to maximally active for the cell to divide every 24 hours (Table 10-2). That is, multiple RNA polymerases must be loaded onto and transcribing each rRNA gene at the same time (see Figure 12-32).

All eukaryotes, including yeasts, contain 100 or more copies of the genes encoding 5S rRNA and the large and small subunit rRNAs. The importance of repeated rRNA genes is illustrated by Drosophila mutants called bobbed (because they have stubby wings), which lack a full complement of the tandemly repeated pre-rRNA genes. A bobbed mutation that reduces the number of pre-rRNA genes to less than «50 is a recessive lethal mutation.

Multiple copies of tRNA and histone genes also occur, often in clusters, but generally not in tandem arrays.

Most Simple-Sequence DNAs Are Concentrated in Specific Chromosomal Locations

Besides duplicated protein-coding genes and tandemly repeated genes, eukaryotic cells contain multiple copies of other DNA sequences in the genome, generally referred to as repetitious DNA (see Table 10-1). Of the two main types of repetitious DNA, the less prevalent is simple-sequence DNA, which constitutes about 3 percent of the human genome and is composed of perfect or nearly perfect repeats of relatively short sequences. The more common type of repetitious DNA, composed of much longer sequences, is discussed in Section 10.3.

Simple-sequence DNA is commonly called satellite DNA because in early studies of DNAs from higher organisms using equilibrium buoyant-density ultracentrifugation some simple-sequence DNAs banded at a different position from the bulk of cellular DNA. These were called satellite bands to distinguish them from the main band of DNA in the buoyant-density gradient. Simple-sequence DNAs in which the repeats contain 1-13 base pairs are often called microsatellites. Most have repeat lengths of 1-4 base pairs and usually occur in tandem repeats of 150 base pairs or fewer. Microsatellites are thought to have originated by "backward slippage" of a daughter strand on its template strand during DNA replication so that the same short sequence is copied twice.

Microsatellites occasionally occur within transcription units. Some individuals are born with a iU larger number of repeats in specific genes than observed in the general population, presumably because of daughter-strand slippage during DNA replication in a germ cell from which they developed. Such expanded microsatellites have been found to cause at least 14 different types of neuromuscular diseases, depending on the gene in which they occur. In some cases expanded microsatellites behave like a recessive mutation because they interfere with the function or expression of the encoded gene. But in the more common types of diseases associated with expanded microsatellite repeats, myotonic dystrophy and spinocerebellar ataxia, the expanded repeats behave like dominant mutations because they interfere with RNA processing in general in the neurons where the affected genes are expressed. I

▲ EXPERIMENTAL FIGURE 10-5 Simple-sequence DNAs are useful chromosomal markers. Human metaphase chromosomes stained with a fluorescent dye were hybridized in situ with a particular simple-sequence DNA labeled with a fluorescent biotin derivative. When viewed under the appropriate wavelength of light, the DNA appears red and the hybridized simple-sequence DNA appears as a yellow band on chromosome 16, thus locating this particular simple sequence to one site in the genome. [See R. K. Moyzis et al., 1987, Chromosoma 95:378; courtesy of R. K. Moyzis.]

Most satellite DNA is composed of repeats of 14-500 base pairs in tandem repeats of 20-100 kb. In situ hybridization studies with metaphase chromosomes have localized these satellite DNAs to specific chromosomal regions. In most mammals, much of this satellite DNA lies near centromeres, the discrete chromosomal regions that attach to spindle microtubules during mitosis and meiosis. Satellite DNA is also located at telomeres, the ends of chromosomes, and at specific locations within chromosome arms in some organisms. These latter sequences can be useful for identifying particular chromosomes by fluorescence in situ hybridization (FISH), as illustrated in Figure 10-5.

Simple-sequence DNA located at centromeres may assist in attaching chromosomes to spindle microtubules during mitosis. As yet, however, there is little clear-cut experimental evidence demonstrating any function for most simple-sequence DNA, with the exception of the short repeats at the very ends of chromosomes discussed in a later section.

DNA Fingerprinting Depends on Differences in Length of Simple-Sequence DNAs

Within a species, the nucleotide sequences of the repeat units composing simple-sequence DNA tandem arrays are highly conserved among individuals. In contrast, differences in the number of repeats, and thus in the length of simple-sequence tandem arrays containing the same repeat unit, are quite common among individuals. These differences in length are

Simple-sequence tandem array

Individual 2

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