= Bacteriophage T2
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\b[Bacteriophage T2] is a <bacteriophage> that infects <Escherichia coli>. Its <DNA> and external <protein> components made it an experimental system for separating attachment from delivery of genetic material in the <Hershey-Chase experiment>.<Independent functions of viral protein and nucleic acid in growth of bacteriophage by A. D. Hershey and Martha Chase>[Experimental material and abstract, points 1–8]{ref}
A complete infectious particle and its individual components need not have the same abilities. In the original experiments, disrupting T2 particles separated much of their <DNA> from sulfur-containing material that retained specific attachment and antigenic properties. Thus observing attachment alone does not establish successful intracellular reproduction.<Independent functions of viral protein and nucleic acid in growth of bacteriophage by A. D. Hershey and Martha Chase>[Abstract, points 1 and 7–8]{ref}
For a conceptual example, compare intact particles with particles whose entry machinery has been impaired. If both attach but only the intact preparation yields progeny, an attachment count alone cannot serve as a progeny count. This separates distinct stages of <virus> infection rather than treating every particle touching a <bacterium> as a completed infection.
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