11.15 A typical C0t curve for a eukaryotic organism contains several steps. The first step in the curve represents DNA renaturing at very low C0t values, because these sequences are present in many copies (highly repetitive). The second step represents DNA renaturing at intermediate C0t values; these sequences are present in an intermediate number of copies (moderately repetitive). The last step represents DNA that renatures slowly; these sequences are present singly or in few copies (unique).
DNA sequences that exist in multiple copies. Although not identical, these copies are similar enough to reanneal.
Moderately repetitive DNA typically consists of sequences from 150 to 300 bp in length (although they may be longer) that are repeated many thousands of times. Some of these sequences perform important functions for the cell; for example, the genes for ribosomal RNAs (rRNAs) and transfer RNAs (tRNAs) make up a part of the moderately repetitive DNA. However, much of the moderately repetitive DNA has no known function in the cell. Moderately repetitive DNA itself is of two types of repeats. Tandem repeat sequences appear one after another and tend to be clustered at a few locations on the chromosomes. Interspersed repeat sequences are scattered throughout the genome. An example of an interspersed repeat is the Alu sequence, each of which consists of about 200 bp. The Alu sequence is present more than a million times in the human genome and makes up about 11% of each person's DNA. Short repeats, such as the Alu sequences, are called SINEs (short interspersed elements). Longer interspersed repeats consisting of several thousand base pairs are called LINEs (long interspersed elements). Most interspersed repeats are transposable genetic elements, sequences that can multiply and move (see next section).
The other major class of repetitive DNA is highly repetitive DNA. These short sequences, often less than 10 bp in length, are present in hundreds of thousands to millions of copies that are repeated in tandem and clustered in certain regions of the chromosome, especially at centromeres and telomeres. Highly repetitive DNA is sometimes called satellite DNA, because it has a different base composition from those of the other DNA sequences and separates as a satellite fraction when centrifuged at high speeds. Highly repetitive DNA is rarely transcribed into RNA. Although these sequences may contribute to centromere and telomere function, most highly repetitive DNA has no known function.
Eukaryotic DNA comprises three major classes: unique-sequence DNA, moderately repetitive DNA, and highly repetitive DNA. Unique-sequence DNA consists of sequences that exist in one or only a few copies; moderately repetitive DNA consists of sequences that may be several hundred base pairs in length and is present in thousands to hundreds of thousands of copies. Highly repetitive DNA consists of very short sequences repeated in tandem and present in hundreds of thousands to millions of copies.
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