Extra DNA in forebrain cortical neurons

Combined cytochemical and biochemical techniques show that neurons from the forebrain cortex of various mammals (rat, mouse, and rabbit) contain near, but not full, 4c DNA levels (c, DNA content of haploid chromosome set). This extra DNA is predominantly synthesized post-natally. More specifically, in rats a wave of DNA synthesis starts a few hours before birth and extends well into the post-natal period, levelling off after 30 days. Density labelling experiments using [5- 3H]-bromo-2′-deoxyuridine (BUdR) suggest that the DNA synthesis proceeds semiconservatively, with the prenatally formed DNA serving as a template for post-natal strand replication. Although all three mammalian DNA polymerases (α, β, and γ) can be detected in the cortical neurons of young rats their developmental course does not allow one to unambiguously identify the enzyme(s) responsible for the observed DNA increase. Density gradient centrifugations of neuronal DNA post-natally labelled with [ 3H]thymidine give no indication of a preferential enrichment of defined segments of the genome. In spite of this, the present data do not rule out the possibility that functionally important sequences might be selectively replicated.