ACCESSION NO:  93-94-1129
       TITLE:  DNA's New Twists
      AUTHOR:  RENNIE, JOHN
     JOURNAL:  Scientific American
    CITATION:  March, 1993, 266: 122-132.
        YEAR:  1993
    PUB TYPE:  Article
 IDENTIFIERS:  MOLECULAR BIOLOGY; GENETICS; DNA; RNA; TRANSPOSABLE GENES; 
               GENE MUTATIONS; FRAGILE X SYNDROME; IMPRINTING; RNA EDITING
ABSTRACT: The science of genetics was born during the mid-19th century in the garden of the Moravian monastery where Gregor Mendel experimented with pea plants. The mathematical rules of inheritance that he and subsequent workers developed became the cornerstone of modern biology. Not until the 1950s was DNA (deoxyribonucleic acid) identified as the primary genetic material, thus opening the era of molecular biology.

Genetic information, researchers learned, was written in the double-strand helix of DNA as a sequence of its four constituent nucleotide bases. Single strands of DNA acted as templates for making complementary molecules of messenger RNA (ribonucleic acid), which carried the information to the organelles called ribosomes. The ribosomes then read the messenger, and interpreting its bases three at a time, popped together appropriate amino acids to make a protein. But as investigators have more closely scrutinized the actions of the genome (the complete set of an organism's genes) and its associated molecules, they have found that their properties and behavior are far more diverse.

One of the first scientists to appreciate the dynamism of the genome was the geneticist Barbara McClintock. In 1947, she made the revolutionary discovery that some genes could "jump" within and between chromosomes. In 1991 Marilyn Houck, a mite specialist now at Texas Tech University, and Margaret Kidwell of the University of Arizona, found hints that a jumping gene, called a P element, moved between two species of fruit fly.

Jumping genes represent a type of mutation unforeseen by the founders of genetics. In recent years, researchers have discovered another mutation: abnormal genes that suddenly balloon in size, with tragic consequences. The discovery of this type of mutation is helping geneticists to fathom the odd patterns of inheritance associated with several diseases, such as fragile X syndrome, the most common inherited cause of mental retardation.

One fundamental assumption of Mendelian genetics is that the effect of a gene is totally independent of its origins-- whether it came from an organism's mother or father. Yet geneticists have found compelling instances in which males and females seem to imprint, or mark, the genes that they pass on. Experiments on mice have shown that embryos with a full set of chromosomes derived from just one sex inevitably fail to reach birth, even though they are genetically identical to normal mice. In humans, researchers have found evidence for the effects of genome imprinting in at least two diseases. Prader-Willi syndrome, characterized by mental retardation and obesity, and Angelman syndrome, characterized by mental retardation and staccato movements.

A still greater molecular genetics controversy concerns the effects of the environment on genomes. Radiation, carcinogenic chemicals and other agents are well known for their ability to induce random mutations. Yet a few biologists are investigating the possibility that environmental stresses can sometimes direct the kinds of mutations that occur.