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DNA plays two roles: (1) provides information for the reproduction, 
growth, and functions of the cell, and (2) preserves and directs replication 
of this information and transfers it to the offspring. These two roles 
of DNA are common to animals, plants, single -cell organisms, and 
many viruses. The DNA of cells is mainly found in organized structures 
called chromosomes. 
Intracellular DNA also occurs outside of the chromosomes as 
separately replicating molecules. Such DNA molecules include the 
plasmids, found in bacteria; the DNA of chloroplasts, common to 
green plants; and the DNA of mitochondria, the energy-producing units 
of the cells of complex organisms. These DNAs, while not strictly 
part of the inherent genetic make-up of a cell, help define the cell's 
functional capability. Another type of DNA commonly found in cells 
is the DNA of infecting viruses. 
During the past 30 years the structure of DNA molecules has been 
studied intensively, and it can now be described in much detail. The 
molecule may be compared to a long but twisted stepladder with 
thousands to millions of rungs (a short piece of DNA is shown in Figure 
IV-2). The sides of the ladder are formed of sugar molecules 
(deoxyribose) attached end to end through phosphate groups. At right 
angles to each sugar molecule is one of four possible bases --adenine, 
guanine, thymine, and cytosine. The precise sequence of these bases, 
the rungs of the ladder, codes the information content. The "reading" 
of the code contained in the sequence of bases results in the formation 
of proteins, which in turn carry out most of the essential functions 
of the cell. 
Figure IV-2 
