Changes in chromosome structure, called rearrangements, encompass several major classes of events. A chromosome segment can be lost, constituting a dele-tion, or doubled, to form a duplication. The orientation of a segment within the chromosome can be reversed, constituting an inversion. Or a segment can be moved to a different chromosome, constituting a translocation. DNA breakage is a major cause of each of these events. Both DNA strands must break at two differ-ent locations, followed by a rejoining of the broken ends to produce a new chromo-somal arrangement ( Chromosomal rearrangements by breakage can be induced artificially by using ionizing radiation. This kind of radia-don, particularly X rays and gamma rays, is highly energetic and causes numerous double-stranded breaks in DNA.
To understand how chromosomal rearrangements are produced by breakage, several points should be kept in mind:
1. Each chromosome is a single double-stranded DNA molecule.
2. The first event in the production of a chromosomal rearrangement is the generation of two or more double-stranded breaks in the chromosomes of a cell (see Figure 16-19, top row at left).
3. Double-stranded breaks are potentially lethal, unless they are repaired.
4. Repair systems in the cell correct the double-stranded breaks by joining broken ends back together (see Chapter 15 for a detailed discussion of DNA repair).
5. If the two ends of the same break are rejoined, the original DNA order is restored. If the ends of two different breaks are joined together, however, one result is one or another type of chromosomal rearrangement.
6. The only chromosomal rearrangements that survive meiosis are those that produce DNA molecules that have one centromere and two telomeres. If a rearrangement produces a chromosome that lacks a centromere, such an acentric chromosome will not be dragged to either pole at anaphase of mitosis or meiosis and will not be incorporated into either progeny nucleus.
Therefore acentric chromosomes are not inherited. If a rearrangement produces a chromosome with two centromeres (a dicentric), it will often be pulled simultaneously to opposite poles at anaphase, forming an anaphase bridge. Anaphase-bridge chromosomes typically will not be incorporated into either progeny cell. If a chromosome break produces a chromosome lacking a telomere, that chromosome cannot replicate