Any long base-pair sequence found at many different places throughout the chromosomes. This excludes DNA coding for structural genes, which with few exceptions are present as single copies. Repeated sequences account for more than 50% of all the DNA present and fall into three categories.
Repetitive DNA usually consists of widely scattered sequences about 300 base pairs long, which are often adjacent to structural genes. It has been proposed that these sequences are functionally equivalent to the *operator genes
of bacteria. Suppose that sequence A is adjacent to three unlinked genes X, Y, and Z, and that A' is an *inducer of A. The presence of A' in the cell would 'switch on' all genes to which A was adjacent, in this case X, Y, and Z. An appropriate mix of repeated sequences, say A, B, C, and D next to structural gene X would make possible an elaborate *transcriptional control system as demanded in a complex organism.
Highly repetitive or satellite DNA is often about 20 base pairs long and is usually found near the centromere. Palindromes, which are often a few hundred base pairs long, are scattered throughout the chromosomes. Such sequences read the same in both directions, i.e. are palindromic. The functions of satellite DNA and palindromic DNA are uncertain.