10.3 Flashcards
Note 1 —-»
After the 1952 Hershey-Chase experiment convinced most biologists that DNA was the material that stored genetic information, a race was on to determine how the structure of this molecule could account for its role in heredity. At the time, the arrangement of covalent bonds in a nucleic acid polymer was well established, and therefore, researchers focused on discovering the three-dimensional shape of DNA. First to the finish line were two scientists who were relatively unknown at the time—American James D. Watson and Englishman Francis Crick.
Note 2 —-»
The partnership that solved the puzzle of DNA structure began soon after Watson, a 23-year-old newly minted Ph.D., journeyed to Cambridge University in England, where the more senior Crick was studying protein structure with a technique called X-ray crystallography. While visiting the laboratory of Maurice Wilkins at King’s College in London, Watson saw an X-ray image of DNA produced by Wilkins’s colleague, Rosalind Franklin. A careful study of the image-enabled Watson to deduce the basic shape of DNA to be a helix (spiral) with a uniform diameter and the nitrogenous bases located above one another like a stack of dinner plates. The thickness of the helix suggested that it was made up of two polynucleotide strands, forming a double helix.
Double Helix
The form of native DNA, referring to its two adjacent polynucleotide strands interwound into a spiral shape.
Note 3 —-»
At first, Watson and Crick imagined that the bases paired like with like—for example, A with A and C with C. But that kind of pairing did not fit the X-ray data, which suggested that the DNA molecule has a uniform diameter. An A-A pair, with two double-ring bases, would be almost twice as wide as a C-C pair, made of two single-ring bases. It soon became apparent that a double-ringed base (purine) on one strand must always be paired with a single-ringed base (pyrimidine) on the opposite strand to produce a molecule of uniform thickness. After considerable trial and error, Watson and Crick realized that the chemical structures of the bases dictated the pairings even more specifically. As discussed in the previous module, each base has protruding functional groups that can best form hydrogen bonds with just one appropriate partner. Adenine can best form hydrogen bonds with thymine and only thymine, and guanine with cytosine and only cytosine. In the biologist’s shorthand, A pairs with T, and G pairs with C. A is also said to be “complementary” to T and G to C. Watson and Crick’s pairing scheme both fit what was known about the physical attributes and chemical bonding of DNA and explained some data obtained several years earlier by American biochemist Erwin Chargaff. Chargaff had discovered that the amount of adenine in the DNA of anyone species was equal to the amount of thymine and that the amount of guanine was equal to that of cytosine. Chargaff’s rules, as they are called, are explained by the fact that A on one of DNA’s polynucleotide chains always pairs with T on the other polynucleotide chain, and G on one chain pairs only with C on the other chain.
Note 4 —-»
Although the Watson-Crick base-pairing rules dictate the allowable combinations of nitrogenous bases that can form the rungs of the double helix, they place no restrictions on the sequence of nucleotides along the length of a DNA strand. In fact, the sequence of bases can vary in countless ways, and each gene has a unique order of nucleotides or base sequence. In April 1953, Watson and Crick rocked the scientific world with a succinct paper in the British scientific journal Nature that explained their molecular model for DNA. In 1962, Watson, Crick, and Wilkins received the Nobel Prize for their work. (Sadly, Rosalind Franklin died of multiple cancers in 1958 at the age of 38 and was thus ineligible for the prize; some suspect that her work with X-ray radiation may have caused her illness.) Few milestones in the history of biology have had as broad an impact as the discovery of the double helix, with its A-T and C-G base pairing. The Watson-Crick model gave new meaning to the words genes and chromosomes—and to the chromosome theory of inheritance. The Watson-Crick model gave new meaning to the words genes and chromosomes—and to the chromosome theory of inheritance.
Along one strand of a double helix is the nucleotide sequence GGCATAGGT. What is the complementary sequence for the other DNA strand?
CCGTATCCA
