So chromosomal rearrangements describe changes in chromosomal structure. This is usually focused on one chromosome, where something unusual happens to it—parts of it may be deleted, inverted, or duplicated. There are various types of rearrangements, such as deletions, duplications, but also specific terms like translocations and emergence. We will cover each of these topics separately. Typically, no matter what type of rearrangement is occurring, we name them based on their proximity to the centromere. Remember, the centromere—while often illustrated in the middle of a chromosome—doesn't have to be centrally located; it can be at the ends, slightly off-center, or anywhere along the chromosome's length.
Rearrangements can lead to "acentric" chromosomes, which lack a centromere due to some chromosomal rearrangement. In such cases, we refer to the resulting structure as an acentric chromosome. Another kind of rearrangement is the "dicentric" chromosome, which features two centromeres due to some alteration, such as an inversion or a translocation. Dicentric chromosomes face unique challenges during cell division. For instance, when discussed cell division, the crucial role centromeres play, where microtubules attach and pull the chromosomes towards opposite poles, becomes complicated. If a chromosome is dicentric, microtubules can potentially attach to each centromere and tug in opposite directions, forming what's known as an "anaphase bridge." This process can tear the DNA, as the chromosome's material is pulled in two opposing directions.
Here's an illustration: consider an acentric chromosome, missing its centromere from one end to the other, and imagine a dicentric one with two centromeres. We categorize rearrangements as either unbalanced, where there is a change in gene dosage (either a gene is deleted or duplicated), or balanced, where the gene order is altered but the gene dosage remains the same. This can occur through sequence inversions or translocations.
For a clear example, consider a normal arrangement with homologous pairs featuring alleles a, b, c, and d, centered around a centromere. An unbalanced rearrangement might involve the deletion of the d allele, affecting the gene dosage. In contrast, a balanced rearrangement changes the order of the genes—like swapping positions—but doesn't necessarily add or remove any genes; the number of genes remains constant, but their sequence is altered.
Now, let's continue exploring this intricate yet fascinating subject.
