Chromosome damage probably occurs continuously in all cells. Types of damage range from single base changes, which result from mistakes made by DNA polymerases during replication, to chromosome breakage. Damage is caused by many factors, including the production of metabolic mutagens within the cells. Certain chemicals in the environment, ionizing radiation, and UV light also damage DNA and chromosomes.
Cells have active repair processes to repair such damage. Repairs occur by direct reversal of damage and by excision of a damaged segment of DNA followed by its replacement. Insects no doubt have many genes involved in DNA repair, with some encoding products that recognize DNA damage, some that can excise the damaged region, and others that repair the damage.
Chromosome breaks can occur at any stage of the cell cycle and generally are repaired by rejoining the broken ends, so that the repaired chromosome appears intact. Unfortunately, not all chromosomal damage is repaired, and chromosomal breaks can lead to large-scale rearrangements of chromatin within chromosomes or exchanges of chromatin between nonhomologous chromosomes.
If the rearranged chromosome lacks a centromere, it is acentric and a cell containing the acentric fragment will be unable to transmit this fragment to its daughter cells during meiosis or mitosis, resulting in loss of significant amounts of genetic information, which is usually lethal. Chromosomes that end up with two centromeres (dicentric) also are unstable, leading to breaks in the chromosomes if the centromeres are distributed to opposite poles during meiosis or mitosis. This results in breakage and loss of genetic information, which is often lethal.
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