Principles of Biochemistry Volume 3 - A. Lehninger 1985

Molecular mechanisms of genetic information transfer
DNA: Structure of chromosomes and genes
Many eukaryotic genes contain intervening untranslated sequences (introns)

Many eukaryotic genes (perhaps even the majority of them) possess a rather puzzling structural feature: their nucleotide sequence is interrupted by a DNA segment that does not encode the Amino Acid Sequence of the polypeptide product. These untranslated insertions disrupt the strictly collinear correspondence between The nucleotide sequence of the remaining Gene regions and The amino acid sequence of the polypeptide encoded by that gene (Figs. 27–29). Such untranslated DNA regions within genes are referred to as intervening sequences, or introns, whereas the regions encoding the amino acid sequence of the polypeptide are called exons. A well-known example is the gene encoding the single polypeptide chain of the egg white protein, Ovalbumin. As shown in Figs. 27–29, this gene contains six introns that divide the ovalbumin gene into seven exons. It is also evident that the introns in this gene are much longer than the exons, accounting for 85% of the total length of the gene's DNA. With few exceptions, all eukaryotic genes studied to date contain introns, which vary in number, position, and the fraction of the total gene length they occupy. For instance, the serum albumin gene contains 6 introns, the chicken conalbumin gene has 17 introns, and the Collagen gene contains over 50 introns. Histone genes are an exception, as they appear to be devoid of introns.

The Biological Significance of introns remains fully unresolved and is the subject of numerous hypotheses. One proposal suggests that introns act as regulatory signals. According to another hypothesis, introns divide genes into distinct, exchangeable segments (“mini-genes”) that can recombine during species evolution to generate novel genes. Whatever their function may be, The Study of introns will undoubtedly help shed light on several issues related to gene METABOLISM/31.html">Transcription (Chapter 28).