CHAPTER SUMMARY
19.1 THE REGULATION OF GENE EXPRESSION IN EUKARYOTES TAKES PLACE AT MANY LEVELS, INCLUDING DNA PACKAGING IN CHROMOSOMES, TRANSCRIPTION, AND RNA PROCESSING.
- Gene expression involves the turning on or turning off of a gene. Gene regulation determines where, when, and how much gene product is made.
- Regulation at the level of chromatin involves chemical modifications of DNA and histones that make a gene accessible or inaccessible to the transcriptional machinery.
- Dosage compensation is the process by which the expression of X-linked genes is equalized in XX individuals (who have two copies of each X chromosome and hence two copies of each X-linked gene) and XY individuals (who have only one copy of each X chromosome and X-linked gene).
- One mechanism of dosage compensation involves the inactivation of one of the two X chromosomes in females; this mechanism, known as X- inactivation, is observed in mammals.
- X-inactivation occurs by the transcription of a noncoding RNA known as Xist, which binds to the X-chromosome inactivation center (XIC) and coats the entire X chromosome, leading to DNA and histone modifications and transcriptional repression.
- Transcriptional regulation controls whether or not transcription of a gene occurs.
- Transcription can be regulated by regulatory transcription factors that bind to specific DNA sequences near genes known as enhancers.
- Further levels of regulation after a gene is transcribed to mRNA include RNA processing, splicing, and editing.
19.2 AFTER AN mRNA IS TRANSCRIBED AND EXPORTED TO THE CYTOPLASM IN EUKARYOTES, GENE EXPRESSION CAN BE REGULATED AT THE LEVEL OF mRNA STABILITY, TRANSLATION, AND POSTTRANSLATIONAL MODIFICATION OF PROTEINS.
- Small regulatory RNAs, especially microRNA (miRNA) and small interfering RNA (siRNA), affect gene expression through their effects on translation (miRNA) or RNA stability (siRNA).
- Translational regulation controls the rate, timing, and location of protein synthesis.
- Translational regulation is determined by many features of an mRNA molecule, including the 5′ and 3′ UTR, the cap, and the poly(A) tail.
- Posttranslational modification comes into play after a protein is synthesized, and includes chemical modification of side groups of amino acids, affecting the structure and activity of a protein.
- Gene regulation is influenced by both genetic and environmental factors.
19.3 TRANSCRIPTIONAL REGULATION IS ILLUSTRATED IN BACTERIA BY THE CONTROL OF THE PRODUCTION OF PROTEINS NEEDED FOR THE UTILIZATION OF LACTOSE, AND IN VIRUSES BY THE CONTROL OF THE LYTIC AND LYSOGENIC PATHWAYS.
- Transcriptional regulation can be positive, in which a gene is usually off and is turned on in response to the binding to DNA of a regulatory protein called an activator, or negative, in which a gene is usually on and is turned off in response to the binding to DNA of a regulatory protein called a repressor.
- Jacob and Monod studied the lactose operon in E. coli as a model for bacterial gene regulation.
- When lactose is added to culture of bacteria, the genes for the uptake of lactose (permease, encoded by lacY) and cleavage of lactose (β-galactosidase, encoded by lacZ) are expressed.
- The lactose operon is negatively regulated by the repressor protein (encoded by lacI), which binds to DNA sequences known as the operator.
- When lactose is added to the medium, it induces an allosteric change in the repressor protein, preventing it from binding to the operator and allowing transcription of lacY and lacZ. In this way, lactose acts as an inducer of the lactose operon.
- An additional level of regulation of the lactose operon is provided by the CRP–cAMP complex, a positive activator of transcription.
- In infection of E. coli cells by bacteriophage λ, predominance of cro protein results in the lytic pathway, whereas predominance of the cI protein results in the lysogenic pathway.
Self-Assessment Question 1
Distinguish between gene expression and gene regulation.
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Model Answer:
Gene expression is when genes are turned on, and gene regulation controls when gene expression happens. Gene regulation makes sure that the gene is turned on in the right place at the right time in the right amount.
Self-Assessment Question 2
Explain what is meant by “different levels” of gene regulation and give some examples.
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Model Answer:
Gene regulation can occur at each level of gene expression. For example, a gene expression can be regulated during transcription, translation or posttranslation.
Self-Assessment Question 3
Give a few examples of how DNA bases or chromatin can be modified to regulate gene expression.
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Model Answer:
Bases can be modified by attaching a methyl group to cytosine. When several cytosines are methylated in close proximity it creates a methylated CpG island. When a methylated CpG island is near a promoter region, that gene is usually repressed or turned off. Chromatin, which is comprised of DNA, RNA, and proteins, is wrapped around an octamer of histone proteins. When chromatin is coiled tightly around the histones, the DNA is not accessible to transcription proteins. However, when the chromatin unravels from the histone through chromatin remodeling, a process in which the nucleosomes are repositioned to expose different stretches of DNA, the transcription proteins can access the DNA and begin transcribing it.
Self-Assessment Question 4
Explain how X-inactivation in female mammals results in patchy coat color in calico cats.
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Model Answer:
The level of expression of X-linked genes is the same for both sexes. This means that one of the X chromosomes needs to be inactivated in female cells (XX). This phenomenon is called X-inactivation and is a means of dosage compensation. Thus females are actually a mosaic of tissues where sometimes the maternal X is expressed and other times the paternal chromosome is expressed. In calico cats, which are always female, the orange and black fur color is due to different alleles of a single gene on the X- chromosome. Based on which X chromosome is active, the cat will have a different color fur in that area.
Self-Assessment Question 5
Explain how one protein-coding gene can code for more than one polypeptide chain.
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Model Answer:
One protein-coding gene can code for more than one polypeptide chain through the mechanism of alternative splicing. By leaving in various exons or splicing them out, the spliceosome can splice the RNA in different ways to yield different proteins.
Self-Assessment Question 6
Name and describe three ways in which gene expression can be influenced after mRNA is processed and leaves the nucleus.
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Model Answer:
One way in which gene expression can be influenced after mRNA is processed and leaves the nucleus is through the actions of small regulatory RNAs or small interfering RNA that can either inhibit or degrade mRNA respectively. A second way is to alter the 5’cap of the mRNA so translation initiation cannot occur. Altering or deleting the poly A tail is a third way in which translation of the mRNA can be inhibited.
Self-Assessment Question 7
Diagram the lactose operon in E. coli with the proper order of the elements lacI, lacO, lacY, and lacZ, and explain how expression is controlled in the presence and absence of lactose.
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Model Answer:
See below diagram.
Self-Assessment Question 8
Describe the role of the CRP–cAMP complex in positive regulation of the lactose operon in E. coli.
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Model Answer:
The CRO-cAMP complex helps regulate which compounds are utilized first by the bacterium. In the absence of glucose, E.coli’s preferred energy source, cAMP levels are high and the CRP-cAMP complex binds to a site near the promoter of the lac operon and activates transcription. In the presence of glucose, cAMP levels are low and the CRP-cAMP complex does not bind, and transcription is not induced to high levels even in the presence of lactose.
Self-Assessment Question 9
Describe what is meant by lysis and lysogeny, and explain how gene regulation controls these two pathways.
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Model Answer:
Lysis is the process in which a cell bursts open to release progeny phage. Lysogeny is the process where the phage DNA is incorporated into the bacterial chromosome. In the lytic pathway, the phage protein cro is produced and inhibits access to the promoter Pm, thereby blocking expression of cI. This allows for transcription of genes in the lytic pathway to take place. In the lysogeny pathway the protein cl is produced which prevents the translation of cro and cll proteins. This action shuts down translation of all genes not associated with lysogeny.