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Molecular Basis of Heredity: Part 1. Nucleic Acids

Author(s): Raye L. Alford, PhD

The DNA Double Helix

Double-stranded DNA is not a straight ladder-like molecule. Rather it turns in a helix, like a spiral staircase. The DNA helix is called a double helix because it is made of two complementary strands of DNA. The paired bases lie in the center of the spiral, forming the steps of the DNA spiral staircase. The sugar phosphate backbones form the railings of the DNA spiral staircase.

The hydrogen bonds between the base pairs that hold the two sides of the helix together can be broken easily. The DNA double helix can be forced to separate into its component strands by increasing the temperature or the pH of the solution in which it is suspended. If the temperature and/or pH is lowered back to physiologic levels, the complementary DNA strands will find one another in solution and re-hybridize into a double helix, in which A again pairs with T, and C pairs with G for the entire length of the strands. This physical property of DNA complementarity is the basis not only for the replication and transcription of DNA, but also for most molecular genetic testing technologies through which the sequence of DNA is investigated to diagnose genetic disease and determine paternity or personal identity. Replication is the process by which a DNA molecule reproduces itself. Transcription is the process through which DNA serves as the template for the synthesis of RNA.