Key Concepts of Section 8.1

• In molecular terms, a gene is the entire DNA sequence required for synthesis of a functional protein or RNA molecule. In addition to the coding regions (exons), a gene includes control regions, and in multicellular animals and plants, most genes include introns.

• A simple eukaryotic transcription unit produces a single monocistronic mRNA, which is translated into a single protein.

• A complex eukaryotic transcription unit is transcribed into a primary transcript that can be processed into two or more different monocistronic mRNAs depending on the choice of splice sites or polyadenylation sites. A complex transcription unit with alternative promoters generates two or more different mRNAs in different cell types (see Figure 8-3b).

• Many complex transcription units (e.g., the fibronectin gene) express one mRNA in one cell type and an alternative mRNA in a different cell type.

• About half the protein-coding genes in vertebrate genomic DNA are solitary genes, each occurring only once in the haploid genome. The remainder are duplicated genes, which arose by duplication of an ancestral gene and subsequent independent mutations (see Figure 8-2b). The proteins encoded by a gene family have homologous but nonidentical amino acid sequences and exhibit similar but slightly different properties.

• In invertebrates and vertebrates, rRNAs are encoded by multiple copies of genes located in tandem arrays in genomic DNA. Multiple copies of tRNA, snRNA, and histone genes also occur, often in clusters, but not generally in tandem arrays.

• Many genes also encode functional RNAs that are not translated into proteins but nonetheless perform significant functions, such as rRNA, tRNA, and snRNA. Among these are micro-RNAs, whose biological significance in regulating gene expression has only recently been appreciated. Understanding the function of thousands of newly discovered nuclear long noncoding RNAs (lncRNAs) is currently an intense area of research.