Molecular mechanisms of production of symmetrical and asymmetrical chromosome exchanges

Abstract

The formation of a heteroduplex is probably the first step leading to chromosome exchanges. Heteroduplexes occur by complementary association of two single DNA strands from different chromosomes. Therefore, repetitive DNA is the most common region involved in heteroduplex formation. DNA repeats are defined as polarized when they run the same, and antipolarized when they run opposite from centromere to telomere in two different chromosomes or chromosome arms. Paracentric inversions may easily account for the origin of antipolarized repeats. Palindromes are a special type of reversed repeats which always run the same from centromere to telomere independently of the existence or not of chromosomal rearrangements. Heteroduplexes leading to symmetrical exchanges can only occur by association of DNA strands with polarized repeats. On the other hand, antipolarized repeats are essential for the occurrence of asymmetrical rearrangements. Accordingly, the frequency of induced symmetrical and asymmetrical exchanges in a cell population may partially depend on the frequency of polarized and antipolarized repeats in the genome.