Do our genes work the same way all the time? How do we regulate the expression of our genes? Explore the various ways organisms control gene transcription through repression and induction of operons.
Genes and Gene Expression
Have you ever wished that you had an identical twin? Sometimes I'm jealous of people who have twins. I think it would be interesting to see how another me would turn out. I mean, identical twins don't turn out completely identical. Usually, they end up slightly different heights, with slightly different facial features and different personalities. Some identical twins are easy to tell apart even though their genes are exactly the same. But that seems odd, doesn't it? If identical twins have the same exact genes, then why are there noticeable differences at all? Are genes really the only things that define our physical features? Or, do genes get tweaked a little by the rest of our cellular functions?
Let's go back to what a gene really is. It's a section of DNA that codes for, or specifies, a particular protein. The gene is first transcribed into mRNA, then translated into a polypeptide chain. That polypeptide is a component of the proteins that make up your body, including your height, facial features, and everything else. When the codes hidden inside our genes come out to light as physical traits, we call it gene expression.
Genes are transcribed into mRNA and then into polypeptide chains
Regulation of Gene Expression
Gene expression is just what it sounds like. It's the act of genes expressing themselves. How do people express themselves? They show off their emotions, right? They put out actions and words that result from the thoughts and feelings they have inside. When genes express themselves, they put out protein molecules that result from the genetic codes they have inside. Genes express themselves by turning the DNA code into a protein by way of transcription and translation. It's basically another way of talking about the central dogma. Gene expression describes how the genetic makeup of an organism is shown as an organism's physical traits. It's the process by which information flows from genes to proteins.
During the life of a living thing, it's important to have control over how much of a gene is expressed at any given time. For example, take the gene for keratin. Keratin is the protein that makes up our skin, hair and nails. We generally need to grow these things at a continuous pace, because our skin, hair and nails get worn down over time. But, if we make too much keratin, we could end up with way too much hair, really long nails or really thick, tough skin. Life would be a pain if this happened to us! So, it's important that we regulate the expression of the keratin gene. Regulation of gene expression describes a variety of mechanisms by which our cells control the amount of protein that's produced by our genes.
Prokaryotic vs. Eukaryotic Transcription
Gene regulation happens differently depending on whether the organism is a prokaryote or a eukaryote. Do you remember the difference between these two? Let's quickly remember. Eukaryotes are organisms, like plants, animals, fungi and protists, that all have cells with nuclei and organelles inside. Most eukaryotes are multicellular. A prokaryote is a single-celled organism, like bacteria, that doesn't have a nucleus or organelles inside. Since a eukaryotic cell has a nucleus, and a prokaryotic cell doesn't, the regulation of transcription is different between the two.
In a eukaryote, the mRNA that is transcribed in the nucleus must pass through the nuclear envelope to be translated in the cytoplasm. Before it can leave, it has to be processed. Some parts are added to the strand, and some are taken out. There's more to it than that, but we'll save eukaryotic RNA processing for another time.
Unlike in eukaryotes, multiple genes get translated together in prokaryotes
In a prokaryote, there's no nuclear envelope, so the mRNA can begin translation right there in the cytoplasm. Transcription and translation overlap with each other. So, the production of proteins actually begins before the mRNA strand is complete. Unlike eukaryotes, prokaryotes have more than one gene on an mRNA strand. So, with the overlap of processes, all the genes on the mRNA end up getting translated together. Usually, an organism doesn't want to translate different proteins at the same time because different proteins are involved in different cellular activities. So, in a prokaryote, genes that are related to each other are found side-by-side on the actual DNA. Clusters of related genes are called operons. When an entire operon is translated, a whole team of proteins is produced. Every protein on the team contributes to the same cellular function.
The Lac Operon
The study of operons was the first way that we learned about the regulation of gene expression. In 1961, two French biologists studied the bacteria E. coli to learn how operons work. Remember that E. coli is an important bacteria that lives in your intestine. It helps you digest certain foods you eat, like the lactose sugar found in milk and dairy products. E. coli has three genes that code for the lactose-digesting enzymes. Without the enzymes, you wouldn't be able to digest the sugar lactose. And without the three genes, you wouldn't be able to make the enzymes. The set of three genes is an example of an operon. Scientists call this one the lac operon because it controls the production lactose-digesting enzymes.
Now, we're going to look inside your intestine for a minute. Let's say that every morning when you wake up, you always drink a full glass of milk. But for the rest of the day, you don't have any more dairy products. Do you think that the lac operon makes the lactose-digesting enzymes at a constant rate, all day long? Of course not! The lac operon makes lots of enzymes in the morning, when you first drink all that milk. But for the rest of the day, it doesn't need to make more enzymes, because you're not having any more dairy. The lac operon regulates the expression of its genes depending on how much the enzymes are needed in different situations.
Induction, Repression and Gene Regulation
The mechanics of gene regulation in the lac operon are pretty complex. But, the short story is that the operon is turned on and off based on the amount of lactose in the bacteria's environment. And by environment, I just mean the inside of your intestine; that's all the environment an E. coli bacteria has. When you drink your milk, the lactose ends up in your intestine and surrounds the E. coli bacteria. The presence of lactose turns on the lac operon, just like a light switch. This is called induction. Transcriptional induction is an increase in gene expression due to the presence of an inducer. An inducer is a molecule that begins gene expression. In the case of E. coli and the lactose-digesting enzymes, the inducer is the sugar, lactose.
The lac operon is turned on in the presence of lactose and controls lactose-digesting enzymes
If an inducer turns an operon on, then what turns an operon off? Well, there's another molecule called a repressor. It's a protein that regulates gene expression by blocking gene transcription. Once you've finished digesting your morning dose of milk, the lac operon turns off so that the bacteria don't waste energy making enzymes you don't need. This is called repression. Transcriptional repression is the blocking of gene expression in response to a repressor.
As you can see, our genes are not completely in charge of defining our physical characteristics. Instead, they're more like basic instructions for different possibilities of protein products. Other cellular functions get to help decide how our genes are expressed by controlling how much, and how often, our proteins are produced. It makes sense now that two identical twins can turn out looking so different. Because they don't always experience the same environments during their lifetimes, their genes are expressed in different ways due to the processes of gene regulation.
A segment of DNA that codes for a specific protein is called a gene. Genes are expressed when they are transcribed into mRNA and translated into protein. Gene expression is carefully regulated by all organisms so that the correct amount of each protein is made. Eukaryotic organisms regulate their gene expression differently than prokaryotes. While eukaryotic RNA is processed in the nucleus, prokaryotic RNA is arranged in clusters of related genes called operons.
By studying the lac operon found in E. coli bacteria, biologists learned about gene regulation and the processes of repression and induction. Repression is a decrease in gene expression. Induction is an increase in gene expression due to the presence of an inducer. While our genes provide all the instructions for the proteins we make, our individual traits are influenced by the regulation of gene expression.
At the end of this video, you should be able to:
- Explain the function of genes
- Describe the process of gene regulation, including induction and repression
- Differentiate between prokaryotic gene regulation and eukaryotic gene regulation