Write a brief note about cis-trans complementation test

The cis-trans complementation test is a classical genetic technique used to determine whether two mutations that cause a similar mutant phenotype are in the same gene (cistron) or in different genes. This test helps in understanding gene function and structure by showing if two mutations complement each other when combined.

The idea of this test comes from classical genetics. It was first introduced by Edward B. Lewis in the mid 1940s during his pioneering work on the genetics of the fruit fly (Drosophila melanogaster). Lewis's work laid the foundation for the concept of complementation, which is essential for grouping mutations based on their function.

Later, in the mid to late 1950s, Seymour Benzer expanded and applied this test to bacteriophage genetics, specifically studying the rII region of bacteriophage T4. Benzer used the cis-trans complementation test to analyze the fine structure of genes at the molecular level. His work helped define the cistron as the smallest functional genetic unit. This made the test a key tool in molecular genetics and gene mapping.

This test is useful only for recessive mutations and it helps scientists to group mutations into different genes or the same gene based on whether the wild-type function is restored or not.

In this test, two organisms or DNA segments having different mutations are brought together in one cell or one system. These mutations are then arranged in two ways, cis and trans, and the resulting phenotype is studied.

1. Cis Configuration

In the cis configuration, both mutations are present on the same DNA molecule and the other molecule has both wild-type alleles.

For example:

[mut1 mut2] / [wild1 wild2]

In this condition, the normal phenotype is always seen because both wild-type alleles are on one side. So, this setup does not give us information about whether the mutations are in the same gene or not. It acts as a control.

2. Trans Configuration

In the trans configuration, one mutation and one wild-type allele are present on each DNA molecule.

For example:

[mut1 wild2] / [wild1 mut2]

Now, we observe the phenotype:
  • If the phenotype is normal, it means the mutations are in different genes and they complement each other.
  • If the phenotype is still mutant, it means the mutations are in the same gene and they fail to complement.
This is the key part and the main result of the complementation test. The result from the trans arrangement tells us whether the mutations are in same gene or different genes.

Importance of the Cis-Trans Complementation Test

The cis-trans complementation test is useful for understanding gene function. It helps to:
  • Identify if two mutations are in the same gene or different genes
  • Group mutations into complementation groups
  • Estimate the number of functional genes involved in a process
  • Define gene function based on complementation results
  • Support the one gene–one enzyme hypothesis
  • Help in gene mapping and study of gene structure before DNA sequencing was available





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