Question

Jacob and Monod discovered a constitutive mutation of the lacI gene (lacIC). Bacteria with the chromosomal genotype lacIC lacO+ lacZ+ lacY+ express high levels of both β-galactosidase and lactose permease in either the presence or absence of lactose. Which of the following mutations could have yielded this phenotype?

The mutant lacIC protein is unable to bind cAMP.

The mutant lacIC protein is unable to bind the lacO+ DNA sequence.

The mutant lacIC protein is unable to bind allolactose.

The mutant lacIC operator cannot bind CAP

Either the mutant lacIC protein is unable to bind the lacO+ DNA sequence or the mutant lacIC operator cannot bind CAP could explain these results.

You have taken a new job in a forensic laboratory, and your task is to use PCR to test for the presence of specific sequences in very small quantities of DNA. What components do you require for this task?

A plasmid vector containing T3 and T7 primer sites.
Genome-specific primers, dNTPs, and Taq polymerase
E. coli DNA polymerase
A thermostable reverse transcriptase enzyme from hot springs bacteria

Answer 1

In the Jacob and Monod experiment question, the constant or high expression levels of B-galactosidase and lactose permease in either the presence or absence of lactose is due to the mutant lacIC protein is unable to bind the lacO+ DNA sequence. Normally in a lac operon the lacIC protein binds the lacO+ and hence inhibits the lac operon when lactose is absent and if this binding is inhibited then the transcription constantly takes place and hence the high levels of both proteins can be seen irrespective of presence or absence of lactose.

In a forensic laboratory, for testing the presence of specific sequences in very small quantities of DNA by PCR, the components that are required are Genome-specific primers, dNTPs and Taq polymerase. The other options are not required as this is only a qualitative test to detect the DNA present in a sample.

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