- Identify general mechanisms that regulate gene expression.
- Describe how gene regulation occurs in prokaryotes.
- Give an overview of gene regulation in eukaryotes.
- gene expression
- homeobox gene
- regulatory element
- regulatory protein
- TATA box
Each of your cells has at least 20,000 genes. In fact, all of your cells have the same genes. Do all of your cells make the same proteins? Obviously not. If they did, then all your cells would be alike. Instead, you have cells with different structures and functions. This is because different cells make different proteins. They do this by using, or expressing, different genes. Using a gene to make a protein is called gene expression.
How Gene Expression is Regulated
Gene expression is regulated to ensure that the correct proteins are made when and where they are needed. Regulation may occur at any point in the expression of a gene, from the start of transcription to the processing of a protein after translation. The focus in this lesson is the regulation of transcription.
As shown in Figure below, transcription is controlled by regulatory proteins. The proteins bind to regions of DNA, called regulatory elements, which are located near promoters. After regulatory proteins bind to regulatory elements, they can interact with RNA polymerase, the enzyme that transcribes DNA to mRNA. Regulatory proteins are typically either activators or repressors.
- Activators promote transcription by enhancing the interaction of RNA polymerase with the promoter.
- Repressors prevent transcription by impeding the progress of RNA polymerase along the DNA strand.
Other factors may also be involved in the regulation of transcription, but these are typically the key players.
Regulation of Transcription. Regulatory proteins bind to regulatory elements to control transcription. The regulatory elements are embedded within the DNA.
Prokaryotic Gene Regulation
Transcription is regulated differently in prokaryotes and eukaryotes. In general, prokaryotic regulation is simpler than eukaryotic regulation.
The Role of Operons
Regulation of transcription in prokaryotes typically involves operons. An operon is a region of DNA that consists of one or more genes that encode the proteins needed for a specific function. The operon also includes a promoter and an operator. The operator is a region of the operon where regulatory proteins bind. It is located near the promoter and helps regulate transcription of the operon genes.
The Lac Operon
A well-known example of operon regulation involves the lac operon in E. coli bacteria (see Figure below and the video at the link below). The lac operon consists of a promoter, an operator, and three genes that encode the enzymes needed to digest lactose, the sugar found in milk. The lac operon is regulated by lactose in the environment. http://www.youtube.com/watch?v=oBwtxdI1zvk
- When lactose is absent, a repressor protein binds to the operator. The protein blocks the binding of RNA polymerase to the promoter. As a result, the lac genes are not expressed.
- When lactose is present, the repressor protein does not bind to the operator. This allows RNA polymerase to bind to the promoter and begin transcription. As a result, the lac genes are expressed, and lactose is digested.
Why might it be beneficial to express genes only when they are needed? (Hint: synthesizing proteins requires energy and materials.)
The three genes of the lac operon are lacZ, lacY, and lacA. They encode proteins needed to digest lactose. The genes are expressed only in the presence of lactose.
Eukaryotic Gene Regulation
In eukaryotic cells, the start of transcription is one of the most complicated parts of gene regulation. There may be many regulatory proteins and regulatory elements involved. Regulation may also involve enhancers. Enhancers are distant regions of DNA that can loop back to interact with a gene’s promoter.
The TATA Box
Different types of cells have unique patterns of regulatory elements that result in only the necessary genes being transcribed. That’s why a skin cell and nerve cell, for example, are so different from each other. However, some patterns of regulatory elements are common to all genes, regardless of the cells in which they occur. An example is the TATA box. This is a regulatory element that is part of the promoter of most eukaryotic genes. A number of regulatory proteins bind to the TATA box, forming a multi-protein complex. It is only when all of the appropriate proteins are bound to the TATA box that RNA polymerase recognizes the complex and binds to the promoter. Once RNA polymerase binds, transcription begins. To see a video showing the role of the TATA box in the initiation of transcription, go to this link: http://www.youtube.com/watch?v=6tqPsI-9aQA.
Regulation During Development
The regulation of gene expression is extremely important during the development of an organism. Regulatory proteins must turn on certain genes in particular cells at just the right time so the organism develops normal organs and organ systems. Homeobox genes are an example of genes that regulate development. They code for regulatory proteins that switch on whole series of major developmental genes. In insects, homeobox genes called hox genes ensure that body parts such as limbs develop in the correct place. Figure below shows how a mutation in a hox gene can affect an insect’s development. You can learn more about homeobox genes at this link: http://www.youtube.com/watch?v=LFG-aLidT8s.
Effect of Hox Gene Mutation. Scientists caused a mutation in a hox gene of this fruit fly. As a result of the mutation, a leg grew out of its head where an antenna should have developed (fly on right). Normal fly (fly on right).
Gene Expression and Cancer
The mutations that cause cancer generally occur in two types of regulatory genes: tumor-suppressor genes and proto-oncogenes (see Figure below). These genes produce regulatory proteins that control the cell cycle. When the genes mutate, cells with mutations divide rapidly and without limits.
How Cancer Develops. This flow chart shows how a series of mutations in tumor-suppressor genes and proto-oncogenes leads to cancer.
- Gene transcription is controlled by regulatory proteins that bind to regulatory elements on DNA. The proteins usually either activate or repress transcription.
- Regulation of transcription in prokaryotes typically involves an operon, such as the lac operon in E. coli. The lac operon is regulated by proteins that behave differently depending on whether lactose is present.
- Regulation of transcription in eukaryotes is generally more complex. It involves unique regulatory elements in different cells as well as common regulatory elements such as the TATA box. Regulation is especially important during development. It may involve regulatory genes such as homeobox genes that switch other regulatory genes on or off. Mutations in regulatory genes that normally control the cell cycle cause cancer.
Lesson Review Questions
1. What is gene expression?
2. Describe how regulatory proteins regulate gene expression.
3. Identify the TATA box and its function in transcription.
4. What is a homeobox gene?
5. Draw a diagram to show how the lac operon is regulated.
6. Sketch how an insect with a mutated hox gene might look. Explain your sketch.
7. Why is gene regulation especially important during development?
Points to Consider
Scientists know more about human chromosomes and genes than they know about the genetic material of most other species. In fact, scientists have identified all of the approximately 20,000-25,000 genes in human DNA.
- What do you know about human chromosomes and genes? For example, do you know how many chromosomes humans normally have?
- Do you know how human characteristics are inherited? Can you identify characteristics that are controlled by a single gene?