Lac operon

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Lac operon


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USMLE® Step 1 questions

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High Yield Notes

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Lac operon

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USMLE® Step 1 style questions USMLE

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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?  

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Escherichia coli p. , 143

Lac operon p. 36

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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.

In the case of the lac operon, the proteins the lac operon produces are all required for transporting lactose into the cell and metabolising it in Escherichia coli as well as other bacteria.

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.

Genes become proteins in two steps: transcription and translation.

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.

Then there’s translation which is where organelles called ribosomes assemble and utilize the mRNA produced during transcription to create proteins from amino acids lying around in the cytoplasm.

Now, the lac operon is a part of E coli’s DNA and it includes structural genes, like lacZ, lacY, and lacA, as well as regulatory genes like the promoter and operator.

The structural genes lacZYA code for the proteins that ultimately allow E coli to transport and metabolize lactose.

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 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.


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