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Molecular Basis of Heredity: Part 1. Nucleic Acids

Author(s): Raye L. Alford, PhD

DNA is Double-Stranded

One of the most significant findings of Watson, Crick, Wilkins, and Franklin was that at the physiological pH and temperature of living cells, DNA usually is found as a double-stranded molecule. When DNA forms a double-stranded molecule, the strands are joined together through hydrogen bonds that form between the bases. In the double-stranded DNA molecule, an A base always pairs with a T base and a C base always pairs with a G base. Once joined, the bases are referred to as base pairs.

As shown, A-T base pairs form through two hydrogen bonds and C-G base pairs form through three hydrogen bonds. Because of the specificity of the hydrogen bonding, A always pairs with, or is complemented by, T, and C always pairs with, or is complemented by, G, and the two strands of a double-stranded DNA molecule are said to be complementary. The complementarity of base pairing means that knowing the nucleotide sequence of one strand of a double-stranded DNA molecule allows one to predict the nucleotide sequence of the other strand.

The length of a double-stranded DNA molecule often is described by the number of base pairs in the helix. For example, a double-stranded DNA molecule containing 27 pairs of A-T and C-G from one end to the other would be said to be 27 base pairs long. Base pairs also can be used as a unit of genetic distance. For example, two genes can be said to be a specified number of base pairs apart in a segment of DNA.

DNA and RNA strands are oriented according to the ribose sugars within them. The 5' end of a strand of DNA (or RNA) is the end toward which the 5' carbon of the pentose sugar lies. The strands of double-stranded DNA run antiparallel, or in opposite directions. That is, as shown in the illustration on the right, the 5' end of one strand of DNA forms hydrogen bonds with the nucleotides at the 3' end of its complementary strand.

RNA molecules within cells typically are single-stranded at physiologic temperature and pH. However, they frequently fold upon themselves to form secondary structures wherein a single RNA molecule forms intramolecular base pairs. As with many other biological molecules, the shape created by folding affects how the molecule functions inside cells. Although one RNA molecule may associate with another in some circumstances, RNA does not typically form long intermolecular double-stranded structures like DNA does.