Introduction
In classical genetics, the gene was viewed as a single, indivisible unit of function. However, with the advancement of molecular biology, researchers discovered that even within a gene, different mutations can affect different functions. This led to the discovery of concepts like intragenic complementation and the cistron.
Intragenic Complementation
Definition
Intragenic complementation refers to the phenomenon where two different mutant alleles of the same gene, when present together in a heterozygous condition, can restore partial or full function of the gene product.
How It Works
- Occurs when the gene encodes a multimeric protein (a protein made of multiple subunits).
- If each mutant affects a different subunit or domain, combining them in one organism may result in some functional protein.
Example
Consider a gene coding for a dimeric enzyme. Mutation A affects domain 1, and Mutation B affects domain 2. Alone, each produces a non-functional subunit. However, in a heterozygote (A/B), each mutation provides a functional copy of one domain. Together, they form a partially functional enzyme.
Significance
- Reveals that genes can have internal functional organization.
- Helps map functional domains within a protein.
Concept of Cistron
Definition
A cistron is a section of DNA that codes for a single polypeptide chain. In simple terms, it is a gene in functional terms. The concept was introduced by Seymour Benzer.
Key Features
- The smallest unit of genetic function.
- Different mutations within the same cistron do not complement each other—unless intragenic complementation occurs.
Difference Between Gene and Cistron
- Traditionally, a gene was a unit of heredity.
- The cistron defines the functional aspect of a gene in terms of protein coding.
Relationship Between Intragenic Complementation and Cistron
- Intragenic complementation occurs within the same cistron.
- It indicates that a single cistron may encode different functional domains that can interact within a multimeric structure.
Conclusion
Both intragenic complementation and the concept of cistron deepen our understanding of gene structure and function. While cistron defines the functional unit of DNA, intragenic complementation reveals the modular nature of proteins and the complexity within a single gene.