Duplication Chromosomal Aberrations

Duplication chromosomal aberrations occur when a segment of a chromosome is repeated two or more times in a chromosome. This may be either a free segment with a centromere or a chromosomal segment of smaller size (acentric). Duplication may occur at the tip of the chromosome or in the middle.

In some instances, the duplicated portion attaches to the chromosome in the same sequence or a reverse sequence. Usually, terminal duplications are rare due to the presence of telomeres. It may be detected cytologically on a polytene chromosome with extra bands. It is also called repetition. This may arise due to unequal crossover. Sometimes unequal crossover may take place within the repeated stalk.

Types of Duplication

The basic types of segment duplications have been reported, such as direct tandem duplication, reverse tandem duplication, displaced direct duplication, displaced reverse duplication, and transposed duplication.

Direct Tandem Duplication: The added segment has the same genetic order as in the original chromosomal segment and is adjacent to the original one in the same chromosome.
Reverse Tandem Duplication: The segment is adjacent to the original segment in the same chromosome, but its genes are in reverse order.
Displaced Direct Duplication: The repeated segment is inserted into a different chromosome and not adjacent to the normal section.
Displaced Reverse Duplication: The duplicated segment has the reverse order of the original gene sequence and is located separately from it.
Transposed Duplication: The duplicated sequence is transposed to another position.

Bridge Breakage Fusion Cycle

Duplication chromosomal aberrations produce an interesting phenomenon called the bridge breakage fusion cycle. It is a form of chromosomal instability that causes those chromosomes without a telomere cap to break during cell division, and the broken ends of sister chromatids fuse to form a bridge. This bridge is subsequently broken down during the next phases of cell division, causing rebreakage and rearrangements.

An extensive study of broken chromosomes in corn has been made by McClintock. She found that when reduplication of chromosomes takes place, two sister chromosomes may adhere at the point of previous breakage. It can be noted that fused sister chromatids are unable to separate readily, so these constitute a single chromatid with two centromeres. Such a chromatid is a dicentric chromatid. This dicentric chromatid stretches out from one pole to another. The bridge thus formed may break at any point. This bridge does not always break at the point of the previous fusion. The acentric fragment may lie behind and get lost.

The individuals show various abnormalities in body characteristics. Drosophila is an example of the genetic effect of duplication. In this case, a gene controlling the shape of the eye is located on the X chromosome.

The normal eye of the fly has 780 facets (ommatidia) or single eyes and a round shape. The other two types, which arise by duplication, have also been noted. These two types of eyes are bar eye and bar double. The first has 325-358 facets, and the second has about 200 facets. Bar and bar double eyes are caused by unequal crossing over during meiosis.

Bar’s eye shows duplication, ie, the gene responsible for this is represented twice in the same chromosome, and the bar double gene is repeated three times.

Conclusion

In contrast to deletion, duplication chromosomal aberrations are certainly of some evolutionary consequence as they can produce viable and heritable morphological traits. Duplication seems to provide a feasible method for the acquisition of new genes and hence new functions. The effects of duplication on viability are less deleterious than those of deficiency.

References

Agarwal, P. V. |. V. (2004). Cell biology, Genetics, Molecular Biology, Evolution, and Ecology: Evolution and Ecology. S. Chand Publishing.
https://ramsadaycollege.com/upload/eclassroom/Botany/GDM_Botany_Chromosomal%20Aberration_sem4_02.pdf
https://bnmv.ac.in/images/uploads/Unit-4%20Chromosomal%20Aberations.pdf
Wu, M., & Rai, K. (2021). Demystifying extrachromosomal DNA circles: Categories, biogenesis, and cancer therapeutics. Computational and Structural Biotechnology Journal, 20, 6011-6022. https://doi.org/10.1016/j.csbj.2022.10.033