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Transformation is one of three basic mechanisms for genetic exchange in bacteria. Transformation may be either a natural processthat is, one that has evolved in certain bacteriaor it may be an artificial process whereby the recipient cells are forced to take up DNA by a physical, chemical, or enzymatic treatment. In both cases, exogenous DNA (DNA that is outside the host cell), is taken into a recipient cell where it is incorporated into the recipient genome , changing the genetic makeup of the bacterium.

Natural Transformation

Natural transformation is a physiological process that is genetically encoded in a wide range of bacteria. Most bacteria must shift their physiology in order to transform DNA; that is, they must become "competent" for taking up exogenous DNA. There appear to be two basic mechanisms by which bacteria can become competent for transformation. In some bacteria, including Streptococcus pneumoniae and Bacillus subtilis, competence is externally regulated. These bacteria produce and secrete a small protein called competence factor that accumulates in the growth medium.

When the bacterial culture reaches a sufficient density, the concentration of competence factor reaches a level high enough to bind receptors on the outside of the cell. This event causes an internal signal to turn on the expression of the genes needed for transformation. Thus, competence development is controlled by cell density. There are a number of other bacterial functions that are similarly regulated, and these processes are collectively called quorum sensing mechanisms. In other bacteria, including Haemophilus influenzae and Pseudomonas stutzeri, competence development is internally regulated. When there is a shift in the growth dynamics of the bacterium, an internal signal triggers competence development.

Once competence is induced, three additional steps are required for natural transformation. After induction of competence, double-stranded DNA is bound to specific receptors on the surface of the competent cells. These receptors are lacking in noncompetent cells. The double-stranded DNA is nicked and one strand is degraded while the other strand enters the cell. This process is called DNA uptake. Finally, the recombination enzymes of the recipient cell will bind the single-strand DNA that has entered it, align it with its homologous DNA on the recipient chromosome, and recombine the new DNA into the chromosome, incorporating any genetic differences that exist on the entering DNA.

Artificial Transformation

While a wide variety of bacteria can transform naturally, many species cannot take up DNA from an outside source. In some cases DNA can be forced into these cells by chemical, physical, or enzymatic treatment. This is especially important in genetic engineering, as artificial transformation is essential for the introduction of genetically altered sequences into recipient cells. One of the two most common methods is a chemical process where cells are heat-shocked, then treated with the DNA and a high concentration of calcium ions. The calcium ions precipitate the DNA on the surface of the cell, where the DNA is forced into the recipient.

More recently a new method, called electroporation, has been used to introduce DNA by artificial transformation. In this process a suspension of recipient bacteria and transforming DNA is placed in a container with metal sides. A high-voltage electrical current is passed through the sample, temporarily creating small pores, or channels, in the membranes of the bacteria. The DNA enters the cells and the pores close. Thus, exogenous (outside) DNA is introduced into the recipient.

Because exogenous DNA is not enclosed within cell walls, it is susceptible to enzymes that degrade DNA, called DNases. A hallmark of transformation is that it is sensitive to DNase, while the other two processes of genetic exchange, transduction and conjugation , are DNase resistant. Transduction is DNase resistant because the DNA is protected inside a viral protein coat. Conjugation is DNase resistant because fusion occurs between donor and recipient cells, meaning the DNA is never exposed to the outside environment or to enzymes.

Discovery of Transformation

The first report of transformation was an example of natural transformation. Dr. Frederick Griffith was a public health microbiologist studying bacterial pneumonia during the 1920s. He discovered that when he first isolated bacteria from the lungs of animals with pneumonia, the bacterial colonies that grew on the agar plates were of reasonable size and had a glistening, mucoid appearance. When he transferred these colonies repeatedly from one agar plate to another, however, mutant colonies would appear that were much smaller and were chalky in appearance. He designated the original strains as "smooth" strains, and the mutants as "rough" strains. When Griffith injected mice with smooth strains they contracted pneumonia, and smooth strains of the bacterium could be reisolated from the infected mice. However, when he infected the mice with rough strains they did not develop the disease. The smooth strains were capable of causing disease, or were "virulent," while the rough strains did not cause disease, or were "aviruluent."

Griffith questioned whether the ability to cause disease was a direct result of whatever product was making the bacterial colonies smooth, or whether rough strains of the bacterium were less capable of establishing disease for some other reason. To investigate this idea, he prepared cultures of both bacterial types. He pasteurized (killed) each of these cultures by heating them for an hour and then injected the heat-treated extracts into mice. His hypothesis was that if the bacteria had to be living to cause disease, heat-treating that killed the bacteria would prevent disease. If, on the other hand, the smooth material was itself a toxin, heating would not destroy it, meaning heated extracts of smooth strains would continue to cause disease. When Griffith injected heated extracts of both smooth and rough strains into mice, neither caused disease. This suggested to him that only living smooth cells could cause disease.

In his next experiment he coinjected unheated, live rough bacteria with heat-treated, dead smooth bacteria into mice. All of the mice developed disease, and when bacteria were isolated from the lungs of the diseased mice, all the isolates were smooth. This led Griffith to propose that there was some "transforming principle" in the heated smooth extract that converted the rough strains back to smooth ones capable of causing diseases. Griffith was not able to determine the nature of this transforming principle, but his experiments suggested that some "inheritable" material present in the heated extract could genetically convert strains from one colony type to another.

Approximately ten years later, another research team, that of Oswald Avery, Colin Munro MacLeod, and Maclyn McCarty, followed up on Griffith's experiments by enzymatically and biochemically characterizing the heated transforming extracts that Griffith had produced. Their studies indicated that the transforming principle was deoxyribonucleic acid (DNA), providing the first definitive evidence that DNA was the inheritable material.

see also Conjugation; Nature of the Gene, History; Recombinant DNA; Transduction.

Gregory Stewart


Curtis, Helen, and N. Susan Barnes. Invitation to Biology, 5th ed. New York: WorthPublishers, 1994.

Ingraham, John, and Catherine Ingraham. Introduction to Microbiology, 2nd ed. PacificGrove, CA: Brooks/Cole Publishing, 1999.

Madigan, Michael T., John Martinko, and Jack Parker. Brock Biology of Microorganisms, 10th ed. Upper Saddle River, NJ: Prentice Hall, 2000.

Streips, Uldis N., and Ronald E. Yasbin. Modern Microbial Genetics, 2nd ed. Hoboken, NJ: John Wiley & Sons, 2002.

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650. Transformation

  1. Actaeon surprised Artemis bathing and was changed by her into a stag. [Gk. Myth.: Jobes, 28]
  2. Adonis killed by a boar, he was changed into an anemone by Venus. [Gk. Lit.: Metamorphoses ]
  3. Alectryon changed into rooster by angry Ares for neglecting to warn against approach of the sun; doomed forever to announce its arrival. [Gk. Myth.: Zimmerman, 17]
  4. Alpheus hunter pursuing Arethusa is turned into a river. [Gk. Myth.: Brewer Dictionary, 26]
  5. Arachne won weaving contest against Athena, who then changed her into a spider. [Gk. Myth.: Jobes, 116]
  6. Arethusa changed into stream by Artemis to save her from river god, Alpheus. [Gk. Myth.: Zimmerman, 29]
  7. Argus hundred-eyed giant ordered slain by Zeus, changed by Hera into a peacock with a tail full of eyes. [Gk. Myth.: Benét, 48]
  8. Ascalaphus turned into an owl by Demeter. [Gk. Myth.: Kravitz, 37]
  9. Atys beloved of Cybele, who changed him into a pine tree as he was about to commit suicide. [Gk. Myth.: Brewer Dictionary, 55]
  10. Bottom, Nick Puck turns his head into that of an ass. [Br. Drama: Shakespeare A Midsummer Nights Dream ]
  11. Breast, The literature professor transformed into a 155-pound breast topped by a football-size nipple. [Am. Lit.: Philip Roth The Breast in Weiss, 55]
  12. Cadmus sows dragons teeth that turn into armed men. [Gk. Myth.: Brewer Dictionary, 180]
  13. Callisto nymph that Zeus transformed into a bear. [Gk. Myth.: Walsh Classical, 28]
  14. Ceyx and Halcyone royal couple are changed into sea-birds. [Gk. Myth.: Bulfinch]
  15. Chelone changed into tortoise for refusing to attend wedding of Zeus and Hera. [Gk. Myth.: Zimmerman, 59]
  16. Circe seductive sorceress who turned Odysseus companions into swine. [Gk. Myth.: Benét, 201]
  17. Clytie ocean nymph, in love with Apollo, was changed into a heliotrope. [Gk. Myth.: Brewer Dictionary ]
  18. Crocus distressed by unrequited love, changed by Hermes into a saffron plant. [Gk. Myth.: Avery, 338]
  19. Cyane turned by Hades into a fountain (or river). [Gk. Myth.: Kravitz, 70]
  20. Daphne turned into laurel tree to escape Apollo. [Gk. Myth.: Kravitz, 75]
  21. Derceto nature deity; became mermaid when Mopsus pursued her. [Philistine Myth.: Jobes, 433; Avery, 389]
  22. Dirce changed by gods into a fountain. [Gk. Myth.: Zimmerman, 88]
  23. Doolittle, Eliza Cockney flower-girl, trained by a professor, gains admission to polite society. [Br. Drama: G. B. Shaw Pygmalion ; Am. Musical: My Fair Lady in On Stage, 373]
  24. Duchesss baby ill-treated infant turns into a pig. [Br. Lit.: Lewis Carroll Alices Adventures in Wonderland ]
  25. Eurydice transformed into a bacchante to suit enamored Zeus. [Fr. Operetta: Offenbach, Orpheus in Hades, Westerman, 271272]
  26. frog prince transformed by a witch, he is turned back into a prince by favor of a princess. [Ger. Fairy Tale: Grimm]
  27. Galatea statue of woman fashioned by Pygmalion and brought to life by Aphrodite. [Gk. Myth.: Jobes, 623]
  28. Hermine her body is shrunk to figurine size. [Ger. Lit.: Herman Hesse Steppenwolf ]
  29. Hippomenes and Atalanta changed into a lion and a lioness for failing to honor Venus after their legendary race. [Gk. Myth.: Bulfinch]
  30. Hulk, the the monster that David Banner becomes when angered. [Comics and TV: Horn, 324]
  31. Io changed into heifer by Zeus because of Heras jealousy. [Gk. Myth.: Zimmerman, 137]
  32. Jekyll, Dr. Henry by means of a drug, changes himself into a repulsive, evil creature. [Br. Lit.: Stevenson Dr. Jekyll and Mr. Hyde in Magill I, 214]
  33. Jurgen middle-aged pawnbroker turned into a young man ready for amorous adventure. [Am. Lit.: Jurgen in Magill I, 464]
  34. Lawford, Arthur dozing in a graveyard, his body but not his mind is replaced by one of the dead. [Br. Lit.: Walter de la Mare The Return in Magill II, 896]
  35. Lots wife disobeyed Gods order not to look back; she became a pillar of salt. [O.T.: Genesis 19:26]
  36. Lucius metamorphosed into an ass, has a series of adventures. [Rom. Lit.: Apuleius Metamorphoses or The Golden Ass in Magill I, 309]
  37. Lycaon king turned into a wolf for having served human flesh. [Gk. Myth.: Brewer Dictionary, 570]
  38. Medusa her face was so hideous that any who saw it were turned to stone. [Gk. Myth.: Brewer Dictionary, 596]
  39. Midas everything he touched turned to gold. [Gk. Myth.: Zimmerman, 167]
  40. Myrmidons originally ants, turned into human beings by Zeus to populate the island of Oenone. [Gk. Myth.: Benét, 697]
  41. Narcissus enamored of his own reflection in a pool, he pines away and is turned into a flower. [Gk. Myth.: Benét, 701]
  42. Niobe her children slain, she is turned to stone by Zeus at her own request. [Gk. Myth.: Benét, 717]
  43. Odysseus crew turned into swine by Circe. [Gk. Lit.: Odyssey ]
  44. Orion slain by Diana, giant hunter becomes a constellation. [Gk. Myth.: Brewer Dictionary, 664]
  45. Orlando born a man in 1588, dies a woman in 1928. [Br. Lit. Orlando, Magill I, 698700]
  46. Periclymenus had the power to assume any form. [Gk. Myth.: Zimmerman, 199]
  47. Petrouchka clown puppet that comes to life. [Russ. Ballet: Petrouchka in Thompson, 1657]
  48. Philemon and Baucis couple turned into an oak and a linden so that they are together in death. [Gk. Myth.: Brewer Dictionary, 698]
  49. Philomela (Philomena) changed by gods into nightingale. [Gk. Myth.: Zimmerman, 205206]
  50. Phoulca bogey-beast taking many forms; e.g., horse, bat, eagle. [Irish Folklore: Briggs, 326327]
  51. Pinocchio changed from mischievous puppet to loving boy. [Ital. Lit.: Pinocchio ]
  52. portrait of Dorian Gray becomes more hideous as Gray grows more vicious. [Br. Lit.: Oscar Wilde The Picture of Dorian Gray ]
  53. Proteus has ability to change shape. [Gk. Myth.: Kravitz, 201]
  54. pumpkin turned into coach by Cinderellas fairy godmother. [Fr. Fairy Tale: Cinderella ]
  55. Red Queen shaken by Alice, she turns into a kitten. [Br. Lit.: Lewis Carroll Through the Looking-Glass ]
  56. Rhinoceros Berenger discovers that Jean is turning into a rhinoceros like all the other townspeople. [Fr. Drama: Weiss, 394]
  57. Samsa, Gregor young man wakes up one day to find that he has turned into an enormous insect. [Ger. Lit.: Kafka The Metamorphosis in Benét, 663]
  58. Syrinx nymph, pursued by Pan, was changed into a reed, from which Pan made his pipes. [Gk. Myth.: Brewer Dictionary, 876]
  59. tarnhelm golden helmet that allowed its wearer to assume any form or even become invisible. [Ger. Opera: Wagner The Ring of the Nibelung ]
  60. Tebrick, Silvia Fox changed from dignified woman into wild fox. [Am. Lit.: Lady into Fox, Magill I, 486]
  61. Tippetarius boy changed into Ozma, Queen of Oz. [Childrens Lit.: The Land of Oz ]
  62. Tiresias saw two snakes copulating and was changed into a woman. [Gk. Myth.: Jobes, 1576]
  63. Tithonus unable to remove him from the earth because of his immortality, Eos changes him into a grasshopper. [Gk. Myth.: Brewer Dictionary, 901]
  64. transubstantiation changing of bread to body of Christ. [Christian Theol.: Brewer Dictionary, 1097]
  65. Veretius Welsh king changed into wolf by St. Patrick. [Br. Legend: Brewer Dictionary, 1148]
  66. Zeus assumed many forms to indulge his passions. [Zimmerman, 292293]

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Transformation is a process in which exogenous DNA is taken up by a (recipient) cell, sphaeroplast, or protoplast. In order to take up DNA, the cells must be competent. Competence is a state of bacterial cells during which the usually rigid cell wall can transport a relatively large DNA macromolecule. This is a highly unusual process, for bacteria normally lack the ability to transport macromolecules across the rigid cell wall and through the cyotplasmic membrane. Several bacteria, such as Bacillus, Haemophilis, Neisseria, and Streptococcus, possess natural competence because their cells do not require special treatment to take up DNA. This process is transient and occurs only in special growth phases, typically toward the end of log phase.

The demonstration of DNA transformation was a landmark in the history of genetics. In 1944, Oswald Avery, Colin MacLeod, and Maclyn McCarty conducted famous Streptococcus pneumoniae transformation experiments. Bacterial pneumonia is caused by the S strain of S. pneumoniae. The S strain synthesizes a slimy capsule around each cell. The capsule is composed of a polysaccharide that protects the bacterium from the immune response of the infected animal and enables the bacterium to cause the disease. The colonies of the S strain appear smooth because of the capsule formation. The strain that does not synthesize the polysaccharide, hence does not have the capsule, is called R strain because the surface of the colonies looks rough. The R strain does not cause the disease. When heat-killed S strain was mixed with live R strain, cultured, and spread on to a solid medium, a few S strain colonies appeared. When S cell extract was treated with RNase or proteinase and mixed with the live R strain, R colonies and a few S colonies appeared. When the S strain cell extract was treated with DNase and mixed with live R strain, there were only R strain colonies growing on the agar plates. These experiments proved fundamentally that DNA is the genetic material that carries genes.

Transformation is widely used in DNA manipulation in molecular biology . For most bacteria that do not possess natural competency, special treatment, such as calcium chloride treatment, can render the cells competent. This is one of the most important techniques for introducing recombinant DNA molecules into bacteria and yeast cells in genetic engineering.

See also Cell membrane transport; Microbial genetics

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trans·for·ma·tion / ˌtransfərˈmāshən/ • n. a thorough or dramatic change in form or appearance: its landscape has undergone a radical transformation. ∎  a metamorphosis during the life cycle of an animal. ∎  Physics the induced or spontaneous change of one element into another by a nuclear process. ∎  Math. & Logic a process by which one figure, expression, or function is converted into another that is equivalent in some important respect but is differently expressed or represented. ∎  Linguistics a process by which an element in the underlying deep structure of a sentence is converted to an element in the surface structure. ∎  Biol. the genetic alteration of a cell by introduction of extraneous DNA, esp. by a plasmid. ∎  Biol. the heritable modification of a cell from its normal state to a malignant state.

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"transformation." The Oxford Pocket Dictionary of Current English. . 11 Dec. 2017 <>.

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1. Another name for function, used especially in geometry.

2. of programs. See program transformation.

3. of statistics data. A change of scale used to improve the validity of statistical analyses. For data in which small values have smaller variance than large values a logarithmic or square-root transformation is often recommended. For data in the form of proportions, a transformation from the scale (0,1) to an infinite scale is advisable before performing analysis of variance or regression analysis. Several transformations exist for proportions, such as the logistic or log-odds-ratio that is used in the analysis of generalized linear models. Appropriate transformations may be suggested by studying residuals in a regression analysis.

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"transformation." A Dictionary of Computing. . 11 Dec. 2017 <>.

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1. A permanent heritable change in a cell, particularly a bacterial cell, that occurs as a result of its acquiring foreign DNA. Nonvirulent bacterial cells can be transformed into virulent forms if cultured in a medium containing killed virulent bacteria.

2. The conversion of a normal cell into a malignant cell (see cancer), which can be brought about by the action of carcinogens or oncogenic viruses.

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"transformation." A Dictionary of Biology. . 11 Dec. 2017 <>.

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transformation, in genetics: see recombination.

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