Repetitive DNA Elements

Satellites (also called classical satellites), which occur in four classes (I-IV), form arrays of 1,000 to 10 million repeated units, particularly in the heterochromatin of chromosomes. They are concentrated in centromeres and account for much of the DNA there. Satellites of one type, called alpha-satellites, occur as repeated units of approximately 171 base pairs (bp) in length, with high levels of sequence variation between the repeated units, as shown in Figure 2.

Table 1.

Minisatellites form arrays of several hundred units of 7 to 100 bp in length. They are present everywhere with an increasing concentration toward the telomeres. They differ from satellites in that they are found only in moderate numbers of tandem repeats and because of their high degree of dispersion throughout chromosomes.

Microsatellites, or simple sequence repeats (SSRs), are composed of units of one to six nucleotides, repeated up to a length of 100 bp or more. One-third are simple "polyadenylated" repeats, composed of nothing but adenine nucleotides. Other examples of abundant microsatellites are (AC)_n, (AAAN)_n, (AAAAN)_n, and (AAN)_n, where N represents any nucleotide and n is the number of repeats. Less abundant, but important because of their direct involvement in the generation of disease, are the (CAG/CTG)_n and (CGG/CCG)_n trinucleotide (or triplet) repeats.

Telomeric and subtelomeric repeats are present at the end of the telomeres and are composed of short tandem repeats (STRs) of (TTAGGG)_n, up to 30,000 bp long. This sequence is "highly conserved," meaning it has changed very little over evolutionary time, indicating it likely plays a very important role. These STRs function as caps or ends of the long linear chromosomal DNA molecule and are crucial to the maintenance of intact eukaryotic chromosomes. Subtelomeric repeats act as transitions between the boundary of the telomere and the rest of the chromosome. They contain units similar to the TTAGGG, but they are not conserved.