With the discovery that DNA is the genetic material, the race to uncover the three-dimensional structure of DNA began. The arrangement of the bonds in a DNA polymer was already known. Each monomer unit, or nucleotide, of the polynucleotide chain is made up of a phosphate group, the sugar deoxyribose, and a nitrogenous base. The phosphate groups and deoxyribose sugars form the backbone of the chain. There are four nitrogenous bases: guanine, cytosine, adenine, and thymine, abbreviated G, C, A, and T.

The Austrian biochemist Erwin Chargaff found that the amounts of adenine and thymine found in DNA were always about equal to each other, and the amounts of guanine and cytosine were always about equal to each other. These observations are known as Chargaff’s Rules. At the time, nobody understood why Chargaff’s rules were true.

Determining the three-dimensional structure of a DNA molecule required data from several groups. In the early 1950’s while working in London, Rosalind Franklin produced photographs of the X-ray diffraction patterns of DNA fibers. In 1953, from Franklin’s X-ray photographs, an American James Watson and an Englishman Francis Crick determined that DNA is a spiraling coil called a helix. They were also able to tell that the helix was about 2 nanometers wide.

This fit with DNA being a helix of two polynucleotide chains-- commonly called a DNA double helix. In a DNA double helix each turn of the coil is 3.4 nanometers long, and each nucleotide is 0.34 nanometers long. This means that there are 10 nucleotides per turn.

The width of the double helix also helped scientists understand Chargaff’s Rules. Adenine and guanine have two rings. They are called purines. Thymine and cytosine have only one ring, and are called pyrimidines. To fit the width of the double helix, a purine on one strand must pair with a pyrimidine on the other strand.

Chargaff’s Rules state there is the same amount of adenine as thymine, so adenine pairs with thymine. Similarly, guanine pairs with cytosine. The purine and pyrimidine are held together by hydrogen bonds.

Another interesting aspect of DNA structure is the orientations of the two strands in the double helix. The strands run in opposite directions. This is called an antiparallel orientation.

The carbon atoms in the deoxyribose sugar are carbon 1 to carbon 5. A phosphate group is bonded to the number 5 carbon, known as the 5 prime carbon, and another phosphate group is bonded to the number 3 carbon, the 3 prime carbon. or simplicity, we’ll place one strand with the 5 prime carbons all oriented to the top of the screen, and the 3 prime carbons to the bottom of the screen. Since the other strand is antiparallel, all of the 5 prime carbons will face the bottom of the screen and the 3 prime carbons will face the top of the screen. As we read the nucleotides down the screen, the strand on the left is 5 prime to 3 prime, and the strand on the right is 3 prime to 5 prime.

This is a piece of double-stranded DNA. When we break the hydrogen bonds holding the strands together, we end up with two single-stranded DNA molecules. We can use the single strand of DNA to figure out the sequence of nucleotides on the second strand.

To determine the sequence of nucleotides on the second strand, remember that adenine always pairs with thymine, and guanine always pairs with cytosine. To find the sequence on the second strand, the original strand is used as a guide, or template. Both new pieces of double-stranded DNA are identical to the original double-stranded DNA.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education