Molecular Basis Of Mutation: Mutation, Repair and Recombination

The molecular basis of gene mutation involves changes in the DNA sequence that can alter gene function or expression. Mutations occur due to errors in DNA replication, exposure to mutagens like radiation or chemicals, or spontaneous changes within cells. These changes can lead to variations in traits, and some mutations may cause genetic disorders or contribute to evolution. In this article, the molecular basis of mutation, gene mutation, types of mutations, and causes of mutations are discussed. Molecular Basis of Mutation is a topic of the chapter Principles of Inheritance and Variation in Biology.

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This Story also Contains

1. What is the Molecular Basis of Mutation?
2. What Is Gene Mutation?
3. Mutation and its types
4. Causes of Mutations

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 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 loses 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.

Mutation and its types

Mutations can be classified according to the nature of change in the DNA sequence.

Substitutions (Transition and Transversion)

The nucleotide, at a particular point, gets replaced by another kind of nucleotide in substitution mutations. The two major types are transitions, where one purine is replaced by another or one pyrimidine is replaced by another, and transversions, in which one purine is replaced by one pyrimidine or vice versa. Substitutions may result in silent mutations where the amino acid composition is precisely the same or missense mutations where a different amino acid may be incorporated into the protein, which may alter function.

Insertion

Insertion mutations involve the addition of one or more nucleotide pairs in the DNA sequence. This can result in a frameshift mutation by disrupting the reading frame of the gene when the total number added is not a multiple of three nucleotides. Insertions can result in a nonfunctional protein being made or previously inactive regions of the genome being turned on.

Deletions

The deletion mutations involve the loss of one or more pairs of nucleotides from DNA. Just like insertions, deletions may also result in a frameshift mutation, provided the deleted no. of nucleotides is not a multiple of three. Deletions can quite possibly result in the loss of certain vital genetic information that would be required to produce either truncated or malfunctioning proteins. Such an event can have serious implications for the organism's survival.

Frameshift Mutation

These introduce or remove any nucleotides from a sequence, creating a different reading frame for a gene. The code of the genes is read in triplets, which means that frameshifts alter how nucleotides are grouped, resulting in an entirely different amino acid chain thereafter. Commonly, the resulting protein has no function, and these effects alone can dramatically impact an organism's phenotype.

Also Read:

• MCQ Practice on Molecular Basis of Mutation
• Principles of Inheritance and Variation
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• Molecular Basis of Inheritance

Causes of Mutations

Mutation can be due to a variety of sources and broadly can be put under spontaneous and induced mutations.

Spontaneous Mutations:

These are naturally occurring mutations without any external influence.

• Errors during DNA Replication: During replication, the enzyme DNA polymerase is prone to errors—for example, mis-incorporation of nucleotides—thereby introducing point mutations. Even though DNA polymerase has proofreading capabilities, on a few occasions these errors slip through to become permanent mutations.

• Tautomeric Shifts: The bases of DNA can, at times, change to tutomers, that can mispair during DNA replication. An example of it is that adenine may change into a form that would pair with cytosine instead of thymine, hence leading to transition mutations.

Induced Mutations:

It is caused by exposure to some exogenous factors called mutagens.

• Chemical Mutagens: Some chemicals can interact directly with the DNA molecule to induce mutations. For example, base analogues are chemicals that very closely resemble the natural bases and may be incorporated into the DNA in the replicative process. Alkylating agents are chemicals that donate alkyl groups to DNA, which can result in an errant pairing of bases and thus mutation.

• Physical Mutagens: Radiation, inclusive of sources such as UV light and X-rays, can induce the formation of mutations. UV light can induce the formation of thymine dimers, which distort the DNA helix and can lead to errors during DNA replication. X-rays and gamma rays can cause breaks in the DNA strands, leading to various mutations.

• Biological Agents: One wide class among them includes viruses and transposable elements, which can integrate into host DNA, interrupting the normal function of genes and thereby leading to a mutation. For example, viral DNA integration could cause insertional mutagenesis in the host genome, either inappropriately inactivating or activating certain genes.

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