Q.98 In which one of the following organisms, it is easiest to distinguish mutations on adjacent base pairs of DNA
through genetic recombination experiments?
(A) Bacteriophages
(B) Yeast
(C) Escherichia coli
(D) Bacillus subtilis
Bacteriophages are the organisms where it’s easiest to distinguish mutations on adjacent base pairs of DNA through genetic recombination experiments. This is due to their small genome size, high recombination frequencies, and ability to form visible plaques for precise mapping.
Why Bacteriophages?
Bacteriophages, like T4 phage, have compact genomes (around 150-170 kb) allowing fine-scale mapping of mutations just 1-2 base pairs apart via cis-trans tests and three-factor crosses. Recombination rates are exceptionally high (~1-10% per kb), enabling clear separation of adjacent markers into recombinant progeny distinguishable by plaque morphology or host range. Classic experiments by Benzer using T4 rII locus resolved mutations to the base-pair level, far surpassing eukaryotes or other bacteria.
Option Analysis
(A) Bacteriophages
Easiest due to haploid genetics, no diploid masking, short generation times, and massive progeny yields (10^10 per ml). Adjacent mutations recombine into wild-type or double-mutant plaques at predictable frequencies, directly measuring map distance.
(B) Yeast
Yeast (Saccharomyces cerevisiae) supports recombination but diploidy complicates mutant detection; tetrad analysis resolves markers, but genome size (~12 Mb) and lower recombination density (~0.3% per kb) limit resolution to ~100 bp, not single bases.
(C) Escherichia coli
E. coli uses Hfr conjugation or P1 transduction for mapping, but recombination hotspots and mismatch repair (MutS/MutL) reduce fine-scale resolution for adjacent bases; transductant selection rarely distinguishes 1-bp separations reliably (~10^4 kb).
(D) Bacillus subtilis
Transformation or conjugation in B. subtilis allows mapping, but larger genome (~4 Mb), lower transformation efficiency for short fragments, and competence variability make distinguishing adjacent base-pair mutations harder than in phages.
| Organism | Genome Size | Recombination Rate | Resolution for Adjacent Mutations |
|---|---|---|---|
| Bacteriophages | ~0.17 Mb | Very high (1-10%/kb) | Base-pair level |
| Yeast | ~12 Mb | Moderate (0.3%/kb) | ~100 bp |
| E. coli | ~4.6 Mb | Low-moderate | >10 bp |
| B. subtilis | ~4 Mb | Low (transformation-limited) | >50 bp |
In genetics and molecular biology, distinguishing mutations on adjacent base pairs of DNA through genetic recombination experiments is crucial for high-resolution mapping. Bacteriophages stand out as the easiest organism for this, thanks to their tiny genomes and superior recombination tools.
Bacteriophages: Gold Standard for Fine Mapping
Bacteriophages like T4 enable base-pair resolution using rII locus crosses, where recombination frequencies directly quantify distances as small as 1 bp. High phage burst sizes amplify rare recombinants for easy phenotyping.
Comparing Other Organisms
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Yeast: Tetrads help, but larger scale limits precision.
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E. coli: Conjugation suits broader maps, not adjacent bases.
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B. subtilis: Transformation works, but efficiency drops for tiny intervals.
This makes bacteriophages ideal for biotech researchers studying microbial genetics.
