Structural Chromosomal Aberrations

What are Structural Chromosomal Aberrations?

The situations in which the chromosome produces an abnormality in the structure are called chromosomal aberrations. Such structural chromosomal aberrations can cause abnormalities in the chromosome structure. Alterations in the sequence as well as changes in the type of genes in the chromosome, are common consequences.

Chromosomes are generally considered to be certain definite units, carrying genes arranged in a linear order. Moreover, the chromosome number of each plant and animal is 2n in somatic cells of sporophytes, while it is n in gametes, spores, and gametophytes. Thus, in an earlier concept of genetics, chromosomes are fixed in their structure, and they occur either in diploid or haploid conditions.

However, several exceptions to this condition have been noticed on various occasions. The chromosome number is not always 2n, nor is their structure always normal. The structural change may remain confined to a single chromosome or may extend to both members of a pair.

Types of Structural Chromosomal Aberrations

Structural modifications of chromosomes are widespread and have played an important role in evolution. Some of these changes occur at the molecular level and are sub-microscopic. These are called gene mutations. Several of them are however concerned with the number and arrangement of genes and are microscopic.

Structural chromosomal aberrations may be of the following types. It can be simple alterations in their gene sequence, a change in the number of genes in them, or a complete alteration of the location of certain genes. Based on these, there are four types of structural chromosomal aberrations.

Deletion: Loss of a segment of a chromosome, which is called a deletion or deficiency.
Duplication: The addition of a segment to the chromosome is known as duplication.
Inversion: An interchromosomal rearrangement by rotation of a segment through an angle of 180 degrees is known as inversion.
Translocation: The exchange of segments between chromosomes is called translocation.

Deletion

Deletion is a gene mutation that is localised. It is often identified as the elimination of a gene or a nucleotide from a sequence that alters the characteristic or expression of the gene. It is a permanent loss as they get detached from the centromere. As a result, the newly formed chromosome will be devoid of certain genes, which may even be proven fatal in some cases.

Duplication

Duplication, as the name suggests, is the duplication of certain genes in the chromosome. Such situations may preserve certain genes and also gain some new ones. In most cases, this situation is mostly harmless.

Inversion

Inversion is a situation where a gene sequence has the reverse order than usual. This does not change the general trait of the chromosome. However, this new sequence may be viable and does not code for the protein it otherwise should have. There are also cases where such a reversal of the sequence has been proven to be beneficial for the organism. Inversions are caused when a gene breaks but reattaches itself in the reverse order.

Translocation

Translocation of genes happens when genes or segments from one homologous chromosome get detached from it and go on to bind with the other homologous chromosome of the pair. Such structural changes are usually lethal.

Gene Mutations

In addition to the above-mentioned four structural chromosomal aberrations, there are two additional gene mutations- substitution and insertion.

Substitution: Here, one base in a codon gets replaced by another codon. This alters the code for the amino acid and thus the gene expression. Sometimes this gene does not cause any change in expression as it could code for the same amino acid. The latter situation is called a silent mutation as it does not cause any alteration to the outcome. At the same time, if this newly formed codon is a stop codon, it causes the protein to be incomplete.
Insertion: This means the insertion of an additional codon into the gene sequence.

Consequences of Structural Aberrations

The structural changes at times produce acentric chromosomes. These acentric fragments, since they lack centromeres, are incapable of orientation and movement, so they perish sooner.
As these changes are heritable, they could cause genetic disorders in the offspring.
It can cause birth defects and/or developmental issues.
It may also cause semi-sterility.

References

Agarwal, P. V. |. V. (2004). Cell biology, Genetics, Molecular Biology, Evolution, and Ecology: Evolution and Ecology. S. Chand Publishing.
http://eagri.org/eagri50/GBPR111/lec21.pdf
Zeiger, E. (2009). Genetic Toxicology Testing. Comprehensive Toxicology (Second Edition), 139-158. https://doi.org/10.1016/B978-0-08-046884-6.00316-X