During the analysis of seven rII mutations in phage T4, mutants 1, 2, and 6 were in cistron A, while mutants 3, 4, and 5 were in cistron B. Of these, mutant 4 was a deletion overlapping mutant 5. The remainder were point mutations. Nothing was known about mutant 7. Predict the results of complementation (+ or -) between 1 and 2; 1 and 3; 2 and 4; and 4 and 5.

Key Concepts

Cistron and Mutations

A cistron is a segment of DNA that encodes a single polypeptide chain, essentially functioning as a gene. In the context of the question, mutations can be classified as point mutations, which are small changes in a single nucleotide, or deletions, which remove segments of DNA. Understanding the nature of these mutations is crucial for predicting how they will interact during complementation tests.

Complementation Test

A complementation test is a genetic tool used to determine whether two mutations that produce a similar phenotype are in the same gene (cistron) or in different genes. If two mutations complement each other, they are in different cistrons, resulting in a positive complementation (+). Conversely, if they do not complement, they are in the same cistron, leading to a negative result (-).

Overlapping Mutations

Overlapping mutations occur when one mutation affects a region of DNA that is also impacted by another mutation. In this case, mutant 4 is a deletion that overlaps with mutant 5, which means they cannot complement each other. This relationship is essential for predicting the outcomes of the complementation tests, as overlapping mutations will yield a negative result.