What is the Difference Between Back Mutation and Suppressor Mutation

The main difference between back mutation and suppressor mutation is that back mutation is a point mutation which restores the original sequence whereas suppressor mutation is a second mutation which either alleviates or reverts the phenotypic effects of an already existing mutation. Furthermore, back mutation restores the true wild type; suppressor mutation only masks the effect of the first mutation while the gene holds the first mutation. In addition to these, the two types of suppressor mutations include intragenic and extragenic mutations.  

Back mutation and suppressor mutation are two types of reverse mutations, which are second mutations following a particular mutation event. Significantly, they revert the effect of the existing mutation and restores the original phenotype. 

Key Areas Covered 

1. What is Back Mutation
     – Definition, Features, Effect
2. What is Suppressor Mutation
     – Definition, Types, Effect
3. What are the Similarities Between Back Mutation and Suppressor Mutation
     – Outline of Common Features
4. What is the Difference Between Back Mutation and Suppressor Mutation
     – Comparison of Key Differences

Key Terms 

Back Mutation, Extragenic Mutation, Intragenic Mutation, Suppressor Mutation

What is Back Mutation 

Back mutation is a type of reverse mutation, restoring the original DNA sequence. Generally, it is a second mutation, which follows the starting mutation. However, the second mutation occurs at the point where the starting mutation exists. Therefore, it can restore the original DNA sequence of the gene. As a result, the original phenotype of the organism or the wild type can be seen in the revertant. Thus, the back mutation is a type of true reverse mutation.

Figure 1: Point Mutations

For example, when a wild type sequence contains GC bases at a particular site, the mutant or the organism with the starting mutation may have AT in the same position. However, this AT can be reversed in a back mutation into GC, which in turn restores the original gene sequence as well as the phenotype in the revertant. 

What is Suppressor Mutation 

Suppressor mutation is the second type of reverse mutation. However, it is not a type of a true reverse mutation. Therefore, it may not restore the original DNA sequence, leading to the true wild type. Furthermore, the site of the second mutation is different from the site of the starting mutation. Therefore, the functional products of both types of suppressor mutations carry the starting or primary mutation. Meanwhile, based on the position of occurrence of the second mutation, there are two types of suppressor mutations. They are intragenic mutations and extragenic mutations. 

Intragenic Mutations 

Generally, intragenic mutations occur within the gene where the starting mutation exists. For example, in intragenic mutations, the primary mutation can occur due to a deletion of a single nucleotide of a particular gene. Significantly, this may alter the open reading frame, leading to a frameshift mutation. Then, the second mutation can add a single nucleotide to another position of the same gene, restoring the original open reading frame.

Figure 2: Frame Shift Mutation

Extragenic Mutations 

Extragenic or intergenic mutations occur within a separate gene other than the gene with the starting mutation. Basically, in extragenic mutations, the restoration of the original phenotype mainly occurs through the alteration of the translation process of the gene with the primary mutation. Thus, the products of the suppressor genes where the second mutation exists are tRNAs. Therefore, they are now mutant tRNAs. Generally, some of the suppressor genes can have nonsense mutations; for example, each of the three codons, amber (UAG), ochre (UAA) and opal (UGA), can serve as chain-terminating codons.  

Similarities Between Back Mutation and Suppressor Mutation 

• Back mutation and suppressor mutation are two types of mutations which occur following a particular mutation. 
• They are known as reverse mutations, restoring point mutations. 
• They revert the effect of the first mutation, restoring the original phenotype to a greater extent. 
• Organisms that have undergone these mutations are known as revertants. 

Difference Between Back Mutation and Suppressor Mutation 

Definition 

Back mutation refers to a point mutation which restores the original sequence while suppressor mutation refers to a second mutation which either alleviates or reverts the phenotypic effects of an already existing mutation. 

Type of Reverse Mutations 

Back mutations are a type of true reverse mutations, while suppressor mutations are not true reverse mutations. 

Effect on the Mutant Phenotype 

While back mutation restores the true wild type,  suppressor mutation just masks the effect of the starting mutation. 

Effect on the Mutant Gene 

Moreover, back mutation restores the original DNA sequence while suppressor mutation occurs a point other than the starting mutation. 

Conclusion 

Back mutation is a type of reverse mutation, restoring the true wild type. Generally, this is by reverting the starting mutation into the original DNA sequence. In comparison, suppressor mutation is the other type of reverse mutation, either alleviating or restoring the original phenotype. Basically, there are two types of suppressor mutations: intragenic and extragenic mutations. However, the second mutation occurs in a place other than the starting mutation, and therefore, suppressor mutations do not restore the original DNA sequence. Hence, the main difference between back mutation and suppressor mutation is the restoration of the original DNA sequence.  

References:

1. Shivangi, K. “Biology Notes on Reverse Mutations: Genetics.” Biology Discussion, 12 Dec. 2016, Available Here.

Image Courtesy:

1. “Different Types of Mutations” By Jonsta247 (CC BY-SA 4.0) via Commons Wikimedia  
2. “Frameshift deletion (13062713935)” By Genomics Education Programme – Frameshift deletion (CC BY 2.0) via Commons Wikimedia