Book Viewer

Short Contents | Full Contents | Other books @ NCBI

Modern Genetic Analysis Chapter 2. The Structure of Genes and Genomes
The Nature of Genes

Genes are diverse in function and size, yet for most genes certain major topographical features can be delineated.

The Main Regions of a Gene

In Chapter 1 a gene was defined simply as a region of DNA capable of being transcribed to produce a functional RNA molecule. To this we can add that not only does the gene need to be capable of being transcribed into the functional RNA (for most genes, an mRNA) but this RNA must also be made at the correct time and in the correct place in the development of the organism. Only then is the gene properly functional. To accomplish this, at one end a gene has a regulatory region, a segment of DNA with a specific nucleotide sequence that enables it to receive and respond to signals from other parts of the genome or from the environment. Ultimately these activation signals are converted into regulatory proteins that bind to the regulatory region of the gene and initiate transcription in the adjacent RNA-encoding region. At the other end of the gene there is a region that contains signals to terminate the transcript.

MESSAGE
A gene is a region of the chromosomal DNA that can be transcribed into a functional RNA at the correct time and place during development.

Trying to pinpoint precisely what genes are is complicated by the fact that (as we saw in Chapter 1) many eukaryotic genes contain mysterious segments of DNA, called introns, interspersed in the transcribed region of the gene. Introns do not contain information for functional gene product such as protein. They are transcribed together with the coding regions (called exons) but are then excised from the initial transcript. Since correct sequence in the introns (as well as in the regulatory region) is necessary in order to generate a properly sized transcript at the right time and place, introns (along with coding and regulatory regions) must be considered part of the overall functional unitin other words, part of the gene (Figure 2-4).

Figure 2-5 shows the number of exons (equal to the number of introns plus one) in samples of genes from three eukaryotic organisms, baker's yeast (Saccharomyces cerevisiae), fruit flies (Drosophila melanogaster), and mammals. As you can see, there are striking differences. In mammals, genes are generally very large and transcription units contain many introns. Table 2-1 shows average gene sizes of protein-coding genes in representative organisms and compares them with the size of the messenger RNA (which is the same size as all the exons combined). The coding regions of the genes in all these organisms are in the 23 kb range, but the gene sizes are vastly differentmuch of this difference is proportional to average intron number.

Genes Are Surrounded by Other DNA

Nucleotide sequencing and other physical studies have shown that between genes there is DNA, mostly of unknown function. The size and nature of this DNA vary with the genome. For example, in fungi there is little, but in mammals the intergenic regions can be huge. Some general chromosomal topographies are illustrated in Figure 2-6. Caught conceptually someplace between genes and this nongenetic DNA are sequences of DNA that exist quite distant from a given gene yet can affect the regulation of that gene. They could thus be considered part of the functional gene unit, even though separated by long segments of DNA having nothing to do with the gene in question. In many eukaryotes some of the DNA between genes is repetitive, consisting of several different types of units repeated throughout the genome. Some of the repetitive DNA is dispersed; some is found in contiguous "tandem" arrays. Repetitive DNA is also found in some introns. The extent of this DNA is different in different species, and indeed there is variation of repeat number within species. The function of repetitive DNA is still a mystery, but there are several hypotheses about its origin. (These topics will be addressed in full in Chapter 12.)

© 1999 by W. H. Freeman and Company. All rights reserved.