Inversion
A genetic inversion is chromosomal mutation that results in an alteration of that chromosome's gene sequence.
Inversions (inversional mutations) occur when a section of a chromosome is broken away and then re-attached to the chromosome in an orientation opposite the original orientation (a 180 degree rotational shift). There are several types of chromosomal inversion that have differentiating (distinguishing) features. If, for example, the inversional mutation includes the centromere it is considered as pericentric inversion. If the inversional mutation does not include the centromeric region (i.e., the area surrounding the centromere) of the chromosome, then the mutation is referred to as a paracentric inversion.
If one of the chromosomes donated by a parent contains an inversion mutation, the offspring organism is an inversion heterozygote. In such heterozygotes, one of the chromosomes contains a reversal in the linear sequence of genes in the inverted segment of the chromosome. The other chromosome contains the normal sequence or preinversion sequence.
Another type of inversion involves the formation of an inversion loop. During meiosis, homologous chromosomes, one containing an inversional mutation, can form circular or looping structures during synapsis.
When a sequence of DNA reads the same in different directions on two strands (i.e., the same forward on one strand as backward on the matching stand) one strand is the inverted repeat of the other strand. For example, if one strand reads CCATGG the inverted repeat reads GGTACC.
Another type of genetic inversion occurs in inverted terminal repeat sequences that are present at the terminus (end) of some transposons. Inverted terminal repeats are short segments with reverse orientations.
Studies of the Drosophila fruit fly aid scientists in the study of inversions. Because Drosophila have large, easily observable, chromosomes that show a high number of inversions, they are an ideal subject for the study of chromosomal inversions. One mechanism to explain inversions involves short DNA sequences called transposons (also called transposable elements) that replicate and insert into chromosomes.
Other models used to explain the occurrence of inversion mutations relate to recombination reactions.
Inversional mutations vary in consequence to the organism. An inversional mutation may be undetectable at the observable (phenotypic) level if the gene expression on the inverted chromosomal segment remains normal or unaffected. Another factor that can act to diminish the impact of inversion mutations is that inversions are able to re-invert, thus returning the chromosome -- or critical genes on the chromosome -- to their original configuration. Because inversions cause a 180° shift, a second inversion restores the original orientation of the genes on the once inverted segment.
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