How Chromosome Mutations Occur

A chromosome mutation is an unpredictable change that occurs in a chromosome. These changes are most often brought on by problems that occur during meiosis (cell division process of gametes) or by mutagens (chemicals, radiation, etc.). Chromosome mutations can result in changes in the number of chromosomes in a cell or changes in the structure of a chromosome. Unlike a gene mutation which alters a single gene or larger segment of DNA on a chromosome, chromosome mutations change and impact the entire chromosome.

Chromosome Structure

cdascher / Getty Images

Chromosomes are long, stringy aggregates of genes that carry heredity information (DNA). They are formed from chromatin, a mass of genetic material consisting of DNA that is tightly coiled around proteins called histones. Chromosomes are located in the nucleus of our cells and condense prior to the process of cell division. A non-duplicated chromosome is single-stranded and is comprised of a centromere region that connects two arm regions. The short arm region is called the p arm and the long arm region is called the q arm. In preparation for the division of the nucleus, chromosomes must be duplicated to ensure that the resulting daughter cells end up with the appropriate number of chromosomes. An identical copy of each chromosome is therefore produced through DNA replication. Each duplicated chromosome is comprised of two identical chromosomes called sister chromatids that are connected at the centromere region. Sister chromatids separate prior to the completion of cell division.

Chromosome Structure Changes

Duplications and breakages of chromosomes are responsible for a type of chromosome mutation that alters chromosome structure. These changes affect protein production by changing the genes on the chromosome. Chromosome structure changes are often harmful to an individual leading to developmental difficulties and even death. Some changes are not as harmful and may have no significant effect on an individual. There are several types of chromosome structure changes that can occur. Some of them include:

• Translocation: The joining of a fragmented chromosome to a non-homologous chromosome is a translocation. The piece of chromosome detaches from one chromosome and moves to a new position on another chromosome. 
• Deletion: This mutation results from the breakage of a chromosome in which the genetic material becomes lost during cell division. The genetic material can break off from anywhere on the chromosome. 
• Duplication: Duplications are produced when extra copies of genes are generated on a chromosome. 
• Inversion: In an inversion, the broken chromosome segment is reversed and inserted back into the chromosome. If the inversion encompasses the centromere of the chromosome, it is called a pericentric inversion. If it involves the long or short arm of the chromosome and does not include the centromere, it is called a paracentric inversion. 
• Isochromosome: This type of chromosome is produced by the improper division of the centromere. Isochromosomes contain either two short arms or two long arms. A typical chromosome contains one short arm and one long arm.

Chromosome Number Changes

A chromosome mutation that causes individuals to have an abnormal number of chromosomes is termed aneuploidy. Aneuploid cells occur as a result of chromosome breakage or nondisjunction errors that happen during meiosis or mitosis. Nondisjunction is the failure of homologous chromosomes to separate properly during cell division. It produces individuals with either extra or missing chromosomes. Sex chromosome abnormalities that result from nondisjunction can lead to conditions such as Klinefelter and Turner syndromes. In Klinefelter syndrome, males have one or more extra X sex chromosomes. In Turner syndrome, females have only one X sex chromosome. Down syndrome is an example of a condition that occurs due to nondisjunction in autosomal (non-sex) cells. Individuals with Down syndrome have an extra chromosome on autosomal chromosome 21.

A chromosome mutation that results in individuals with more than one haploid set of chromosomes in a cell is termed polyploidy. A haploid cell is a cell that contains one complete set of chromosomes. Our sex cells are considered haploid and contain 1 complete set of 23 chromosomes. Our autosomal cells are diploid and contain 2 complete sets of 23 chromosomes. If a mutation causes a cell to have three haploid sets, it is called triploidy. If the cell has four haploid sets, it is called tetraploidy.

Sex-Linked Mutations

Mutations can occur on genes located on sex chromosomes known as sex-linked genes. These genes on either the X chromosome or the Y chromosome determine the genetic characteristics of sex-linked traits. A gene mutation that occurs on the X chromosome can be dominant or recessive. X-linked dominant disorders are expressed in both males and females. X-linked recessive disorders are expressed in males and can be masked in females if the female's second X chromosome is normal. Y chromosome linked disorders are expressed only in males.

Sources:

• "Mutations and Health." Help Me Understand Genetics. US National Library of Medicine. Web. Updated 5 July 2016. https://ghr.nlm.nih.gov/primer#mutationsanddisorders.