Molecular Basis Of Mutation: Mutation, Repair and Recombination

Edited By Irshad Anwar | Updated on May 31, 2025 07:58 AM IST

The molecular mechanism of mutation consists of the alteration of the sequence of DNA, which can transform gene function or expression. They occur spontaneously during DNA replication by errors committed by DNA polymerase or tautomeric transitions of nucleotide bases. Despite proof-reading capabilities, approximately 1 in 10⁷ base pairs might suffer permanent alteration. Such spontaneous mutation is an important source of genetic variation and is responsible for driving evolution. But some can cause deleterious consequences, like genetic diseases.

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What is the Molecular Basis of Mutation?
What Is Gene Mutation?
Mutation and Its Types
Causes of Mutations And Their Types
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Molecular Basis Of Mutation: Mutation, Repair and Recombination

Induced mutations happen when DNA is subjected to an external mutagen, like UV radiation, X-rays, chemicals, or biological agents. For example, UV radiation induces thymine dimers, and alkylating agents mispair bases in DNA. Biological agents like viruses could integrate themselves into the genome of the host, disturbing gene function. These mutations could lead to disorders like cancer or inherited diseases.

What is the Molecular Basis of Mutation?

The molecular basis of mutation is changes in the nucleotide sequence of DNA, manifested in alterations of the genetic information it carries. These can be caused by spontaneous errors during DNA replication, by chemicals, radiation, and also by viruses. The results of mutations can be innocuous or injurious and can moreover influence the attributes of an organism, thus attaining genetic diversity in populations or genetic disorders.

What Is Gene Mutation?

A gene mutation is a permanent alteration within the DNA sequence that makes up the gene. Such mutations can happen in several ways, such as substitutions, insertions, deletions, and frameshift mutations. These changes can perturb gene function in a way that the expression of either a nonfunctional gene occurs or its production. Gene mutations are either inherited or acquired during an organism's lifetime and may make important contributions to a variety of diseases and evolutionary processes.

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Mutation and Its Types

Mutations can be classified according to the nature of the change in the DNA sequence. All 4 types of mutation, along with their comparison, are discussed below in the table:

Feature	Substitution (Transition & Transversion)	Insertion	Deletion	Frameshift Mutation
Definition	Another replaces one nucleotide	One or more nucleotides are added	One or more nucleotides are removed	Shifts the reading frame due to the insertion or deletion of nucleotides
Sub-types	Transition (purine↔purine, pyrimidine↔pyrimidine); Transversion (purine↔pyrimidine)	Not applicable	Not applicable	Not applicable
Effect on Reading Frame	No	Yes, if insertion is not in multiples of 3	Yes, if deletion is not in multiples of 3	Yes
Impact on Protein Sequence	May be silent (no change), missense (amino acid change), or nonsense (stop codon)	Alters the amino acid sequence, can introduce a frameshift	Alters amino acid sequence, can introduce a frameshift	Alters the entire downstream amino acid sequence
Possibility of Frameshift	No	Yes, if not in multiples of 3	Yes, if not in multiples of 3	Always
Functional Consequences	Protein may be normal, altered, or truncated	Protein may be nonfunctional or gain a novel function	May lead to loss of essential protein function	Usually results in nonfunctional or harmful proteins
Severity of Effect	Mild to severe, depending on position and type	Often severe if it causes a frameshift	Often severe if it causes a frameshift	Usually severe due to a major change in protein structure
Example	Sickle Cell Anaemia (missense mutation in β-globin gene)	Tay-Sachs disease (insertion in the HEXA gene)	Cystic Fibrosis (3-base pair deletion in the CFTR gene)	Huntington’s disease (triplet repeat expansion leads to a frameshift in the coding region)

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Causes of Mutations And Their Types

Mutations can be due to a variety of sources and can be broadly categorised into spontaneous and induced mutations. A basic comparison between both of them is discussed below:

Feature	Spontaneous Mutations	Induced Mutations
Definition	Mutations that occur naturally without any external influence	Mutations that occur due to exposure to external physical, chemical, or biological agents (mutagens)
Major Cause	Internal cellular processes (e.g., DNA replication errors, spontaneous base changes)	Environmental mutagens such as chemicals, radiation, or biological agents
Sub-types	1. Errors during DNA replication 2. Tautomeric shifts	1. Chemical mutagens 2. Physical mutagens 3. Biological agents
Mechanism	DNA polymerase misincorporates bases Base tautomerism leads to incorrect base pairing	Mutagens alter base structure or cause strand damage Foreign DNA integration alters gene function
Example of Mechanism	Tautomeric shift: Adenine pairs with cytosine instead of thymine DNA replication errors cause point mutations	Base analogues: mimic real bases and get incorporated UV radiation causes thymine dimers Viruses: cause gene disruption
Type of Mutation Caused	Often point mutations (e.g., transition or transversion)	Can cause point mutations, insertions, deletions, frameshifts, or chromosomal aberrations
Repair Possibility	May be repaired by proofreading or mismatch repair systems	It may overwhelm repair systems depending on the dose or intensity of the mutagen
Examples	Spontaneous base deamination Tautomeric base pairing	UV radiation (thymine dimers) Alkylating agents Retrovirus integration
Significance	Source of natural genetic variation	Used in experimental mutagenesis and linked to mutagen-induced diseases (e.g., cancer)