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182 Chapter 6 DNA Structure, Replication, and Recombination

Figure 6.1 Ancient DNA still carries information. Molecular we can only have a very sketchy understanding of life it-
biologists have successfully extracted and determined the sequence self.” For this reason, we shift our perspective in this
of DNA from a 38,000-year-old Neanderthal skull. These findings chapter to an examination of DNA, the molecule of which
attest to the chemical stability of DNA, the molecule of inheritance. genes are made.
© DEA Picture Library/De Agostini/Getty Images
As we extend our analysis to the molecular level, bear
in mind two important themes. First, DNA’s genetic func-
tions flow directly from its molecular structure—the way
its atoms are arranged in space. Second, all of DNA’s ge-
netic functions depend on specialized proteins that interact
with it and read the information it carries, because DNA
itself is chemically inert. In fact, DNA’s lack of chemical
reactivity makes it an ideal physical container for long-
term maintenance of genetic information in living organ-
isms, as well as their nonliving remains.

After purifying DNA from the nuclein by chemical
6.1 Experimental Evidence for means, researchers established that contains only four dis-
DNA as the Genetic Material tinct chemical building blocks linked in a long chain (Fig.
6.2). The four individual components belong to a class of
compounds known as nucleotides; the bonds joining one
learning objectives nucleotide to another are covalent phosphodiester bonds;
and the linked chain of building block subunits is a type of
1. Describe the chemical components of DNA. polymer.
2. Summarize the methods that located DNA in A procedure first reported in 1923 made it possible to
chromosomes. discover where in the cell DNA resides. Named the Feulgen
3. Explain how Avery and his colleagues demonstrated reaction after its designer, the procedure relies on a chemical
bacterial transformation, and explain the significance which stains DNA red. In a preparation of stained cells, the
of this finding. chromosomes redden, while other areas of the cell remain
4. Describe the blender experiments of Hershey and Chase relatively colorless. The reaction shows that DNA is localized
and what the results revealed about DNA’s function. almost exclusively within chromosomes.
The finding that DNA is a component of chromosomes
does not itself prove that the molecule has anything to do
At the beginning of the twentieth century, geneticists did with genes. Typical eukaryotic chromosomes also contain
not know that DNA was the genetic material. It took a co- an even greater amount of protein by weight. Because pro-
hesive pattern of results from experiments performed over teins are built of 20 different amino acids, whereas DNA is
more than 50 years to convince the scientific community made of only four different nucleotides, many researchers
that DNA is the molecule of heredity. We now present key thought proteins had greater potential for diversity and
pieces of the evidence. were better suited to serve as the genetic material. These
same scientists assumed that even though DNA was an im-
portant part of chromosome structure, it was too simple to
Chemical Studies Locate contain the complexity of genes.
DNA in Chromosomes

In 1869, Friedrich Miescher extracted a weakly acidic, Bacterial Transformation Implicates
phosphorus-rich material from the nuclei of human white DNA as the Genetic Material
blood cells and named it nuclein. It was unlike any chemical
compound reported previously. Nuclein’s major compo- Several studies eventually promoted the idea that DNA
nent turned out to be DNA, although it also contained would be the chemical substance that carries genetic infor-
some contaminants. The full chemical name of DNA is mation. The most important of these used single-celled
deoxyribonucleic acid, reflecting three characteristics of bacteria as experimental organisms. Bacteria carry their ge-
the substance: One of its constituents is a sugar known as netic material in a single circular chromosome that lies within
deoxyribose; it is found mainly in cell nuclei; and it is acidic. the cell without being enclosed in a nuclear membrane. With