UNRAVELING THE MYSTERIES OF THE DOUBLE HELIX

UNRAVELING THE MYSTERIES OF
THE DOUBLE HELIX
                                   In the 20th century, scientists discovered more about heredity, especially about DNA. In the 1920s, Dr. Fred Griffith, a British microbiologist, studied two strains, or types, of pneur nococcus—the bacteria that can cause pneumonia. One strain has a smooth outer coating on its cells, and the other has a rough outer coating

                             When Griffith injected the smooth strain into laboratory mice, they developed pneumonia and (lied. Aninjection of the strain with the rough outer coating, however, did not harm the mice. Griffith then killedsome of the smooth strain bacteria and injected them into mice. The animals lived, and the scientist concluded that killing the bacteria rendered them harmless.An unexpected development took place, however,wheti Griffith took the (lead, smooth strain of bacteria and injected it into mice along with the harmless, live,rough strain of bacteria. To his amazement, the animals died. In 1928, Griffith concluded that something in the dead, harmful strain had transformed the harmless strain into a dead lv one..

                                In the 1940s, Oswald T. Avery and a team of res earchers at Rockefeller University in New York Citytried to determine just what had caused that transform ation. In 1944, after years of experiments based onGriffith’s findings, Avery and his team announced that the transforming substance was DNA. Spurred on by these developments, other scientists began to study DNA. Scientists already understoodmany facts about its composition, but no one knew the molecule’s exact structure. DNA was too small tobe photographed by normal means or even viewed through a microscope. In the early 1950s, however,Maurice Vilkins and Rosalind Franklin, working at Kings College in Cambridge, England, used X-ray diffraction photography to study DNA. This type of photography does not produce the kind of picture that comes from a regular camera. Instead, it makes an outline of the objects by passing X rays through them.Using all these intriguing yet incomplete clues, James Watson, an American, and Francis Crick, an

Englishman, built three-dimensional models of possib le structures for this molecule, based on the X-ray diff

raction photographs of Wilkins and Franklin. Working at Cambridge University in England, the team finally

came up with the concept of a double helix, a shape that looked like two spiral staircases wound around

each other. In oilier words, it resembled a twisted ladder, with “rungs” made up of the nucleotide bases—

adenine, thymine, cytosine, and guanine, A, C, T, and G. On April 25, 1953, Watson and Crick announced

their findings in Nature magazine, a respected British publication. In 1962 Watson, Crick, and Wilkins won

the Nobel urize. IRosalind Franklin. who had con-tributed so much to this discovery, died before she