Mutation class 2: Different types of mutation

What are the different types of mutation?

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Any alteration in a nucleotide sequence of DNA is called as a mutation. At a molecular level, single nucleotide to several nucleotides may alter nevertheless at the chromosomal level, entire segment or several gene segments can alter. The alteration may be addition, deletion, substitution or translocation. We will discuss it later in this part.

Spontaneous vs Induced mutation

A mutation which occurred without any known cause is called as a spontaneous mutation. It arises due to metabolic errors, replication errors or during development. Spontaneous mutations are rare and occurred without any reason. Spontaneous mutations are generally come of by birth.

Larger genes are more prone to spontaneous mutation because larger the gene, the chance of error in polymerisation or DNA replication is higher. However, it is not always true. The rate of spontaneous mutation is 10-5 per gene per generation.

The image represents induced vs spontaneous mutation.

Induced mutation is resulting from exposure of an organism to mutagenic agents. The general mutagenic agents are radiation, UV light and chemicals.

UV light is responsible for xeroderma pigmentation and skin cancer by penetrating into the skin. Because UV light cannot penetrate inside the body due to the lower energy, it cannot damages other tissue of the body. Chemicals such as alkylating agents and base analogues are common chemical mutagens. However, every chemical is somehow mutagenic to our DNA.

Additionally,  environmental changes and lifestyle of an organism may result in some of the mutations. Spontaneous mutations are occurred infrequently in every generation but, are stable and frequently increases into the population depending upon its inheritance pattern.

“Under the influence of environment and other adverse conditions, any alteration in nucleotide sequences of DNA is termed as mutations”. 

Although spontaneous mutation does not show any sign or symptoms in the parental organism (remain recessive), it stably expressed in further consecutive generations.

In contrast, induced mutation immediately shows their effect in an organism who is being affected with mutagens.

Somatic vs germline mutation

Mutations which are occurred in the somatic cells of an organism is called as a somatic mutation. Somatic mutations are non inherited because only germ cells (sperm or egg ) can undergo fertilization. Though it is non inherited, it can still cause some life-threatening phenotypes.

Somatic mutation is restricted to tissue or some part of the body. But if it occurs in progenitor cells it can be spread to other daughter cells. Progenitor cells are the group of cells which have the capacity to divide.

The commonest type of somatic mutation results in cancer. In contrast, germline mutation inherits from one generation to other generation. Two types of germ cells sperm and egg, have haploid numbers of a genome and go through the fertilization process. Hence if it has occurred in germ cells it is inherited into the next generation.

In somatic mutation, if a mutation occurs in cells after metaphase, it can not be spread to the other cells. The somatic mutation occurs during the mitosis whereas germline mutation occurred during meiosis.

Germline mutations may or may not affect the parental organism but it will surely affect the offsprings. However, germline mutations are generally non-curable. The parental organism remains unaffected in almost all cases. All the inherited disorders are germline mutations.

The image represents somatic vs germline mutation

Actually, germline mutation provides additional diversity in population but it may be harmful to the organism, sometime. In contrast, somatic mutation always affects an organism since the origin of mutation.

Somatic mutations are restricted to an individual, if it is lethal, an individual may die but it cannot spread to other organisms in the population. Whereas germline mutation is silent, it remains in a recessive condition in some organism and steadily spreads throughout the population.

Chromosomal mutation vs mitochondrial mutation

Mitochondria are an important powerhouse organelle of our cell. Mitochondrial DNA (mtDNA) is present in mitochondrial (not in the nucleus), and it has its own replication and transcription machinery. 

Genomic mutations are inherited in Mendelian fashion. It is inherited in a syndromic manner. Whereas mitochondrial mutations are non-syndromic and do not follow Mendelian inheritance.

One of the interesting thing about mtDNA is, it is inherited from mother. Though all the cells of our body have mitochondria, it is inherited from our mother to us. So it is called as maternal inherited DNA. If the mutation is inherited to male, it cannot further be transmitted.


Read related article: Mutation class 1: What is mutation


Chromosomal mutations are inherited in Mendelian fashion. It inherited to any of the offspring depending upon the type of inheritance. Cry du chat, down syndrome and patau syndrome are some of the common types of chromosomal disorders.

As long as mitochondrial DNA is inherited maternally, it is rarest among rare. Kearns- syre syndrome, Leigh syndrome and non-syndromic hearing loss are some of the mitochondrial disorders which arise due to the mutation in mtDNA.

Deletion, duplication, translocation and inversion is a common type of mutations observed in mtDNA same as genomic DNA. Different types of gene mutations are explained in our previous article: what is mutation.

Chromosomal mutation:

At the molecular level, single nucleotides to several hundred nucleotides are altered but at the chromosomal level large segment of the chromosome with one gene to many, are deleted or added. Depending upon the type of action, chromosomal mutations are categorized into addition, deletion, duplication, translocation, inversion, ring chromosome.

Addition: when some of the extra segment is added to a chromosome, is called as an addition.

Deletion: when some segment of an entire chromosome is deleted and losses several genes, is called as a deletion.

The image is a representation of chromosomal mutation

Duplication: one segment of a chromosome is duplicated in a manner which alters the structural hierarchy of the entire chromosome and creates chromosomal abnormality.

Translocation

Balanced translocation: when two segments (nearly exact) are exchanged, resulting in balanced translocation. Here the two segments are nearly the same in size hence it is not possible to detect such type of translocation by cytogenetic analysis.

The image represents a general type of chromosomal translocation.

Generally, balanced translocation results in recurrent abortion in females and it results in abnormalities in the fetus. Balance translocation can be screened by FISH.

Reciprocal translocation: translocation occurred between two non-homologous chromosomes, is called as reciprocal translocation. This type of translocation is most common in nature.

Robertsonian translocation: Robertsonian translocation occurs between two acrocentric chromosomes. Acrocentric chromosomes are small short chromosome with one long q-arm and a short or very smaller p-arm.

In humans 13, 14, 15, 21, 22 and Y chromosomes are acrocentric. When translocation between two acrocentric chromosomes occurs, one nearly metacentric and one “only centromeric” chromosomes are formed.

Here long arms of both chromosome are fused together. Hence one chromosome become larger metacentric and another chromosome remain without any arms or only with centromere.

Different type of chromosomal translocation.

Though the “only centromeric” chromosome does not have any chromatid, still it contains many genes of both chromosomes. Balanced Robertsonian translocation does not cause any physical abnormality but recurrent abortion in a female is common.

Imbalanced Robertsonian translocation results in mental retardation, trisomy and other structural abnormalities.

Inversion: a segment of a chromosome is inverted and rearranged back to the parental chromosome. Here if the inversion occurs in only one arm of the chromosome, this type of inversion is called as a paracentric inversion. The centromere is not involved in paracentric inversion.

If the inversion occurs between two arms of a chromosome, this type of inversion is called as a pericentric inversion. The centromere is involved in pericentric inversion.

The image represents a general type of chromosomal inversion.

Ring chromosome: when both ends of a chromosome are fused together, it results in ring-like structure and so-called as ring chromosome. A ring chromosome is generally observed in the long metacentric chromosome.

Telomeres of each chromosome have specific sequences which prevent them to fuse together. When some of this sequences (telomeric sequences are losses) results in fusion of chromosome.

Ring chromosome results in wide varieties of phenotypic abnormalities like microcephaly, mental retardation and epilepsy.

Read related articles:

Mutation class 1: What is mutation

Article written by- Tushar Chauhan 

Article reviewed by- Ravi Parmar

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