Mutation: Definition, Types, Causes and Effects

A Mutation is defined as a sudden, random and directionless change in the genetic sequence. As we know the different alleles present in different gene pools cause phenotypic variations like these. Alleles are the mutated copies of a single gene responsible for the particular character. In this article, we will learn about the definition of mutation, its types, significance and much more.

A few types of mutations are namely, Silent mutation, Nonsense mutation and Missense mutation. Genetic variation occurs as a result of the mutation. Positive mutations are passed down across generations. Continue reading to know more interesting things about Mutation.

What is Mutation?

A mutation is a sudden change in the genetic sequence, i.e. change in the sequence of nucleotides on DNA. Mutations occur randomly and are directionless. Such mutations can occur for a single nucleotide or for a large number of nucleotides or chromosomal segments. the Father of Mutation is Hugo de Vries, as he gave mutation theory while working on plant evening primrose, i.e. Oenothera lamarckiana.

Terminologies Associated with Mutation

1. Saltation: Single-step large mutation that leads to speciation is called saltation.
2. Aneuploidy: When the number of chromosomes in an organism are not the exact multiple of the basic set of chromosomes, i.e. \(n\), it is called aneuploidy.
3. Frames: The set of three nitrogenous bases that codes for a codon is called frames.
4. Mutagens: Mutation causing agents are called mutagens.

Types of Mutations

Mutations form the basis of evolution by producing variations. Hence the study of mutations interests scientists in many fields of biology, ranging from evolution to modern genomics and they classify mutations differently. Thus, mutations can be classified as-

1. Frameshift Mutations: During protein synthesis or translation, the amino acid is added based on the sequence of three nitrogenous bases called codons. This sequence of three nitrogenous bases forms a frame. Insertion, deletion or duplication of nucleotides between any of these nucleotides, results in reading of these nucleotides sequences or frames. Such a mutation is called frameshift mutation.

Fig: Illustration of Frameshift Mutations

2. Point Mutations: When a single nucleotide is changed in a DNA, such a mutation is called a point mutation. Point mutations are also referred to as point substitution mutations. A point mutation can be further classified into two types, i.e. synonymous point mutation and non-synonymous point mutation.
a. Synonymous point mutation or silent mutation: Where the nucleotide gets replaced in such a way that a similar amino acid gets coded. This kind of mutation is generally seen in case of codons showing degeneracy.
b. Non-synonymous mutations: When a point mutation codes for an entirely different amino acid, then such a mutation is called a nonsynonymous mutation. The subtypes of non-synonymous mutations are- missense mutation (entirely different amino acids), nonsense mutation (substitution results in formation of STOP codon) and nonstop mutation (otherwise STOP codon is converted into an amino acid codon).

3. Small-scale Mutations: These mutations affect a gene in a single or few nucleotides. They are further classified as:
a. Insertion: One or more pairs of nucleotides are added in the sequence.
b. Deletion: One or more pairs of nucleotides are lost from the sequence.
c. Substitution: One or more nucleotides are substituted. Substitution may be of transition or transversion type.
i. Transition: It is a substitution of purines for purines and pyrimidines for pyrimidines.
ii. Transversion: It is a substitution of a purine for pyrimidine and vice versa.

4. Large-scale Mutations: these are the mutations at the chromosomal level and referred to as chromosomal rearrangements or chromosomal aberrations. Chromosomal rearrangements are broadly categorized as balanced and imbalanced.
a. Balanced chromosomal rearrangements change the gene order but do not delete or duplicate any genes.
b. Imbalanced chromosomal rearrangements involve deletions or duplications of genes.
5. Structural Mutations: Any change in the structure of chromosomes, small or large, will affect the organism at many levels. Such mutations are called structural mutations. Structural mutations can be further classified as inversion, translocation, deletion and duplication.
a. Chromosomal inversion: When a chromosome segment is broken from two points, and the broken segment rotates by \({180^o}\) and rejoin the same chromosome, then such a mutation is called chromosomal inversion mutation.

Fig: Schematic Representation of Chromosomal Inversion

b. Chromosomal Translocation: When a chromosome segment is interchanged between non-homologous chromosomes, then such a mutation is called chromosomal translocation mutation.

Fig: Schematic Representation of Chromosomal Translocation

c. Chromosomal Deletion: When a DNA segment is lost from one of the arms of a chromosome and two segments on either side of the deleted part are joined together, then such a mutation is called chromosomal deletion mutation.

Fig: Schematic Representation of Chromosomal Deletion

d. Chromosomal Duplication: When a DNA segment is duplicated within an arm of a chromosome, the segment gets copied or duplicated, such a mutation is called chromosomal duplication mutation. In the simplest type of duplication, the two segments are next to each other (tandem duplication).

Fig: Schematic Representation of Chromosomal Duplication

6. Numerical Mutations: Change in the number of chromosomes in an organism is called numerical chromosomal mutations. Based on the numbers of chromosomes, the chromosomal aberrations can be:
a. Euploidy: Itis a variation in the complete sets of chromosomes in an organism. It can be of the following subtypes.
i. Haploidy (monoploidy): Only one set of chromosomes is present \((n)\).
ii. Polyploidy: Multiple sets of chromosomes are present.
iii. Triploidy: Three sets of chromosomes are present \((3n)\).
iv. Tetraploidy: Four sets of chromosomes are present \((4n)\).
v. Pentaploidy: Five sets of chromosomes are present \((5n)\).
vi. Hexaploidy: Six sets of chromosomes are present \((6n)\).
vii. Septaploidy: Seven sets of chromosomes are present \((7n)\).
viii. Octoploidy: Eight sets of chromosomes are present \((8n)\).
b. Aneuploidy: When the number of chromosomes in an organism is not the exact multiple of the basic set of chromosomes, i.e. \(n\), it is called aneuploidy. These types can be
i. Monosomy: Loss of one chromosome in a diploid organism. It is mathematically represented as \((2n – 1)\).
ii. Double Monosomy: Loss of one chromosome from two different chromosome pairs in a diploid organism. It is mathematically represented as \((2n – 1 – 1)\).
iii. Nullisomy: Loss of one pair of chromosomes in a diploid organism. It is mathematically represented as \((2n – 2)\).
iv. Trisomy: Addition of one extra chromosome in a pair of chromosomes in a diploid organism. It is mathematically represented as \((2n + 1)\).
v. Double trisomy: Addition of one extra chromosome in two different chromosome pairs in a diploid organism. It is mathematically represented as \((2n + 1 + 1)\).
vi. Tetrasomy: Addition of one extra pair of chromosomes in a diploid organism. It is mathematically represented as \((2n + 2)\).
7. Somatic Mutations: Those mutations that take place in somatic cells and are not inherited from or to the generations. Tumour cells that become malignant are examples of cells with somatic mutations.
8. Germline Mutations: Those mutations that take place in reproductive cells and are inherited from or to the generations.  Germline mutations give rise to a constitutional mutation in the offspring,

Causes of Mutations

Mutations are caused mainly by four types of factors, including natural processes and man-made things. The sources of mutations can be:
a. Spontaneous Changes: Mutations occurring due to oxidative damage to the DNA are called spontaneous mutations. They are characterized by some specific changes such as:
i. Depurination: Depurination can take place owing to spontaneous hydrolysis of glycosidic bonds. It creates an apurinic site (AP site).
ii. Deamination: Spontaneous hydrolysis can also lead to the replacement of an amine group from the nitrogenous bases to a keto group resulting in atypical bases.
iii. Tautomerism: A base is spontaneously changed to its structural isomer due to the rearrangement of hydrogen bonds within its structures.
iv. Modification of bases: Endogenous cellular processes such as methylation, phosphorylation, acetylation, etc. modify the bases. Reactive oxygen species and other metabolites form adducts with DNA modifying the bases. Such modifications may cause errors in DNA replication leading to mutations.
v. Slipped strand mispairing: During DNA replication, the newly formed strand get spontaneously denatured and renatured, but during renaturation mispairs leading to inversions, deletions or additions of segments
b. Induced Mutations: Chemicals and radiations are responsible for causing mutations. Such agents are called mutagens. Induced mutations can be caused by:
i. Chemicals: Many types of chemicals are responsible for induced mutations. Few examples are base analogues, alkylating agents, DNA cross-linking agents, agents that form DNA adducts, reactive oxygen species, aromatic amines, deaminating agents, polycyclic aromatic hydrocarbons (PAHs). Industrial products, paints, resins, chemical fertilizers are the sources of many of such chemicals.
ii. Radiations: Prolonged exposures to non-ionizing radiations (UV rays, microwaves)  and ionizing radiations ( gamma rays, \({\rm{X}}\)-rays etc) lead to induced mutations.
iii. Biological mutagenic agents: Some of the biological agents such as viruses, bacteria, transposons etc are responsible for induced mutations.

Significance of Mutations

1. They have major roles to play in various areas like diseases, carcinogenesis and prions.
2. They are sources of variation in an organism and thus contribute to evolution and speciation.

Diseases Due to Mutations

Diseases caused due to any kind of mutations explained in this article can cause a wide range of disorders in humans. A few of these examples are given in the table below:

Role in Carcinogenesis

1. Carcinogenesis means causing cancer.
2. Point mutations arising from spontaneous mutations during DNA replications, error-prone DNA repair mechanisms, upon exposure to mutagens such as radiation, chemicals and extreme heat etc. lead to malignant transformation of normal cells.  
3. All these causes exert their effect on special genes called cancer genes or oncogenes. These oncogenes exist as proto-oncogenes which are like deactivated forms.
4. Exposure to mutagens initiates a mutation in them and proto-oncogenes get activated to become oncogenes.
5. Proto-oncogenes may carry normal cellular functions, but the mutated forms, oncogenes, promote the characteristic features of cancer i.e. uncontrolled cell division (tumour formation) and metastasis.

Fig: Mutation Leading to Malignant Transformation of Cells

Role of Mutations in Evolution

1. Environment plays a role in variations among individuals, but the mutation is the ultimate source of all the variations that are fuel for the evolutionary changes.
2. The table below shows types of mutations and their effects with respect to evolution.

3. The location of mutation decides the phenotypic effect of those mutations.
4. Mutation in the coding region may alter the function of protein but mutation in regulatory regions of the gene may alter the timing and  expression levels
5. Mutations in regulatory regions of a gene may alter the developmental or environmental context of its expression
6. All such scenarios present the perfect raw material for the environmental factors to act upon, leading to speciation.

Figure: Speciation emerges from variation

Summary

A mutation is a change in the DNA at a particular locus in an organism. It is the alteration of the nucleotide sequence in the genome of an organism. Different criteria exist for classifying the mutations. The most common are point mutations, structural mutations and numerical mutations. Sources of mutations range from spontaneous natural cellular processes to man-made mutagens. Mutations are scary as they cause many heritable diseases and they may activate the oncogenes causing cancer. But mutation also forms the first step for evolution as it creates new DNA sequences for a particular gene, creating new alleles.

Frequently Asked Questions

Q.1. What are the four types of structural mutations?
Ans: The four main types of structural mutations are insertion, deletion, inversion and translocation.

Q.2. What is mutation?
Ans: A mutation is a sudden change in the genetic sequence. It is a change in the sequence of building blocks of DNA, i.e. nucleotides.

Q.3. What are the three causes of mutations?
Ans: Spontaneous cellular processes, radiations and chemical mutagens are the \({\rm{3}}\) causes of mutations.

Q.4. Is mutation good or bad?
Ans: Mutations can be beneficial, harmful or neutral. Harmful mutations may lead to disease conditions or may be fatal. Beneficial mutations give some advantage to the organism while neutral mutations are neither harmful nor beneficial.

Q.5. What are the effects of mutation?
Ans: Mutations give rise to variations in the organisms. Harmful mutations may lead to disease conditions or may be fatal to the organisms. Beneficial mutations give some survival advantage.

Q.6. Can humans mutate?
Ans: Yes, like all other organisms, humans also undergo mutations. Sickle cell anaemia, Down’s syndrome, haemoglobin, etc. are examples of mutations.

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