The Structure of Eukaryotic Genes

For each polypeptide chain that an organism produces, there exists a corresponding gene. The nucleotide sequence in that gene dictates the amino acid sequence of the protein which, in turn, defines the protein's secondary, tertiary, and quaternary structures.

Introns/Exons - In contrast to the simple organization of prokaryotic genomes, in which individual genes are present as single, uninterrupted coding sequences of DNA, the genes in eukaryotic genomes are organized in a more complex manner. For example, within most eukaryotic genes are DNA sequences that are never translated into polypeptides. These noncoding regions, called introns, alternate with regions called exons that are expressed in the polypeptide sequence.

Globin Gene Structure - Figure 7.20 shows how the exon - intron structure of the hemoglobin gene is related to the structure of the globin polypeptide. Only the parts of the gene shown in color correspond to portions of the polypeptide chain.

The process by which eukaryotic mRNA is made is therefore more complicated than the analogous process in prokaryotes.

pre-mRNA - Eukaryotic transcription first produces a primary transcript, or pre-mRNA, corresponding to the whole gene-exons, introns, and portions of flanking regions (Figure 7.20). The pre-mRNA, while still in the cell nucleus, is cut and spliced to remove the regions corresponding to introns, thereby producing an mRNA that codes correctly for the polypeptide chain. See here for more information.

See also: Eukaryotic Transcription (from Chapter 28), Processing of mRNA (from Chapter 28), Introns (from Chapter 28)