A bacteriophage T4 DNA “synapsis model” proposes that the bacteriophage T4 terminase small subunit (TerS) apposes two pac site containing dsDNA homologs to gauge concatemer maturation adequate for packaging initiation. N-terminus, C-terminus, or both ends modified fusion Ter S proteins retain function. Replacements of the TerS gene in the T4 genome with fusion genes encoding larger (18–45 kDa) TerS-eGFP and TerS-mCherry fluorescent fusion proteins function without significant change in phenotype. Co-infection and co-expression by T4 phages encoding TerS-eGFP and TerS-mCherry shows in vivo FRET in infected bacteria comparable to that of the purified, denatured and then renatured, mixed fusion proteins in vitro. FRET of purified, denatured-renatured, mixed temperature sensitive and native TerS fusion proteins at low and high temperature in vitro shows that TerS ring-like oligomer formation is essential for function in vivo. Super-resolution STORM and PALM microscopy of intercalating dye YOYO-1 DNA and photoactivatable TerS-PAmCherry-C1 fusions support accumulation of TerS dimeric or multiple ring-like oligomer structures containing DNA and gp16-mCherry in vivo as well as in vitro to regulate pac site cutting. Bacteriophage T4 DNA packaging is shown to involve a eukaryotic meiotic-like synapsis of two homologous pac site DNAs to gauge adequate DNA concatemer synthesis for cutting and packaging of a full genome's amount of DNA. Single molecule fluorescence and super-resolution optical microscopy of fully functional terminase small subunit fusion proteins in vitro and in vivo support a TerS twin ring model for the synapsis. TerS gene pac site to gene 19 pac site gene amplifications can be selected in phage T4 only with functional TerS protein. This provides genetic evidence for synapsis of the two homologous pac site containing duplex DNA segments by TerS double rings. The results can also explain the divergent and highly variable TerS protein monomer and multimer crystal structures found among bacteriophages: Dimeric protein rings binding two homologous dsDNA duplexes based on many TerS monomer structures can supply the same function.