The display of the 46 human chromosomes at mitosis is called the human karyotype. If parts of chromosomes are lost or are switched between chromosomes, these changes can be detected either by changes in the banding patterns or—with greater sensitivity—by changes in the pattern of chromosome painting (Figure 4–12). Cytogeneticists use these alterations to detect inherited chromosome abnormalities and to reveal the chromosome rearrangements that occur in cancer cells as they progress to malignancy (discussed in Chapter 20).
Chromosomes Contain long Strings of genes
Chromosomes carry genes—the functional units of heredity. A gene is often defined as a segment of DNA that contains the instructions for making a particular protein (or a set of closely related proteins), but this definition is too narrow. Genes that code for protein are indeed the majority, and most of the genes with clear-cut mutant phenotypes fall under this heading. In addition, however, there are many “RNA genes”—segments of DNA that generate a functionally significant RNA molecule, instead of a protein, as their final product. We shall say more about the RNA genes and their products later.
As might be expected, some correlation exists between the complexity of an organism and the number of genes in its genome (see Table 1–2, p. 29). For example, some simple bacteria have only 500 genes, compared to about 30,000 for humans. Bacteria, archaea, and some single-celled eukaryotes, such as yeast, have concise genomes, consisting of little more than strings of closely packed genes. However, the genomes of multicellular plants and animals, as well as many other eukaryotes, contain, in addition to genes, a large quantity of interspersed DNA whose function is poorly understood (Figure 4–13). Some of this additional DNA is crucial for the proper control of gene expression, and this may in part explain why there is so much of it in multicellular organisms, whose genes have to be switched on and off according to complicated rules during development (discussed in Chapters 7 and 21).
Differences in the amount of DNA interspersed between genes, far more than differences in numbers of genes, account for the astonishing variations in genome size that we see when we compare one species with another (see Figure 1–32). For example, the human genome is 200 times larger than that of the yeast Saccharomyces cerevisiae, but 30 times smaller than that of some plants and amphibians and 200 times smaller than that of a species of amoeba. Moreover, because of differences in the amount of noncoding DNA, the genomes of closely related organisms (bony fish, for example) can vary several hundredfold in their DNA content, even though they contain roughly the same number of genes. Whatever the excess
Figure 4–13 The arrangement of genes in the genome of S. cerevisiae compared to humans. (A) S. cerevisiae is a budding yeast widely used for brewing and baking. The genome of this single-celled eukaryote is distributed over 16 chromosomes. A small region of one chromosome has been arbitrarily selected to show its high density of genes. (b) A region of the human genome of equal length to the yeast segment in (A). The human genes are much less densely packed and the amount of interspersed DNA sequence is far greater. Not shown in this sample of human DNA is the fact that most human genes are much longer than yeast genes (see Figure 4–15).
DNA may do, it seems clear that it is not a great handicap for a eukaryotic cell to carry a large amount of it.