What is the Difference Between Induced and Spontaneous Mutation

The main difference between induced and spontaneous mutation is that an induced mutation arises due to the influence of environmental agents called mutagens whereas a spontaneous mutation arises due to the natural changes in the DNA structure. Furthermore, radiation, oxidative damage, intercalating agents, base analogs, hydroxylamine, nitrous oxide result in induced mutations while errors in DNA replication, depurination, deamination, and transposable genetic elements result in spontaneous mutations. 

Induced and spontaneous mutations are two types of heritable changes in the DNA structure. Mutations are an essential tool in the study of genetics. 

Key Areas Covered 

1. What is an Induced Mutation
     – Definition, Mutagens, Mechanisms
2. What is a Spontaneous Mutation
     – Definition, Facts, Mechanisms
3. What are the Similarities Between Induced and Spontaneous Mutation
     – Outline of Common Features
4. What is the Difference Between Induced and Spontaneous Mutation
     – Comparison of Key Differences

Key Terms 

Base Analogs, Base Damage, Errors in DNA Replication, Induced Mutation, Mispairing, Spontaneous Mutation, Spontaneous Lesions 

What is an Induced Mutation 

Induced mutations are the heritable structural changes in DNA, occurring due to the environmental agents called mutagens. In general, two types of mutagens give rise to mutations: chemical mutagens and physical mutagens. Some examples of chemical mutagens include hydroxylamine, base analogs, alkylating agents, DNA adducts, intercalating agents, DNA crosslinkers, oxidative damage, nitrous acid, etc. In contrast, the two types of physical mutagens are UV radiation and ionizing radiation. 

Figure 1: Mutation Due to UV Radiation

Furthermore, three mechanisms are involved in the formation of induced mutations. They are the incorporation of base analogs, specific mispairing, and base damage.  

Incorporation of Base Analogs 

Base analogs are the chemical compounds with similar structural properties to the nitrogenous bases in the DNA. They can incorporate into DNA, resulting in mutations. For example, 5-bromouracil (5-BU) is a base analog of thymine. Also, 2-amino-purine (2AP) is a base analog of adenine.  

Specific Mispairing 

Some mutagens alter the nitrogenous bases in DNA, leading to mispairing. Alkylating agents like ethyl methanesulfonate (EMS) and nitrosoguanidine (NG) add oxygen to the 6th position of guanine in DNA and the 4th position of thymine, causing a GC to AT transition. Also, intercalating agents including proflavin, acridine orange are planner molecules, which can slip between nitrogenous bases in the DNA.  

Base Damage 

Some mutagens damage one or several nitrogenous bases in DNA causing replication block. In bacteria, the SOS system is responsible for repairing these types of damages in DNA. 

What is a Spontaneous Mutation 

Spontaneous mutations are another type of heritable changes in the structure of DNA, occurring due to natural factors. The three sources of spontaneous mutations are the errors in DNA replication, spontaneous lesions, and the transposable genetic elements.  

Errors in DNA Replication 

The addition of incorrect bases to the newly synthesized DNA strand by DNA polymerase results in errors in DNA replication in the form of base substitutions. Base substitutions can be transition mutations in which a purine base is substituted by another purine base or transversion mutations, which substitute purines by pyrimidines and vice versa. Also, frameshift mutations arise due to replication errors. Also, deletions account for a notable fraction of spontaneous mutations. Moreover, replication errors can cause duplication of segments of DNA as well. 

Figure 2: Different Structural Changes in DNA

Spontaneous Lesions 

Depurination and deamination are the two main types of spontaneous lesions which can occur in DNA. Depurination is the subsequent loss of purine residues from the structure of DNA, resulting in apurinic sites. In deamination, cytosine converts into uracil, which then pairs with adenine during replication. Therefore, this also causes GC to AT transition. However, there is a third type of spontaneous lesions called oxidatively damaged bases. Here, active oxygen bases like superoxide radicals, hydrogen peroxide, and hydroxyl radicals cause oxidative damage to the DNA, causing mutations.  

Transposable Genetic Elements 

Some genetic elements can move from one to another position of the same or a different chromosome. They mediate chromosomal rearrangements.  

Similarities Between Induced and Spontaneous Mutation 

• Induced and spontaneous mutations are two types of structural changes occur in DNA. 
• Moreover, induced and spontaneous mutations are heritable. 
• Also, both types of mutations are important in the study of functional genomics to find out the function of a particular gene in the genome. 

Difference Between Induced and Spontaneous Mutation 

Definition 

Induced mutation refers to a mutation that is produced by treatment with a physical or chemical agent that affects the deoxyribonucleic acid molecules of a living organism while spontaneous mutation refers to a mutation that arises naturally and not as a result of exposure to mutagens. Thus, this is the main difference between induced and spontaneous mutation.

Origin 

Another difference between induced and spontaneous mutation is that the induced mutations occur due to mutagens from the environment while the spontaneous mutations occur due to natural causes. 

Mechanisms  

Mechanisms involved in them is also a major difference between induced and spontaneous mutation. Induced mutations occur due to the incorporation of base analogs, base mispairing, and base damage while spontaneous mutations occur due to the errors in DNA replication, spontaneous lesions like depurination and deamination, and transposable genetic elements. 

Functional Genomics 

While induced mutations are important in reverse genetics, spontaneous mutations are important in forward genetics. Hence, this is another difference between induced and spontaneous mutation.

Conclusion 

An induced mutation is a type of structural change in the DNA, occurring due to mutagens. The three main mechanisms of induced mutations are the incorporation of base analogs, mispairing, and base damage. On the other hand, a spontaneous mutation occurs due to natural processes like errors in DNA replication, spontaneous lesions, and transposable genetic elements. Thus, the main difference between induced and spontaneous mutation is the origin of the agent that causes the mutation. 

References:

1. Griffiths AJF, Miller JH, Suzuki DT, et al. An Introduction to Genetic Analysis. 7th edition. New York: W. H. Freeman; 2000. Induced mutations. Available Here 
2. Griffiths AJF, Miller JH, Suzuki DT, et al. An Introduction to Genetic Analysis. 7th edition. New York: W. H. Freeman; 2000. Spontaneous mutations. Available Here

Image Courtesy:

1. “DNA UV mutation” By NASA/David Herring – NASA, [1] (Public Domain) via Commons Wikimedia  
2. “Chromosomes mutations-en” By GYassineMrabetTalk✉This W3C-unspecified vector image was created with Inkscape. – Own work based on Chromosomenmutationen.png (Public Domain) via Commons Wikimedia