USMLE® Step 1 style questions USMLE
A study is conducted on the effects of media on the production of B-galactosidase in E. Coli. When the bacteria are inoculated in medium A, the production of B-galactosidase is minimal. However, when the bacteria are inoculated in medium Y, many B-galactosidase is produced. Which of the following is likely present in medium Y?
Content Reviewers:Rishi Desai, MD, MPH
With the lac operon, lac refers to lactose, which is a sugar found in milk, and an operon is a portion of DNA where genes with related functions are grouped together and are controlled by the same promoter.
Although glucose is the preferred carbon source for most bacteria, the lac operon allows these bacteria to use lactose when glucose isn’t available.
Gene regulation of the lac operon is well studied, and that’s why it has become a classic example of gene regulation in bacteria.
Now before getting into the details of the lac operon and how it functions, let’s review gene expression.
DNA is made up of genes, and each gene is basically a specific part of the DNA that codes for a protein.
In transcription a segment of DNA is copied into RNA, specifically messenger or mRNA, by the enzyme RNA polymerase.
RNA polymerase unwinds the DNA double helix to produce the complementary mRNA, which is like a blueprint on what protein to build.
LacZ, produces the enzyme beta galactosidase, also called lactase, which break down lactose into glucose and galactose.
LacY produces beta-galactosidase permease, which allows lactose to enter, or permeate into the cell, and lacA encodes beta-galactoside transacetylase, and its function isn’t clearly understood.
Now, in addition to those structural genes, there’s the promoter and operator, which tell the operon when to start and stop transcription.
The promoter is a nucleotide sequence on the DNA where RNA polymerase binds and begins transcribing mRNA.
The operator, on the other hand, is located in between the promoter and the structural genes and it works together with the lac repressor protein which is encoded by the lacI gene - which is found upstream of the promoter.
When there’s glucose available there’s no need to metabolize lactose, so the lac repressor protein binds to the operator region, and it acts like a bouncer, physically blocking RNA polymerase from attaching to and transcribing the lacZYA genes.
During the good times, there’s plenty of glucose available for E. coli to use for energy, so it doesn’t need to produce lacZYA proteins.
This happens even when there’s glucose and lactose around, because glucose metabolism is more efficient.
The lac operon is a genetic regulatory sequence found in bacteria that codes for the production of enzymes necessary for lactose metabolism. The operon is controlled by a repressor protein, LacI, which binds to an operator site on the DNA upstream of the genes and prevents their expression. When lactose is present, it binds to LacI and displaces it from the operator site, allowing transcription of the genes to proceed.