identifying a criminal.[104] However, identification can be complicated if the scene is contaminated with DNA from several people.[105] DNA profiling was developed in 1984 by British geneticist Sir Alec Jeffreys,[106] and first used in forensic science to convict Colin Pitchfork in the 1988 Enderby murders case.[107] People convicted of certain types of crimes may be required to provide a sample of DNA for a database. This has helped investigators solve old cases where only a DNA sample was obtained from the scene. DNA profiling can also be used to identify victims of mass casualty incidents.[108]
BioinformaticsFurther information: Bioinformatics Bioinformatics involves the manipulation, searching, and data mining of DNA sequence data. The development of techniques to store and search DNA sequences have led to widely-applied advances in computer science, especially string searching algorithms, machine learning and database theory.[109] String searching or matching algorithms, which find an occurrence of a sequence of letters inside a larger sequence of letters, were developed to search for specific sequences of nucleotides.[110] In other applications such as text editors, even simple algorithms for this problem usually suffice, but DNA sequences cause these algorithms to exhibit near-worst-case behaviour due to their small number of distinct characters. The related problem of sequence alignment aims to identify homologous sequences and locate the specific mutations that make them distinct. These techniques, especially multiple sequence alignment, are used in studying phylogenetic relationships and protein function.[111] Data sets representing entire genomes' worth of DNA sequences, such as those produced by the Human Genome Project, are difficult to use without annotations, which label the locations of genes and regulatory elements on each chromosome. Regions of DNA sequence that have the characteristic patterns associated with protein- or RNA-coding genes can be identified by gene finding algorithms, which allow researchers to predict the presence of particular gene products in an organism even before they have been isolated experimentally.[112]
DNA nanotechnology

Further information: DNA nanotechnology DNA nanotechnology uses the unique molecular recognition properties of DNA and other nucleic acids to create self-assembling branched DNA complexes with useful properties. DNA is thus used as a structural material rather than as a carrier of biological information. This has led to the creation of two-dimensional periodic lattices (both tile-based as well as using the "DNA origami" method) as well as three-dimensional structures in the shapes of polyhedra. Nanomechanical devices and algorithmic self-assembly have also been demonstrated, and these DNA structures have been used to template the arrangement of other molecules such as gold nanoparticles and streptavidin proteins.
History and anthropologyFurther information: Phylogenetics and Genetic genealogy Because DNA collects mutations over time, which are then inherited, it contains historical information and by comparing DNA sequences, geneticists can infer the evolutionary history of organisms, their phylogeny.[113] This field of phylogenetics is a powerful tool in evolutionary biology. If DNA sequences within a species are compared, population geneticists can learn the history of particular populations. This can be used in studies ranging from ecological genetics to anthropology; for example, DNA evidence is being used to try to identify the Ten Lost Tribes of Israel.[114][115]
DNA has also been used to look at modern family relationships, such as establishing family relationships between the descendants of Sally Hemings and Thomas Jefferson. This usage is closely related to the use of DNA in criminal investigations detailed above. Indeed, some criminal investigations have been solved when DNA from crime scenes has matched relatives of the guilty individual.[116]

History

Further information: History of molecular biology 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".[117] In 1919 this discovery was followed by Phoebus Levene's identification of the base, sugar and phosphate nucleotide unit.[118] 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.[119]
In 1928, Frederick Griffith 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.[120] This system provided the first clear suggestion that DNA carried genetic information, when Oswald Avery, along with coworkers Colin MacLeod and Maclyn McCarty, identified DNA as the transforming principle in 1943.[121] 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.[122]
In 1953, based on X-ray diffraction images[123] taken by Rosalind Franklin and the information that the bases were paired, James D. Watson and Francis Crick suggested[123] what is now accepted as the first accurate model of DNA structure in the journal Nature.[5] Experimental evidence for Watson and Crick's model were published in a series of five articles in the same issue of Nature.[124] Of these, Franklin and Raymond Gosling's paper was the first publication of X-ray diffraction data that supported the Watson and Crick model,[125][126] this issue also contained an article on DNA structure by Maurice Wilkins and his colleagues.[127] In 1962, after Franklin's death, Watson, Crick, and Wilkins jointly received the Nobel Prize in Physiology or Medicine.[128] However, debate continues on who should receive credit for the discovery, as the Watson and Crick article in Nature was based on Franklin's data without either acknowledgment or her knowledge.[129]
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".[130] Final confirmation of the replication mechanism that was implied by the double-helical structure followed in 1958 through the Meselson-Stahl experiment.[131] 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.[132] These findings represent the birth of molecular biology.
See alsoGenetic disorder Plasmid DNA sequencing Southern blot DNA microarray Polymerase chain reaction Phosphoramidite Junk DNA Triple-stranded DNA Nucleic acid analogues

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Further readingClayton, Julie. (Ed.). 50 Years of DNA, Palgrave MacMillan Press, 2003. ISBN 978-1-40-391479-8 Judson, Horace Freeland. The Eighth Day of Creation: Makers of the Revolution in Biology, Cold Spring Harbor Laboratory Press, 1996. ISBN 978-0-87-969478-4 Olby, Robert. The Path to The Double Helix: Discovery of DNA, first published in October 1974 by MacMillan, with foreword by Francis Crick; ISBN 978-0-48-668117-7; the definitive DNA textbook, revised in 1994, with a 9 page postscript. Ridley, Matt. Francis Crick: Discoverer of the Genetic Code (Eminent Lives) HarperCollins Publishers; 192 pp, ISBN 978-0-06-082333-7 2006 Rose, Steven. The Chemistry of Life, Penguin, ISBN 978-0-14-027273-4. Watson, James D. and Francis H.C. Crick. A structure for Deoxyribose Nucleic Acid (PDF). Nature 171, 737–738, 25 April 1953. Watson, James D. DNA: The Secret of Life ISBN 978-0-375-41546-3. Watson, James D. The Double Helix: A Personal Account of the Discovery of the Structure of DNA (Norton Critical Editions). ISBN 978-0-393-95075-5 Watson, James D. "Avoid boring people and other lessons from a life in science" (2007) New York: Random House. ISBN 978-0-375-41284-4 Calladine, Chris R.; Drew, Horace R.; Luisi, Ben F. and Travers, Andrew A. Understanding DNA, Elsevier Academic Press, 2003. ISBN 978-0-12155089-9

External links

- http://en.wikipedia.org/wiki/DNA, from Wikipedia, the free encyclopedia
The Secret Life of DNA - DNA Music compositions
[2] Crick's personal papers at Mandeville Special Collections Library, Geisel Library, University of California, San Diego DNA Interactive This site from the Dolan DNA Learning Center included dozens of animations as well as interviews with James Watson and others (requires Adobe Flash) DNA from the Beginning Another DNA Learning Center site on DNA, genes, and heredity
from Mendel to the human genome project. Double Helix 1953–2003 National Centre for Biotechnology Education Double helix: 50 years of DNA, Nature Rosalind Franklin's contributions to the study of DNA U.S. National DNA Day — watch videos and participate in real-time chat with top scientists Genetic Education Modules for Teachers — DNA from the Beginning Study Guide Listen to Francis Crick and James Watson talking on the BBC in 1962, 1972, and 1974 PDB Molecule of the Month pdb23_1 DNA under electron microscope DNA at the Open Directory Project DNA coiling to form chromosomes DISPLAR: DNA binding site prediction on protein Dolan DNA Learning Center Olby, R. (2003) "Quiet debut for the double helix" Nature 421 (January 23): 402–405. Basic animated guide to DNA cloning DNA the Double Helix Game From the official Nobel Prize web site [3] download DNA data of various species (such as homo sapiens DNA - 2.5 GB) DNA Lab, demonstrates how to extract DNA from wheat using readily available equipment and supplies.