1. Gene Regulation and Expression
Read the lesson title aloud to students.

2. Learning Objectives Describe gene regulation in prokaryotes.
Explain how most eukaryotic genes are regulated.
Relate gene regulation to development in multicellular organisms.
Click to show each of the learning objectives.
Ask: Do you recall what happens to mRNA after it is transcribed, but still in the nucleus?
Answer: The mRNA is edited; introns are cut out and exons are spliced together.
Ask: How do you define gene expression?
Answer: Gene expression is the way that DNA, RNA, and proteins put genetic information into action in living cells.
Ask: Based on what you know, how do you think mRNA editing affects gene expression?
Answer: Sections of mRNA are cut out and not translated into proteins. Therefore, only the parts of the gene left in are expressed.
Tell students that in this lesson, they will learn about other ways gene expression is controlled.

3. Prokaryotic Gene Regulation
DNA-binding proteins in prokaryotes regulate genes by controlling transcription.
One of the keys to gene transcription in bacteria is the organization of genes into operons.
Ask: How are prokaryotic genes regulated?
Answer: DNA-binding proteins in prokaryotes regulate genes by controlling transcription.
Click to reveal this answer to students.
Explain to students that, to conserve energy and resources, prokaryotes regulate their activities, using only those genes necessary for the cell to function.
By regulating gene expression, bacteria can respond to changes in their environment; for example, the presence or absence of nutrients.
Ask: How does an organism know when to turn a gene on or off?
Answer: One of the keys to gene transcription in bacteria is the organization of genes into operons.
Click to reveal this answer.
Tell students: An operon is a group of genes that are regulated together.
The genes in an operon usually have related functions.
Tell students: The E. coli shown has operons within its DNA.
The 4288 genes that code for proteins in E. coli include a cluster of three genes that are regulated together for the metabolism of lactose. Lactose is a sugar. To digest lactose, E. coli needs certain enzymes.
The three genes that code for such enzymes in E. coli are called the lac operon.

4. The Lac Operon
When lactose is not present, the lac genes are turned off by regulatory proteins that bind to DNA and block transcription.
Ask: Why must E. coli be able to switch the lac genes on and off?
Answer: Lactose is a compound made of two simple sugars, galactose and glucose. To use lactose for food, the bacterium must transport lactose across its cell membrane and then break the bond between glucose and galactose.
Explain to students that these tasks are performed by proteins coded for by the genes of the lac operon.
This means that if the bacterium grows in a medium where lactose is the only food source, it must transcribe these genes and produce these proteins.
If grown on another food source, such as glucose, it would have no need for these proteins.
Point out that the bacterium is able to detect the presence of lactose and therefore when the products of these genes are needed. When lactose is not present, the lac genes are turned off by regulatory proteins that bind to DNA and block transcription.

5. Promoters and Operators
Located in front of the operon’s three genes are two regulatory regions:
A promoter
An operator
Tell students: On one side of the operon’s three genes are two regulatory regions: a promoter and an operator.
Tell students: The promoter is a site where RNA polymerase can bind to begin transcription.
Click to highlight the promoter.
Tell students: The operator site is where a DNA-binding protein known as the lac repressor can bind to DNA.
Click to highlight the operator site.
Tell students: When the repressor is bound to the operator, RNA polymerase cannot move along the DNA and reach the genes.
In other words, the repressor is like a roadblock.
Tell students: The gene that codes for the repressor protein is located before the promoter region.
Click to highlight the repressor.
Ask: What is the function of the repressor?
Answer: It can bind to the operator, blocking RNA polymerase and stopping transcription (and therefore gene expression).
Ask a volunteer to locate the lac genes in the figure.
Click to confirm the volunteer’s selection.

6. The Lac Repressor Blocks Transcription
When the lac repressor binds to the O region, RNA polymerase cannot reach the lac genes to begin transcription.
Make sure students know that this illustration shows the same segment of DNA in an E. coli bacterium as the previous slide.
Ask: What is being shown in the picture?
Answer: The repressor has bound to the operator and is blocking RNA polymerase from transcribing the genes.
Ask a volunteer to point out the repressor protein.
Click to verify the volunteer’s selection.
Ask: Is lactose absent or present in this situation?
Answer: absent, which is why the repressor is blocking transcription
The repressor binds to the operator, preventing RNA polymerase from binding to the promoter.
Repressor protein

7. Lactose Turns On the Operon
When lactose is added to the medium, it diffuses into the cell and attaches to the lac repressor.
RNA polymerase
Make sure students know that this illustration shows the same segment of DNA in an E. coli bacterium as the previous two slides.
Ask: If the repressor protein is always present, how can the lac genes ever be switched on?
Answer: The lac repressor protein has a binding site for lactose.
Point out that lactose is present in this illustration.
Click to highlight the lactose.
Ask: What happens when lactose is present?
Answer: Lactose binds to the repressor, and this causes a change in the repressor structure.
With lactose bound to it, the repressor can no longer bind to the operator. This allows RNA polymerase to bind to the promoter.
Step students through the process.
When lactose is added to the medium, it diffuses into the cell and attaches to the lac repressor.
This changes the shape of the repressor protein in a way that causes it to fall off the operator.
Click to highlight this step.
With the repressor no longer bound to the O site, RNA polymerase can bind to the promoter and transcribe the genes of the operon.
Point out that, in the presence of lactose, the operon is automatically switched on.
Ask: How is the way lactose turns genes on and off similar to the way cold air signals a furnace to turn on or off?
Answer: Cold air causes a furnace to turn on. When the air is no longer cold, the warmer temperature causes the furnace to turn off. Lactose works in a similar way. The presence of lactose causes lac genes to turn on. When lactose is no longer present, the absence of lactose causes lac genes to turn off.
Lactose
Repressor protein with changed shape

8. Eukaryotic Gene Regulation
A typical eukaryotic gene has a TATA box.
Explain to students that the general principles of gene regulation in prokaryotes also apply to eukaryotes, but that there are some differences.
Tell students: Eukaryotic genes are controlled individually and have more complex regulatory sequences than those of the lac repressor system.
Direct students to the illustration and point out the TATA box.
Click to highlight the TATA box.
Tell students: One of the most interesting features of a typical eukaryotic gene is the TATA box.
Explain that the TATA box is a short region of DNA, about 25 or 30 base pairs before the start of a gene, containing the sequence TATATA or TATAAA.
The TATA box binds a protein that helps position RNA polymerase by marking a point just before the beginning of a gene.
Click to illustrate this process.
Explain to students that getting transcription started, which is shown in this illustration, is just one way that eukaryotic genes can be regulated. They can also be regulated by transcription repression.

9. Transcription Factors
By binding DNA sequences in the regulatory regions of eukaryotic genes, transcription factors control the expression of those genes.
Ask: How are genes regulated in eukaryotic cells?
Answer: By binding DNA sequences in the regulatory regions of eukaryotic genes, transcription factors control the expression of those genes.
Explain that gene expression in eukaryotic cells can be regulated at a number of levels.
One of the most critical is at the level of transcription by means of DNA-binding proteins known as transcription factors.
Click to highlight transcription factors.
Tell students: Some transcription factors enhance transcription by opening up tightly packed chromatin.
Others help attract RNA polymerase.
Still others block access to certain genes, much like prokaryotic repressor proteins.

10. Cell Specialization
Complex gene regulation in eukaryotes is what makes differentiation and specialization possible.
Ask: Why is gene regulation in eukaryotes more complex than in prokaryotes?
Answer: because of the way genes are expressed in a multicellular organism
Complex gene regulation in eukaryotes is what makes differentiation and specialization possible.
Encourage students to consider the following:
Genes that code for liver enzymes are not expressed in nerve cells.
Keratin, an important protein in skin cells, is not produced in blood cells.
Cell differentiation requires genetic specialization, yet most of the cells in a multicellular organism carry the same DNA in their nucleus.

11. RNA Interference
Small RNA molecules that do not belong to any of the major groups of RNA play a powerful role in regulating gene expression.
They do so by interfering with mRNA.
Tell students that it is easier to understand new material by relating it to something they already know. Point out that RNA interference depends on something they already know about: complementary base pairing.
Call on volunteers to explain how base pairing works.
Ask: What are the processes in which base pairing occurs?
Answer: DNA replication, transcription, and translation

12. RNA Interference
The small interfering RNA molecules fold into double-stranded hairpin loops. The dicer enzyme cuts the double strands into microRNA (miRNA).
Explain to students that in the last decade, a series of important discoveries has shown that, after they are produced by transcription, the small interfering RNA molecules fold into double-stranded hairpin loops.
Click to highlight the double-stranded loops.
An enzyme called the “dicer” enzyme cuts, or dices, these double-stranded loops into microRNA (miRNA), each about 20 base pairs in length.
Click to illustrate this process.

13. RNA Interference The two strands of the loops separate.
One of the miRNA pieces attaches to a cluster of proteins, forming a silencing complex.
Tell students: The two strands of the RNA separate.
Then, one of the miRNA pieces attaches to a cluster of proteins to form a silencing complex.
Click to highlight the silencing complex.
Explain that the silencing complex binds to and destroys any mRNA containing a sequence that is complementary to the miRNA.
Click to highlight the complimentary sequence.

14. RNA Interference
Blocking gene expression by means of an miRNA silencing complex is known as RNA interference.
Explain to students that the silencing complex effectively shuts down the expression of the gene whose mRNA it destroys.
Ask: What happens to the mRNA sequence that is complementary to the bound miRNA?
Answer: That mRNA sequence is destroyed and not translated.
Click to highlight the destroyed mRNA sequence.
Point out that at first, RNA interference (RNAi) seemed to be a rare event, found only in a few plants and other species. It’s now clear that RNA interference is found throughout the living world and that it even plays a role in human growth and development.

15. The Promise of RNAi Technology
The discovery of RNAi has made it possible for researchers to switch genes on and off at will, simply by inserting double- stranded RNA into cells.
It may provide new ways to treat and perhaps even cure diseases.
Explain to students that the silencing complexes block the expression of genes producing mRNA complementary to the miRNA.
This technology is a powerful way to study gene expression in the laboratory.
Point out to students that RNAi technology also holds the promise of allowing medical scientists to turn off the expression of genes from viruses and cancer cells, and it may provide new ways to treat and perhaps even cure diseases.
Explain that Huntington’s disease is caused by a single autosomal dominant mutant gene. The gene produces a protein that causes brain abnormalities, which in turn interfere with coordination, speech, and mental abilities.
Ask: How might RNA interference technology be used to treat Huntington’s disease?
Answer: An miRNA molecule complementary to the mutant gene that causes Huntington’s disease might be injected into a person with the gene. The miRNA would prevent the expression of the gene so that its protein could not be produced. This would prevent the disease from developing.

16. Genetic Control of Development
Regulating gene expression is important in shaping how a multicellular organism develops.
Each of the specialized cell types found in the adult originates from the same fertilized egg cell.
Ask: What controls the development of cells and tissues in multicellular organisms?
Answer: As an embryo develops, different sets of genes are regulated by transcription factors and repressors that bind to DNA.
Explain that these, in turn, regulate transcription and the production of RNA molecules.
Regulation of both DNA and RNA in this way helps cells undergo differentiation, becoming specialized in structure and function.

17. Homeotic, Homeobox, and Hox Genes
Homeotic genes regulate organ development.
Homeobox genes code for transcription factors.
Hox genes determine the identities of each body segment.
Tell students: The work of American biologist Edward B. Lewis made it clear that a set of master control genes, known as homeotic genes, regulate body parts that develop in specific parts of the body.
Click to display the summary point.
Explain that the molecular studies of homeotic genes show that they all share a very similar 180-base DNA sequence, which was given the name homeobox. Homeobox genes code for transcription factors that activate other genes that are important in cell development and differentiation.
Point out that, in flies, a group of homeobox genes known as Hox genes are located side by side in a single cluster. Hox genes determine the identities of each segment of a fly’s body.
Ask: What section of the bodies of flies and mice is coded by the genes shown in blue?
Answer: the back of the body; posterior portion of the abdomen of the fruit fly and rump of the mouse

18. Environmental Influences
Environmental factors can affect gene regulation.
Metamorphosis is an example of how organisms can regulate gene expression in response to change in their environment.
Explain to students that in prokaryotes and eukaryotes, environmental factors like temperature, salinity, and nutrient availability can regulate gene expression.
Make the connection for students with the lac operon in E. coli, which is switched on only when lactose is the only food source in the bacteria’s environment.
Tell students: Metamorphosis is another example of how organisms can regulate gene expression in response to change in their environment.
Click to reveal this statement.
Ask: What happens to a tadpole under less-than-ideal conditions such as a drying pond, a high density of predators, or low amounts of food?
Answer: If the tadpole’s environment changes for the worse, its genes will direct the production of hormones to speed the transformation of the tadpole to the adult bullfrog.
Explain that the speed of metamorphosis is determined by various environmental changes that are translated into hormonal changes, with the hormones functioning at the molecular level.
Ask: What other environmental influences might impact the metamorphosis of a tadpole to a bullfrog?
Answer: temperature and population size