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A blotting membrane is a thin support onto which separated biological molecules are transferred so they can be retained and probed. In a Southern blot, DNA fragments separated by gel electrophoresis keep their relative spatial pattern during transfer. A complementary labelled probe then detects fragments carrying a matching sequence. ^[ref]

The membrane and the probe do different jobs: the first immobilizes the sample pattern, while the second supplies sequence-selective detection. A signal at the same migration position establishes neither complete sequence identity nor equal copy number without appropriate controls. Transfer efficiency, sample loading and probe binding can all affect intensity. The support therefore turns a transient separation into a reusable surface for molecular recognition.

Latent viral DNA has regularly spaced nucleosome protection, whereas much of the nuclear viral DNA during productive infection lacks that regular protection. In the latent sample, increasing digestion resolves a ladder of protected fragments, as expected when micrococcal nuclease cuts exposed linker DNA between nucleosomes. In the productive sample, viral signal is lost rapidly into heterogeneous small material rather than accumulating as a comparable ladder. That is consistent with a large pool of newly replicated or otherwise differently packaged viral DNA being more accessible.

The total-DNA ladders show that host chromatin remains nucleosomal and that the assay can recover protected fragments in each cellular condition. The virus-specific Southern blot distinguishes viral DNA from this abundant host background, and the uninfected lane controls probe background. The conclusion concerns the dominant detectable viral population: productive cultures can contain both chromatinized templates in viral latency and accessible replication products. Absence of a ladder does not establish absence of every histone molecule, and differences in DNA recovery or nuclease accessibility should be considered before assigning a precise fraction of chromatinized genomes.