  We characterize the equilibrium thermodynamics of a thick polymer confined in
a spherical region of space. This is used to gain insight into the DNA
packaging process. The experimental reference system for the present study is
the recent characterization of the loading process of the genome inside the
$\phi$29 bacteriophage capsid. Our emphasis is on the modelling of
double-stranded DNA as a flexible thick polymer (tube) instead of a
beads-and-springs chain. By using finite-size scaling to extrapolate our
results to genome lengths appropriate for $\phi$29, we find that the
thickness-induced force may account for up to half the one measured
experimentally at high packing densities. An analogous agreement is found for
the total work that has to be spent in the packaging process. Remarkably, such
agreement can be obtained in the absence of any tunable parameters and is a
mere consequence of the DNA thickness. Furthermore, we provide a quantitative
estimate of how the persistence length of a polymer depends on its thickness.
The expression accounts for the significant difference in the persistence
lengths of single- and double-stranded DNA (again with the sole input of their
respective sections and natural nucleotide/base-pair spacing).
