In vertebrates and invertebrates, the genes encoding ribosomal RNAs and some other nonprotein-coding RNAs, such as those involved in RNA splicing, occur as tandemly repeated arrays. These multiple tandemly repeated genes are distinguished from the duplicated genes of gene families in that they encode identical, or nearly identical, proteins or functional RNAs. Most often, copies of these sequences appear one after the other, in a head-to-tail fashion, over a long stretch of DNA. Within a tandem array of rRNA genes, each copy is nearly exactly like all the others. Although the transcribed portions of the genes are the same, the nontranscribed regions between the transcribed regions can vary in length and sequence.

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These tandemly repeated rRNA genes have evolved to meet the great cellular demand for their transcripts. To understand why, consider that a fixed maximal number of rRNA molecules can be produced from a single gene during one cell generation when the gene is fully loaded with RNA polymerase molecules. If more RNA is required than can be transcribed from one gene, multiple copies of the gene are necessary. For example, during early embryonic development in humans, many embryonic cells have a doubling time of about 24 hours and contain 5 million–10 million ribosomes. To produce enough rRNA to form this many ribosomes, an embryonic human cell needs at least 100 copies of genes encoding the large and small rRNA subunits, and most of these genes must be close to maximally active for the cell to divide every 24 hours; that is, multiple RNA polymerases must be transcribing each rRNA gene at the same time (see Figure 10-39). Indeed, all eukaryotes, including yeasts, contain 100 or more copies of the 5S rRNA gene and of the genes encoding the other rRNAs.

The genes encoding tRNA and the genes encoding the histone proteins are also present in multiple copies. As we will see later in this chapter, histones bind and organize nuclear DNA. Just as the cell requires multiple rRNA and tRNA genes to produce sufficient numbers of ribosomes and tRNAs, multiple copies of the histone genes are required to produce sufficient histone protein to bind the large amount of nuclear DNA produced in each round of cell replication. While tRNA and histone genes often occur in clusters, they generally do not occur in tandem arrays in the human genome.