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Bacteriophage T4 is a tailed, double-stranded DNA bacteriophage that infects Escherichia coli. Its viral capsid encloses the genome, while the tail and attachment structures participate in contact with the host and DNA delivery. A complete virion, a detached enzyme such as bacteriophage T4 DNA polymerase, and DNA remaining inside a shell are distinct experimental objects.

A primary study by Bo Hu and collaborators used cryo-electron tomography to examine phage–cell complexes. It observed different T4 infection intermediates, including tail contraction and particles with different amounts of DNA remaining in their heads.^[ref] These frozen three-dimensional observations connect structure with an inferred sequence of entry events. They do not continuously film the same particle through every transition.

A constructed counting example shows why one structural marker cannot replace another. Suppose 100 attached particles include 60 with contracted tails. Among those 60, suppose 40 have empty heads and 20 retain DNA. Then the contracted fraction is $60/100=0.60$, whereas the empty-head fraction among contracted particles is $40/60=2/3$. Neither count alone gives the number of progeny produced. These stipulated counts illustrate different denominators and endpoints; they are not observations from the cited study.

The primary work is especially useful because it separates attachment, structural remodeling and genome delivery instead of treating them as one instantaneous event. A shell or tail image supplies evidence about particle structure; successful genome transfer and subsequent reproduction require their own observations. This distinction also explains why the biochemical activity of a purified T4 enzyme is not a measurement of intact-phage infectivity.