Operon: Definition & Sequencing

Operon: Definition & Sequencing
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  • 0:03 The Operon: The Master Switch
  • 0:24 Operon Sequence
  • 1:36 Examples of Operons
  • 3:31 Lesson Summary
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Lesson Transcript
Instructor: Amanda Robb
This lesson explains the basics of bacterial genes called operons. We'll learn what operons are and how they help bacteria be more efficient in making things they need.

The Operon: The Master Switch

In bacteria, groups of genes are clustered together in a unit called an operon. An operon makes it easy for the bacteria to turn on all the genes needed for a task all at once. Think of it as being able to flip a switch and turn on all the appliances you need for dinner at once, instead of going over to them and turning them on one by one.

Operon Sequence

Operons are controlled by an 'on' switch, called the promoter. The promoter is a place where a special protein called RNA polymerase binds. When a protein binds to something else, it attaches, like a lock and key, to make another process happen. When RNA polymerase binds DNA, it makes a copy of the DNA in the form of RNA. This step is called transcription, which is the first step to making a protein. (Remember that proteins are the building blocks of cells and give cells all their structures and functions.)

The promoter, or 'on' switch, is followed by an 'off' switch called an operator. The operator DNA is the place where a repressor protein can bind to stop, or repress, transcription of the genes. After the promoter and the operator, the next part of the operon is the structural gene. Structural genes have the information necessary to make the actual proteins needed for a job. Here is an image of the different structures of an operon:


When a bacteria wants to turn on an operon, the repressor protein must be released and RNA polymerase must bind to the promoter ('on' switch).

Examples of Operons

A classic example of an operon in action is the lac operon. The lac operon regulates how bacteria metabolize, or break down, lactose. When lactose isn't around, the bacteria don't waste energy making proteins to break down lactose. Pretty smart, huh? When lactose is around, the bacteria waste no time in creating the proteins needed to break down lactose to get energy.

Here's how it works: A protein called a repressor is bound to the operator ('off' switch). When lactose is present, it binds to the repressor protein. The repressor protein changes shape and falls off the operator. Then, RNA polymerase is free to bind the promoter ('on' switch) and make the three proteins needed to break down lactose.

lac operon

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