William E. Hahn

Transcription of Nonrepeated DNA in Mouse Brain

Under normal conditions of DNA renaturation, about 60 percent of mouse DNA fragments renature at a rate consistent with their being present only once per sperm. These nonrepeated sequences (also called single-copy or unique) may be used in RNA-DNA hybridization experiments to provide quantitative estimates of RNA diversity. About 10 percent of the mouse single-copy sequences are transcribed in mouse brain tissue. Estimates of about 3 percent were obtained for mouse liver and kidney RNAs. If only one of the complementary DNA strands is transcribed, this hybridization value implies that the equivalent of at least 300,000 different sequences of 1000 nucleotides are expressed in mouse brain tissue. It is suggested that the large amount of DNA in mammals is functionally important, and that a substantial proportion of the genome is expressed in the brain.

Synthesis of RNA containing polyadenylic acid sequences in preimplantation rabbit embryos

Abstract Messenger RNA synthesis has been estimated by assaying polyadenylic acid (poly A)-rich sequences in heterogeneous RNA from preimplantation rabbit embryos. Poly A containing RNAs are synthesized at least as early as the 16-cell stage and continue to be made through blastocyst formation and maturation. Sixty to 78% of the heterogeneous polysomal RNA in blastocysts contain poly A sequences. The portion of the heterogeneous RNA containing poly A sequences does not appear to change markedly between cleavage and blastocyst stages of development. Poly Arich sequences are greater than 4 S and consist of at least 84% adenine residues. RNA molecules ranging from 6 S to greater than 28 S contain poly A sequences.

GENETIC PERSPECTIVES ON BRAIN DEVELOPMENT AND COMPLEXITY

The detection and identification of macromolecular species amidst a vastly complex background is a major problem in neurobiology. Therefore, we wish to begin by making a few introductory remarks pertaining to present estimates of macromolecular complexity of the brain. Genes and their transcripts are the first and second orders respectively of the vast molecular complexity of the brain. The first measurements relevant to estimating the extent to which “single” copy DNA (scDNA or DNA which encodes most of the different proteins) is transcribed in mammalian organs were made about 14 years ago (Hahn, 1970; Hahn & Laird, 1971). These initial measurements, although confirmed by others (Brown & Church, 1971; Grouse, Chilton, & McCarthy, 1972; reviewed by Kaplan & Finch, 1982), were underestimates. But they nonetheless showed that very complex arrays of RNA species are present in eukaryotic cells and organs. We now know that in the mammalian brain (mouse and rat) at least 18–20% of the scDNA is transcribed as nuclear RNA (=~40% of the haploid coding capacity) (Bantle & Hahn, 1976; Chikaraishi, Deeb & Sueoka, 1978). Most of these different transcripts apparently reside in the nuclear RNA of neurons (Ozawa, Kushiya & Takahashi, 1980).

Genes expressed in the brain: evolutionary and developmental considerations

Publisher Summary This chapter focuses on evolutionary and developmental considerations on genes expressed in the brain. It is evident from measurements of the sequence complexity of messenger RNA (mRNA) that a substantial portion of genetic information in mammals and invertebrate animals is apparently required for development and function of the brain. Many of the genes expressed in the brain are expressed in a variety of other organs, but quantitative differences in expression of many of these shared genes are evident. In other words, the relative abundance of a given messenger RNA species can differ markedly among various tissues and organs. Of greater interest regarding functions unique to the brain are measurements that indicate the presence of a wide variety of putatively brain specific mRNAs. Presumably, these mRNAs encode for proteins that have presently evolved such that they are of specific adaptive value in the development and function of the brain. The chapter discusses several aspects of genetic expression in the brain. It describes the complexity of gene expression in the brain. The chapter also highlights recombinant DNA and the isolation of genes specifying brain proteins.