What is a chromosomal aberration?
Chromosomal aberration is a change in the chromosomes that is visible under the light microscope. In contrast, there are gene mutations, these changes are much smaller and can only be detected by more precise genetic diagnostics. Chromosomal aberrations can be divided into two forms.
There are structural and numerical aberrations. The structural abberations were discussed in this chapter and are also called chromosomal mutations. They are accompanied by a change in the chromosomes themselves.
Numerical aberration means that there is an incorrect number of chromosomes. The normal human karyotype, i.e. the number of chromosomes, is 46.XY in men and 46.XX in women. It should be noted that the chromosomes are duplicated, the only exception being the sex chromosomes in men.
The sex chromosomes, which are the only difference in the genome in men and women, are indicated by the letters. In numerical aberrations, chromosomes may be more common or missing. The best known example of this is Down’s syndrome, trisomy 21, the name indicates that the 21st chromosome is present three times instead of twice.
The karyotype of a person then looks like this: 47,XY+21 in men and 47,XX+21 in women. An example of the absence of a chromosome is the Turner syndrome or monosomy X, in which an X chromosome is missing, resulting in the karyotype 45,X0. The affected people are women with certain physical abnormalities.
The Klinefelter syndrome is not a chromosomal mutation. Chromosomal mutations are accompanied by a change in structure. In Klinefelter’s syndrome, there is an incorrect number of chromosomes.
This change is called numerical chromosome aberration. Only men can be affected by Klinefelter’s syndrome. Normally, men have one X chromosome and one Y chromosome in addition to the 44 other chromosomes (22 chromosomes, each in two copies).
In Klinefelter’s syndrome there is one X chromosome too many. This change has the greatest impact on the reproductive organs. The affected persons have small testicles and in most cases no functioning sperm production.
In addition, the testosterone production of the testicles is limited. Klinefelter’s syndrome is often first diagnosed during a fertility consultation. It remains undiscovered in the overwhelming majority of those affected. Breast development can occur during puberty, and usually there is only a sparse development of body hair. However, the affected persons are not restricted in their intelligence.
What is a point mutation?
A point mutation is a mutation of genes that are present in the form of DNA. The DNA is made up of four different nucleotides. In a point mutation, one nucleotide is replaced by another.
The DNA of the genes is read in a complex process. The end product of this process is a protein. Which protein is produced is essentially determined by the sequence of the nucleotides.
Three successive nucleotides determine which amino acid is attached to the protein chain that is formed. A large number of amino acids are then combined to form a protein. If one nucleotide is replaced by another, another amino acid may be incorporated into the protein.
The consequence is a so-called missense mutation. The resulting protein can remain fully functional by replacing the amino acid or lose its function. Another possibility is the integration of a so-called stop codon.
This type of mutation is called the nonsense mutation. The stop codon marks the end of the protein. If it is inserted at the wrong place, the protein is not complete.
With these forms of point mutations, the number of nucleotides to be read remains the same. The correct amino acids are inserted before and after point mutations. The “reading frame” therefore remains the same.
More serious are mutations in which a nucleotide is inserted or removed, i.e. insertion or deletion. In this mutation, three other nucleotides are combined after the missing or inserted nucleotide, thus inserting different amino acids and creating a completely different protein. Our next article might also be interesting for you: Telomeres – Anatomy, Function & Diseases