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A gene is a portion of the DNA molecule which codes for the manufacture 
of a single protein. In higher organisms, much of the DNA may not serve 
as genes in this sense, but may regulate the activity of nearby genes. It 
is possible to break open cells and isolate DNA, free of other cellular 
constituents. 
The genetic make-up of cells and organisms is basically stable, 
thus ensuring the proper functioning of the organism and its offspring. 
This stability is not absolute, however, and while changes are relatively 
rare, they occur frequently enough to have profound effects on the 
nature of living things. Such changes, called "mutations, " are basic 
to genetics and essential to evolution. From a chemical point of view, 
all mutations can be described as changes in the structure of DNA. 
Changes of several different types are known to occur. If one of 
the bases in the portion of the DNA molecule coding for a given gene 
is changed to another base, that gene is thereby a mutant gene; and 
the protein manufactured by it will usually differ from the protein 
manufactured by the original gene. Similarly, the protein will be 
different (or perhaps will not be made at all) if a series of bases in the 
original gene are simply deleted, or if a new series of bases is inserted 
within the original sequence of the gene. The latter two types of mutation- - 
deletions and insertions --occur in connection with the phenomenon known 
as crossing-over, or recombination. 
Recombination in Nature . This phenomenon has long been known to 
occur within cells "of a given species of both simple, single -celled 
organisms and more complex animals and plants. Genetic information, 
or rather segments of DNA molecules bearing information, can be 
exchanged between chromosomes and between different regions of a 
given chromosome. Such genetic recombination contributes importantly 
to biological diversity. 
Recombination results from breaking and then joining of two separate 
DNA molecules. The two molecules involved may be from the chromo- 
somes of the organism itself, or one or both of them may be the DNA 
of a plasmid or virus. Generally, these events have been observed 
only within a given species: they may involve the DNA of the single 
organism and that of plasmids or viruses that infect only that species. 
For example, genetically determined resistance to antibiotics is trans- 
ferred readily between free-living microorganisms. Recombination 
between different species of single -celled microorganisms has also 
been described in a limited number of cases. It is not known how 
widespread such interspecies recombination is in nature. 
