E. coli and the lactose operon

Well, we’re back in business! I’ve decided to focus a bit on microorganisms, since school has started again and I need to bone up on my micro before I have to lecture about it. In light of that, here’s today’s question!

In E. coli, induction of the lactose operon occurs when allolactose binds to:

A)Galactosidase
B)lac mRNA
c)the operator
D)the promoter
E)the repressor

This question tests your knowledge of how gene expression works, but tries to confuse you by giving specifics about E. coli. First things first–how does gene expression work?

Genes are some of those super complex things in biology–and one of those things that has a million names attached. Fun for us! Anyhow, sections of DNA code for proteins. They do this by creating messenger RNA (or mRNA) that tells the cell what to make. However, the DNA doesn’t just randomly make messenger RNA and code for proteins…specific conditions must be met for this to happen. Of course this causes things to be much more complex.

DNA has specific sequences that tell the cell when to make mRNA, when to stop making mRNA, and when to prevent mRNA from being made. All these sequences together with the actual genes are called the “operon.” Often time, in addition to the operon is a regulator gene called the repressor or co-repressor that allows the operon to be turned on.

If a repressor is present, it usually stops the gene from being expressed (keeps the proteins from being made); hence the name “repressor.” Certain criteria must be met for the repressor to allow gene expression.

So, lets look at E. coli specifically: this bacterium loves lactose (anyone lactose intolerant? You know that horrible feeling you get after drinking milk? That’s because the E. coli in your gut loves the undigested lactose and poops out acid and gas. Thanks microbes!) and therefore has a gene dedicated to breaking down this sugar. This gene is one of those with a repressor, however. It wouldn’t make much sense for E. coli to try and break apart lactose if there wasn’t in the environment, now wouldn’t it? When lactose is present, a metabolite of lactose called allolactose is present. Allolactose binds to the repressor, which then allows the gene to be expressed. If there is no allolactose in the environment (and therefore no lactose) the repressor stays in effect and keeps the gene dormant.

Well, back to our question:

In E. coli, induction of the lactose operon occurs when allolactose binds to:

A)Galactosidase
B)lac mRNA
c)the operator
D)the promoter
E)the repressor

Since we know that induction means “get started” and an operon is that group of DNA that includes the operator, promoter and genes, we can now answer the question. The answer is “E” the repressor. Yay!

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