9. In E. coli the complementation test is done by
(1) Transformation
(2) Obtaining merozygotes
(3) Obtaining heterokaryons
(4) Making them diploid
Concept
E. coli is normally haploid, so it has only one chromosomal copy of each gene. To perform a complementation test, two different mutant alleles must coexist in the same cell so that functional gene products can potentially complement each other. This is achieved by making the cell partially diploid (merozygous) for the region of interest (e.g., mutant allele on chromosome, wild‑type or another mutant allele on an F′ plasmid). If the phenotype becomes wild type, the alleles complement; otherwise, they are in the same functional unit (cistron).
Option-by-option explanation
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Transformation
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Transformation introduces foreign DNA into bacteria, but by itself it does not guarantee a stable partial diploid for the locus being tested.
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It is a DNA uptake mechanism, not specifically the standard way the classic complementation test is set up in E. coli genetics.
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Obtaining merozygotes – correct
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A merozygote is a bacterial cell that is partially diploid for certain genes (e.g., chromosome + F′ factor carrying those genes).
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Having two copies of the region allows different alleles to be present together so their ability to restore function (complementation) can be tested. This is exactly how classical E. coli complementation analysis is done.
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Obtaining heterokaryons
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Heterokaryons are cells with different nuclei in a common cytoplasm, a concept from fungi and some eukaryotes.
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Bacteria like E. coli do not form heterokaryons; this term does not apply to standard bacterial complementation tests.
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Making them diploid
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E. coli as a whole is not made fully diploid for its entire genome.
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Instead, only a specific chromosomal segment is duplicated (via F′ or similar), so the accurate term is merozygote (partial diploid), not complete diploidization.
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Hence, in E. coli, the complementation test is performed by obtaining merozygotes (option 2).
