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verted 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: Worth Publishers, 1994.

Ingraham, John, and Catherine Ingraham. Introduction to Microbiology, 2nd ed. Pacific Grove, 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. Hobo-ken, NJ: John Wiley & Sons, 2002.

Transgenic Animals

The term "transgenics" refers to the science of inserting a foreign gene into genome the total an organism's genome. Scientists do this, creating a "transgenic" organism, genetic material in a to study the function of the introduced gene and to identify genetic elements cell or organism J . ,.,. , , c -

that determine which tissue and at what stage of an organism s development a gene is normally turned on. Transgenic animals have also been created to produce large quantities of useful proteins and to model human disease.

In the early 1980s Frank Ruddle and his colleagues created the first transgenic animal, a transgenic mouse. Researchers making transgenic mice use a very fine glass needle to inject pieces of DNA into a fertilized mouse pronuclei egg and egg. They inject the DNA into one of the egg's two pronuclei, before the pronuclei fuse to become the nucleus of the developing embryo's first cell. After the DNA is injected, multiple copies, usually joined end-to-end, insert randomly into the host organism's nuclear DNA.

Multiple injected embryos are then transferred to a surrogate mother mouse to develop to term. Only a small percentage of the embryos survive the injection, and even of those that survive, not all have successfully incorporated the foreign DNA into their genome. Once the mice are born, researchers must identify which mice have the foreign gene in their genome. The animals that contain the added foreign DNA, or transgene are referred to as transgenics.

sperm nuclei before they fuse during fertilization

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