What are chromosomes made of? How do the atoms and molecules of the genetic material carry information? How is the information on chromosomes expressed to result in the phenotypes of living things? The answers to these questions are the subject of the next three topics - DNA structure, gene expression, and biotechnology.

Early attempts to identify the kinds of molecule that carry biological information involved asking questions about what kind of molecules are capable of changing the genetic makeup of an organism. Two classic experiments answer this question. First, Griffith found that bacterial cells that had been killed by exposure to high temperature retained the ability to reprogram other bacterial cells (Figure 11.2). Griffith reasoned that some compound in the heat-killed S cells was capable of transforming R cells after injection into the mouse. Avery and his coworkers quickly narrowed the possible molecules to DNA and protein. It was assumed that protein was the genetic material because of the greater potential information density in a protein polymer. Although Avery went on to show that DNA alone was capable of transformation, most researchers did not accept this result.

Hershey and Chase, working with another example of bacterial reprogramming, noted that viruses must inject genetic information into the cell. Since viruses are composed of only DNA and protein, they could tell which type of molecule was injected into the bacterial cell by labeling each type with a different radioisotope (Figure 11.4). Like Avery, Hershey and Chase found that DNA was the genetic material. Even though this is something that most people take for granted, it is important to remember that it was not so long ago that no one really knew anything about the molecular basis of inheritance.

As shown in Figure 11.6, DNA is actually two polymers. Each of the two polymer strands is composed of DNA bases, and, since the bases have a directionality, so does the polymer. The two strands of a DNA molecule are arranged in opposite orientation. The sequence of bases along one strand is complementary to the sequence of bases along the other strand, with A always pairing with T and C always pairing with G. It is the sequence of bases along the DNA polymer that carries biological information.

Why have two strands, if the information on one strand is just the mirror image of the information on the other? There are several reasons, but two of the most important have to do with chemical stability and ease of replication. Since DNA carries irreplaceable genetic information, having two strands arranged in a helix allows the actual information to spend most of its time on the inside of the molecule, away from the possibility of chemical damage. In addition, having a complementary strand allows DNA to be repaired using the information from the complement. In S phase, when the DNA is duplicated, having two strands means that each strand can be the template for the synthesis of a new polymer (the yellow molecules in figure 11.7). The double-stranded nature of DNA also influences the process of gene expression, as we will see in the next topic.