c/7. 2 — Introduction • 37 
Figure 9.— Replication of DNA 
Old Old 
When DNA replicates, the original strands unwind and 
serve as templates for the building of new complementary 
strands. The daughter molecules are exact copies of the 
parent, with each having one of the parent strands. 
SOURCE: Office of Technology Assessment. 
The genetic code 
Proteins are the basic materials of cells. Some 
proteins are enzymes, which catalyze reactions 
within a cell. In general, for every chemical re- 
action in a lix’ing organism, a specific enzyme is 
required to trigger the process. Other proteins 
are structural, comprising most of the raw ma- 
terial that forms cells. 
Ironically, proteins are far more complex and 
diverse than the four nucleotides that help 
create them. Proteins, too, are long chains made 
up of small units strung together. In this case, 
however, the units are amino acids rather than 
nucleotides— and there are 20 different kinds of 
amino acids. Since an average protein is a few 
