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DNA Chemistry

You have already learned that DNA, or deoxyribonucleic acid, is a large molecule consisting of two long chains of nucleotides that are wrapped around one another in a right- handed helix. Using this animation, you can now learn more about the chemical structure of this fascinating molecule. If any of the terms in this advanced section confuse you, refer back to the "Basic DNA Structure" animation for a review.

Nucleotides, which are the basic structural units of DNA, are composed of three parts - a five-carbon sugar, a nitrogen- containing base, and a phosphate group. These parts are shown in detail in the first part of this animation.

The five-carbon sugar, shown in light blue, gives DNA part of its name. This sugar is called a deoxyribose because its 3' carbon has a hydroxyl group, while its 2' carbon does not. Note that the 5' carbon of each sugar attaches to a phosphate group (dark blue circle).

Each DNA chain is held together by covalent bonds (called phosphodiester bonds) between the 5' phosphate group of one sugar and the 3' hydroxyl of the adjacent sugar. The "sugar phosphate backbone," which is shown in the second part of this animation, consists of these phosphodiester bonds. They are very strong and help to make DNA a stable molecule.

As the individual chains wrap around one another to form the double helix, they are held together by hydrogen bonds between the complimentary bases on opposite strands. When a base on one strand is hydrogen bonded to its complimentary base on the other strand, the structure is called a base pair. As you can see in the second part of this animation, an A-T base pair is held together by two hydrogen bonds, while a G-C base pair is held together by three. Thus, G-C base pairs are more stable than A-T base pairs.

Finally, the end of each chain is named for the chemical structure found at its terminus. The 5' end of a chain has a 5' phosphate, while the 3' end has a 3' hydroxyl group. As you can see, the two chains in a DNA molecule are oriented such that the 5' end of one chain is opposite the 3' end of the other chain (and vice versa). Thus, DNA is referred to as an antiparallel molecule.