Chromosomes are frequently subject to modifications, either in number ( euploidies and aneuploidies ) or in their structure. Structural changes refer to modifications that affect the morphology of the chromosome, such as the loss of part of the chromosomes, duplication of pieces and even exchange of portions between chromosomes.
→ Types of structural chromosomal alterations
Different factors can cause changes in chromosomal structure, such as radiation and meiosis errors. Structural changes can happen in different ways, the main ones being:
- Deletion: happens when a portion of the chromosome is lost. Because a part is missing, this chromosome doesn’t have all the genes it should have. These deletions can occur in terminal portions or in more internal portions. The latter are called interstitial deletions. Deletions occur most frequently during meiosis.
- Duplication: happens when a portion of a segment of the chromosome is repeated . This situation occurs when the deleted chromosome fragment binds to a sister chromatid, forming an extra segment. Like deletions, duplications occur most frequently during meiosis.
- Inversion: is a process in which part of the chromosome separates and reorganizes itself in an inverted way. In this case, we do not have a loss of genetic material, but a change in its order. There are pericentric and paracentric inversions. The first concerns inversions involving the centromere. Paracentric refers to inversions in which the centromere is not involved.
Translocation: occurs when a portion of a chromosome is transferred to another non-homologous chromosome . When we observe the exchange between two chromosomes, we say that the translocation is reciprocal. There is also the so-called Robertsonian translocation , which occurs in an acrocentric chromosome (a chromosome that has the centromere near the end of one of the arms, ensuring the formation of an arm much larger than the other). In this case, one suffers the breakage of a long arm, and the other, of a short arm. After the exchange, a chromosome with two long arms and another with two short arms are formed. The chromosome with the two short arms is usually lost.
Important: Structural chromosomal alterations can be harmful and are often responsible for changes in the individual’s phenotype.