Genetic expression in the developing brain

The adult mouse brain contains complex populations of polyadenylated [poly(A)⁺] and nonpolyadenylated [poly(A)^-] messenger RNA's (mRNA's). These mRNA's are separate sequence populations, similar in complexity, and in combination are equivalent to ~ 150,000 different mRNA sequences, of average length. Essentially all of the 'adult' poly(A)⁺ mRNA's are present in the brain at birth. In contrast, most of the poly(A)^- mRNA's are absent. Brain poly(A)^- mRNA's begin to appear soon after birth, but the full adult complement is not reached until young adulthood. This suggests that these poly(A)^- mRNA's specify proteins required for the biological capabilities of the brain that emerge during the course of postnatal development.