BIOLOGY SEMESTER ONE
UNIT 18
Checklist unit 18: Regulation of Gene Expression
In this module you will investigate how the expression of genes is regulated in both the prokaryotic and eukaryotic cell. Gene regulation is crucial for organism development, responses to environmental changes, and the allocation of metabolic resources. Organisms must be able to turn protein synthesis on and off; this is accomplished through the regulation of gene expression.
As we learned in last week’s module, prokaryotes lack a nucleus, and mRNA transcription of DNA is immediately translated into a polypeptide (protein). As there is no additional processing of the mRNA in prokaryotic organisms, transcription is the only process during which gene regulation can occur. In prokaryotes, gene regulation is accomplished through the use of positive and negative means. Negative gene regulation reduces the amount of gene expression through the use of repressible and inducible operons. An operon is a portion of DNA that codes for an entire molecular pathway, including the operator, the promoter, and the genes they control (in prokaryotes, all genes related to specific pathway are found in one continuous strip of DNA). A mechanism for positive gene regulation (an increase in gene expression) is the activator protein, which relies on the aid of a regulatory protein to bind to DNA in order to stimulate transcription.
In multicellular eukaryotes, gene expression is particularly important in the development of an embryo and its associated specialized cells and tissues. Recall that each cell houses the entire genome of an individual and, with it, the genetic code for every protein in the body. The regulation of gene expression is responsible for the turning on and off of selected genes in order to create the specialized cells that comprise tissues and organs. It also determines body organization in developing embryos. In completely developed organisms, the regulation of gene expression can allow an individual to maintain homeostasis and react to changes in the environment. In addition, it is becoming apparent that being able to control the turning on and off of genes may lead to breakthroughs in cancer research.
Eukaryotic protein synthesis involves multiple steps and provides multiple points during which it can be regulated: Gene expression can be hindered before transcription begins (by keeping DNA tightly packed in heterochromatin); it can be facilitated or inhibited during initial transcription (by modifying enzymes); mRNA can be spliced into alternative molecules (after transcription) that can be active or inactive; mRNA can be degraded by enzymes before it is translated; translation initiation can be blocked by regulatory proteins; and, finally, once a protein has been synthesized, it can be degraded or altered to render it inactive. This may seem like a lot of regulation, but it allows the organism to have fine control over the molecules it makes. The combination of these controls allows a rapid response to environmental change.
The regulation of gene expression is not infallible: Genes normally regulate cell growth and development. When mutations occur in somatic cells that alter any of these regulatory genes, it can lead to cancer. Normal cells have safeguards in place, in the form of tumour-suppressor genes that will stop cell division if a mutation has occurred. However, if a mutation occurs in the safeguard gene itself, it can cause the cell to increase its cell division. Generally, more than one mutation is needed to cause cancer. Over time, cancer can result from the accumulation of mutations, which may explain why, as we age, we are increasingly likely to develop cancer.
Learning objectives
At the end of this module you should be able to do the following:
1. Briefly describe strategies that cells use to control metabolism.
2. Explain the adaptive advantage of bacterial genes grouped into an operon.
3. Explain how repressible and inducible operons differ and how those differences reflect differences in the pathways they control.
4. Distinguish between positive and negative controls, and give examples of each.
5. Discuss differential gene expression.
6. Explain how DNA methylation and histone acetylation affects chromatin structure and the regulation of transcription.
7. Discuss epigenetic inheritance.
8. Define control elements and explain how they influence transcription.
9. Explain the role of promoters, enhancers, activators, and repressors in transcriptional control.
10. Describe the controls for gene expression from pre-transcription to post-translation.
11. Distinguish between determination and differentiation. Explain why determination precedes differentiation.
12. Explain how maternal effect genes affect polarity and development, using an example.
13. Explain how mutations in tumour-suppressor and other genes can contribute to cancer.
Check List
ü Read Chapter 18: Regulation of Gene Expression, of Campbell and Reece’s Biology, 9th Ed.
ü As you are reading, address each of the learning objectives listed above.
ü Make flash cards for the terminology list provided. This will be beneficial for studying for the midterm and final exams later in the semester.
ü You may be able to review the PowerPoint Lecture and other resources for this unit. Refer to your instructor’s notes for more details.
ü Discussion Post: Respond to the following Scientific Inquiry (question 13, pg. 380) at the end of Chapter 18 in your text. Outline a scientific experiment to test the theory in 250 words or less. Be sure to include a hypothesis and null hypothesis, and do not forget to identify your test and control groups.
o Prostate cells usually require testosterone and other androgens to survive; however, some prostate cancer cells thrive despite treatments that eliminate androgens. One hypothesis is that estrogen, often considered a female hormone, may be activating genes normally controlled by an androgen in this type of cancer cell. Outline an experiment to test this hypothesis.
ü For extra practice, try the Self Quiz or Practice Test on the Mastering Biology Website. To log onto the website, use the access code provided in your textbook. You will also find other resources, such as downloadable MP3 tutorials for each chapter, a glossary, and an electronic copy of your text—you can catch up on your reading anywhere!
Key Terms
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BIOLOGY SEMESTER ONE
UNIT 18
activator
bicoid
cell differentiation
cyclic AMP (cAMP)
differential gene expression
epigenetic inheritance
feedback inhibition
homeotic gene
inducer
induction
micro-RNA (miRNA)
morphogenesis
oncogene
operator
operon
proteasome
regulatory gene
repressor
RNA interference (RNAi)
small interfacing RNA (siRNA)
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BIOLOGY SEMESTER ONE
UNIT 18
Root Words to Know[1]
morph- = form; -gen = produce (morphogen: a substance that provides positional information in the form of a concentration gradient along an embryonic axis)
proto- = first, original; onco- = tumour (proto-oncogene: a normal cellular gene corresponding to an oncogene)
sources
Campbell, N. A. (2008). Biology, Eighth Edition. San Francisco: Pearson, Benjamin Cummings.
Pearson Education. (2010). Retrieved 2010, from Mastering Biology : http://session.masteringbiology.com
University of Leicester. (2009, Feb 11). Gene Regulation and Expression. Retrieved May 2010, from Virtual Genetics Education Centre: http://www.le.ac.uk/genetics/genie/vgec/he/expression.html
NANSLO Biology Core Units and Laboratory Experiments
by the North American Network of Science Labs Online,
a collaboration between WICHE, CCCS, and BCcampus
is licensed under a Creative Commons Attribution 3.0UnportedLicense;
based on a work at rwsl.nic.bc.ca.
Funded by a grant from EDUCAUSE through the Next Generation Learning Challenges.
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[1] (Pearson Education, 2010)