Gene Regulation

5/29/13

Gene Regulation & Viruses

Biol 1002 - Spring 2013

Signed in as Nora Trejos

Gene Regulation & Viruses

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Gene Regulation & Viruses Due: 3:00pm on Monday, May 6, 2013 Note: You will receive no credit for late submissions. To learn more, read your instructor's Grading Policy

Regulation of Gene Expression in Bacteria The operon model describes how bacteria control the production of groups of enzymes. In this model, synthesis of the messenger RNA coding for these enzymes is switched on or off by regulatory proteins.

Part A - Operon vocabulary Can you match terms related to operons to their definitions? Drag the terms on the left to the appropriate blanks on the right to complete the sentences.

Hint 1. Overview of operon control An operon consists of a group of related genes that are transcribed into a single messenger RNA molecule from a single promoter. The switch that controls the transcription of an operon is a segment of DNA called an operator, which is usually part of the promoter. An operon is regulated by a regulatory protein that is transcribed from a regulatory gene outside of the operon DNA. The regulatory protein can be an activator, which in its active form stimulates gene transcription, or a repressor, which switches off the operon. However, a repressor can be inactivated when a small molecule called an inducer binds to the repressor, altering its conformation.

Hint 2. Review of transcription Transcription is the synthesis of RNA using DNA as a template. In bacteria, RNA polymerase binds to a stretch of DNA called a promoter. RNA polymerase then moves along a continuous stretch of DNA, synthesizing RNA.

Hint 3. What are the parts of an operon? Drag the labels to their appropriate locations on the diagram of an operon. Labels can be used once or not at all. ANSWER:

ANSWER:

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Correct An operon is made up of a promoter and the genes of the operon. The promoter, which includes an operator, is the stretch of DNA where RNA polymerase binds. Regulatory proteins bind to the operator. The genes of the operon code for a related set of proteins. A regulatory gene located away from the operon codes for a protein that controls the operon.

Part B - Regulation of model operons The trp and lac operons are regulated in various ways. How do bacteria regulate transcription of these operons? Sort the statements into the appropriate bins depending on whether or not each operon would be transcribed under the stated conditions.

Hint 1. How are operons regulated? Drag the terms on the left to the appropriate blanks on the right to complete the sentences. ANSWER:

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Hint 2. How is the trp operon regulated? How is the trp operon regulated? Select the three statements that apply. ANSWER: The trp operon undergoes negative control. When glucose is present, it binds to catabolite activator protein (CAP), which stimulates transcription of the operon. When tryptophan is absent, the regulatory protein is active. When tryptophan is absent, the regulatory protein is inactive. When glucose is absent, cAMP binds to catabolite activator protein (CAP), which decreases transcription of the operon. When tryptophan is present, it binds to the regulatory protein, activating it.

Hint 3. How is the lac operon regulated? How is the lac operon regulated? Select the four statements that apply. ANSWER: When lactose (and, therefore, allolactose) is absent, the regulatory protein is active. When glucose is absent, cAMP binds to catabolite activator protein (CAP), activating CAP to decrease transcription of the operon. When glucose is absent, cAMP binds to catabolite activator protein (CAP), activating CAP to increase transcription of the operon. The lac operon undergoes negative control. When lactose (and, therefore, allolactose) is absent, the regulatory protein is inactive. The lac operon undergoes positive control.

ANSWER:

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Correct The trp operon is regulated through negative control only. When tryptophan is present, the operon genes are not transcribed. The lac operon is regulated through both negative control and positive control. Negative control: When lactose is absent, the repressor protein is active, and transcription is turned off. When lactose is present, the repressor protein is inactivated, and transcription is turned on. Positive control: When glucose is absent, another regulatory protein (CAP) binds to the promoter of the lac operon, increasing the rate of transcription if lactose is present.

Part C - Regulation of a hypothetical operon You are studying a bacterium that utilizes a sugar called athelose. This sugar can be used as an energy source when necessary. Metabolism of athelose is controlled by the ath operon. The genes of the ath operon code for the enzymes necessary to use athelose as an energy source. You have found the following: The genes of the ath operon are expressed only when the concentration of athelose in the bacterium is high. When glucose is absent, the bacterium needs to metabolize athelose as an energy source as much as possible. The same catabolite activator protein (CAP) involved with the lac operon interacts with the ath operon. Based on this information, how is the ath operon most likely controlled? Drag the labels onto the diagram to identify the small molecules and the states of the regulatory proteins. Not all labels will be used.

Hint 1. What terms describe operon control? Drag the terms on the left to the appropriate blanks on the right to complete the sentences. Not all terms will be used. ANSWER:

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Hint 2. How is the ath operon controlled? How do the various small molecules interact with regulatory protein(s) of the ath operon to regulate its expression? You have found the following: The genes of the ath operon are expressed only when the concentration of athelose in the bacterium is high. When glucose is absent, the bacterium needs to metabolize athelose as an energy source as much as possible. The same catabolite activator protein (CAP) involved with the lac operon interacts with the ath operon. Sort the small molecules into the appropriate bins depending on their direct effect on ath operon transcription. ANSWER:

ANSWER:

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Correct Metabolism of the sugar athelose in this hypothetical system is controlled by an operon that exhibits both positive control and negative control. Transcription of the ath operon is turned on when athelose is present (negative control), and sped up when the bacterium runs out of glucose and must rely on athelose for energy (positive control).

Regulation of Gene Expression in Eukaryotes In all organisms, certain genes are expressed at any given time while other genes are not. Both prokaryotes and eukaryotes regulate gene expression at the transcription stage. However, the greater complexity of eukaryotic cells makes it possible for gene expression to be regulated at many other stages as well. The diagram below shows different stages at which gene expression may be regulated in eukaryotes.

Part A - Modification of chromatin structure Which statements about the modification of chromatin structure in eukaryotes are true? Select all that apply.

Hint 1. Chromatin structure Eukaryotic DNA is packaged with proteins in a complex known as chromatin. Most of the proteins in chromatin are called histones. The attraction between DNA and histones is partially due to the charges of the molecules: DNA is negatively charged, whereas histones are positively charged because they contain positively charged amino acid residues, such as lysine. The basic structural unit of chromatin is the nucleosome, which consists of DNA wound twice around a cluster of eight histones. The amino

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Gene Regulation & Viruses end, or “tail,” of each histone extends from the nucleosome. Chromatin may exist in either a condensed or a decondensed form, as shown below. Chemical modifications of the chromatin determine which form it has.

Hint 2. Do you understand acetylation and methylation of histones? Sort the following phrases based on whether they pertain to acetylation of histone tails, methylation of histone tails, or both. Drag each phrase to the appropriate bin. ANSWER:

Hint 3. Epigenetic inheritance Epigenetic inheritance is the inheritance of traits transmitted by mechanisms that do not directly involve differences in the genome sequence. Chromatin modifications, such as acetylation and methylation of histone tails, may be passed on to future generations of cells. Whereas mutations in the nucleotide sequence of DNA are permanent, modifications to chromatin structure may be reversed. ANSWER: Some forms of chromatin modification can be passed on to future generations of cells. DNA is not transcribed when chromatin is packaged tightly in a condensed form. Acetylation of histone tails in chromatin allows access to DNA for transcription. Deacetylation of histone tails in chromatin loosens the association between nucleosomes and DNA. Methylation of histone tails in chromatin can promote condensation of the chromatin. Acetylation of histone tails is a reversible process.

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Correct One of the mechanisms by which eukaryotes regulate gene expression is through modifications to chromatin structure. When chromatin is condensed, DNA is not accessible for transcription. Acetylation of histone tails reduces the attraction between neighboring nucleosomes, causing chromatin to assume a looser structure and allowing access to the DNA for transcription. If the histone tails undergo deacetylation, chromatin can recondense, once again making DNA inaccessible for transcription.

Recent evidence suggests that methylation of histone tails can promote either the condensation or the decondensation of chromatin, depending on where the methyl groups are located on the histones. Thus, methylation can either inactivate or activate transcription, and demethylation can reverse the effect of methylation. Changes in chromatin structure may be passed on to future generations of cells in a type of inheritance called epigenetic inheritance.

Part B - Regulation of transcription initiation The diagram below shows two stretches of DNA in the genome of an imaginary eukaryotic cell. The top stretch of DNA includes the fantasin gene, along with its promoter and one of its enhancers. The bottom stretch of DNA includes the imaginin gene, its promoter, and one of its enhancers. The slash marks (//) indicate that more than 1,000 nucleotides separate the promoter and enhancer of each gene.

Which statements about the regulation of transcription initiation in these genes are true? Select all that apply.

Hint 1. Can you define terms related to transcription initiation? Drag the terms on the left to the appropriate blanks on the right to complete the sentences. ANSWER:

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Hint 2. Activators and repressors Specific transcription factors are proteins that either promote or inhibit the transcription of particular genes. Those that promote transcription are called activators. Many activators work by interacting with a specific distal control element and with one or more other proteins involved in transcription. These interactions are required for particular genes to be transcribed at high levels. Specific transcription factors that inhibit transcription are called repressors. Many repressors work by preventing activators from interacting with control elements or with other proteins. ANSWER: Control elements C, D, and E are distal control elements for the imaginin gene. The fantasin gene will be transcribed at a high level when activators specific for control elements A, B, and C are present in the cell. Control elements A, B, and C are proximal control elements for the fantasin gene. The imaginin gene will be transcribed at a high level when repressors specific for the imaginin gene are present in the cell. The fantasin gene and the imaginin gene have identical enhancers. Both the fantasin gene and the imaginin gene will be transcribed at high levels when activators specific for control elements A, B, C, D, and E are present in the cell. Both the fantasin gene and the imaginin gene will be transcribed at high levels whenever general transcription factors are present in the cell.

Correct Only certain genes are transcribed in a eukaryotic cell at any particular time. The regulation of transcription initiation depends on the interaction of specific transcription factors with specific control elements in enhancers. In the imaginary eukaryotic cell used as an example here, the enhancers for the fantasin gene and imaginin gene are unique because they contain different sets of control elements (A, B, and C for the fantasin gene; C, D, and E for the imaginin gene). Each gene will be transcribed at a high level when activators specific for all of the control elements in its enhancer are present in the cell.

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Part C - Alternative RNA splicing The diagram below shows a segment of DNA containing an imaginary gene (Z) and the primary RNA transcript that results from the transcription of gene Z. Exons are represented in green and introns are represented in blue.

Which of the following choices represent mRNA molecules that could be produced from the primary RNA transcript by alternative RNA splicing? (In each choice, the yellow part on the left represents the 5' cap, and the yellow part on the right represents the poly-A tail.) Select all that apply.

Hint 1. Exons and introns Eukaryotic genes are composed of regions called exons and introns. Exons are regions of a gene that can be expressed because they can be part of a final mRNA molecule. Introns are intervening regions of a gene that are not expressed because they are never part of a final mRNA molecule. A cell can produce different final mRNA molecules from the same gene by controlling which of the gene’s exons are included in each mRNA molecule.

Hint 2. Can you describe the process of alternative RNA splicing? Which choice(s) accurately describe(s) the process of alternative RNA splicing? Select all that apply. ANSWER: All introns of the primary RNA transcript are removed. Some exons of the primary RNA transcript may be removed. All exons of the primary RNA transcript are removed. Some introns of the primary RNA transcript are removed.

ANSWER:

Correct Alternative RNA splicing produces different mRNA molecules from the same primary RNA transcript. During alternative RNA splicing, all introns are removed, and some exons may also be removed. The removal of different exons produces different mRNA molecules, which are then translated into different proteins. Alternative RNA splicing can greatly expand the number of proteins produced from the same gene.

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Activity: Control of Transcription

Click here to complete this activity. Then answer the questions.

Part A _____ bind(s) to DNA enhancer regions. ANSWER: RNA polymerase Promoters Introns Activators Exons

Correct Activators are a type of transcription factor that bind to enhancer regions.

Part B What is the event that IMMEDIATELY follows the last event of this animation? To view the animation, click here. Then click on the image to start the animation. ANSWER: unbending of the DNA and the release of RNA polymerase from the promoter binding of the activators to enhancers transcription binding of mRNA to the smaller ribosomal subunit binding of RNA polymerase to the promoter

Correct The bending of the DNA allows for the interaction of transcription factors and RNA polymerase.

Part C Which of these indicates an enhancer region?

ANSWER:

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Gene Regulation & Viruses A B C D C and D

Correct This is an enhancer region.

Part D Which of these directly bind(s) to the promoter?

ANSWER: A B C D C and D

Correct Both RNA polymerase and transcription factors bind with the promoter.

Activity: Overview: Control of Gene Expression

Click here to complete this activity. Then answer the questions.

Part A The process of transcription is indicated by the letter _____.

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ANSWER: A B C D E

Correct Transcription is the process by which information encoded in DNA is converted to information encoded in RNA.

Part B The letter E is indicating a process of gene expression that involves _____.

ANSWER: transcription breakdown of mRNA translation of mRNA protein breakdown unpacking of DNA

Correct This is the process indicated by the letter E.

Part C RNA processing is indicated by the letter _____.

ANSWER:

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Gene Regulation & Viruses A B C D E

Correct As you can see, introns have been removed and a cap and a tail added to the RNA molecule.

Part D Which of the following processes is NOT indicated by the label A, B, C, D, or E?

ANSWER: translation RNA transcription export of mRNA DNA unpacking RNA processing

Correct This process is indicated by the arrow that precedes arrow A.

Chapter 18 Pre-Test Question 5 Part A miRNAs can control gene expression by what action?

Hint 1. At what step in gene expression do miRNAs act? ANSWER: binding to DNA and preventing transcription of certain genes binding to mRNAs and degrading them or blocking their translation inhibiting the catalytic activity of rRNA seeking out viral DNA and destroying it degrading proteins as soon as they are formed

Correct miRNAs can effectively "silence" genes by binding to mRNA transcripts. The mRNAs are either broken down by enzymes or are unable to physically interact with the ribosomes to complete translation.

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Chapter 18 Question 27

Part A The phenomenon in which RNA molecules in a cell are destroyed if they have a sequence complementary to an introduced double-stranded RNA is called ANSWER: RNA interference. RNA targeting. RNA disposal. RNA blocking. RNA obstruction.

Correct

Chapter 18 Question 29

Part A Among the newly discovered small noncoding RNAs, one type reestablishes methylation patterns during gamete formation and blocks expression of some transposons. These are known as ANSWER: miRNA. snRNA. RNAi. siRNA. piRNA.

Correct

Chapter 18 Pre-Test Question 9 Part A Which of the following would be most likely to lead to cancer?

Hint 1. Consider which of these would promote unregulated cell division. ANSWER: amplification of a proto-oncogene and inactivation of a tumor-suppressor gene hyperactivity of a proto-oncogene and activation of a tumor-suppressor gene failure of a proto-oncogene to produce a protein and amplification of a tumor-suppressor gene failure of both a proto-oncogene and a tumor-suppressor gene to produce proteins hyperactivity of both a proto-oncogene and a tumor-suppressor gene

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Correct Excess copies of the proto-oncogene could stimulate cell division abnormally, and the inactivation of a tumor-suppressor gene would eliminate a brake on cell division.

Chapter 18 Question 1 Part A If a particular operon encodes enzymes for making an essential amino acid and is regulated like the trp operon, then ANSWER: the enzymes produced are called inducible enzymes. the amino acid acts as a corepressor. the amino acid turns on transcription of the operon. the repressor is active in the absence of the amino acid. the amino acid inactivates the repressor.

Correct

Misconception Question 86

Part A Which statement(s) about inducible operons is/are correct? Select all that apply. ANSWER: Inducible enzymes generally function in synthetic pathways that produce end products from raw materials. In an inducible operon, the repressor is synthesized in an active form. In an inducible operon, an inducer inactivates the repressor.

Correct An inducible operon is usually off but can be turned on when an inducer interacts with and inactivates the repressor. The enzymes in an inducible operon generally function in catabolic pathways. Read about inducible operons.

Misconception Question 87

Part A Which statement(s) about repressible operons is/are correct? Select all that apply. ANSWER: Repressible enzymes generally function in anabolic pathways. A repressible operon is on unless a corepressor is present. In a repressible operon, the repressor is synthesized in an active form.

Correct A repressible operon is usually on but can be turned off when a corepressor interacts with and activates the repressor. The enzymes in an inducible operon generally function in anabolic pathways. Read about repressible operons.

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Misconception Question 88

Part A A gene on human chromosome 15 is expressed throughout the body. However, in the brain, only the maternal copy of the gene is expressed, whereas the paternal copy of the gene is silent and not transcribed. What accounts for this pattern of expression this gene displays in the brain? ANSWER: In the brain, the maternal copy of the gene is methylated whereas the paternal copy is un-methylated. The copies of this gene in brain cells are coordinately controlled with the copies of the gene in body cells. Through genomic imprinting, methylation regulates expression of the paternal copy of the gene in the brain.

Correct Imprinting of the paternal copy of the gene by DNA methylation inactivates it and prevents its expression in brain cells. Read about DNA methylation and genomic imprinting.

Misconception Question 90

Part A Which noncoding RNAs are correctly matched with their function? Select all that apply. ANSWER: MicroRNAs (miRNAs) bind to complementary sequences in mRNA and block its translation. Piwi-associated RNAs (piRNAs) reestablish appropriate methylation patterns in the genome during gamete formation. Small interfering RNAs (siRNAs) bind to complementary sequences in mRNA and block its translation.

Correct Read about the multiple roles of noncoding RNAs.

Chapter 18 Pre-Test Question 1 Part A Why is the lac operon said to be an inducible operon?

Hint 1. Is it normally on or normally off? ANSWER: When activated, the lac operon induces the production of lactose-digesting enzymes. When allolactose is present, it induces the activation of the lac repressor. When allolactose is present, it induces the inactivation of the lac repressor. When allolactose is present, it induces repression of gene expression. When activated, the lac operon induces repression of gene expression.

Correct The lac repressor keeps the production of lactose-digesting enzymes turned off. When allolactose is present, the lac repressor is inactivated, allowing the expression of lactose-digesting enzymes.

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Chapter 18 Pre-Test Question 2 Part A DNA methylation is a mechanism used by eukaryotes to do what?

Hint 1. DNA methylation accounts for genomic imprinting. ANSWER: increase the rate of transcription cause apoptosis facilitate the binding of DNA to intermediate filaments inactivate genes terminate transcription

Correct DNA methylation, involving the attachment of methyl groups to certain bases, is a mechanism for the long-term inactivation of genes during development.

Chapter 18 Question 16

Part A Which of the following mechanisms is (are) used to coordinate the expression of multiple, related genes in eukaryotic cells? ANSWER: A specific combination of control elements in each gene’s enhancer coordinates the simultaneous activation of the genes. A single repressor is able to turn off several related genes. The genes share a single common enhancer, which allows appropriate activators to turn on their transcription at the same time. Environmental signals enter the cell and bind directly to promoters. The genes are organized into large operons, allowing them to be transcribed as a single unit.

Correct

Chapter 18 Question 20

Part A Two potential devices that eukaryotic cells use to regulate transcription are ANSWER: histone amplification and DNA acetylation. DNA methylation and histone modification. DNA acetylation and methylation. DNA methylation and histone amplification. DNA amplification and histone methylation.

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Chapter 18 Question 34

Part A You are given an experimental problem involving control of a gene's expression in the embryo of a particular species. One of your first questions is whether the gene's expression is controlled at the level of transcription or translation. Which of the following might best give you an answer? ANSWER: An analysis of amino acid production by the cell shows you that there is an increase at this stage of embryonic life. You measure the quantity of the appropriate pre-mRNA in various cell types and find they are all the same. You use an antibiotic known to prevent translation. You explore whether there has been alternative splicing by examining amino acid sequences of very similar proteins. You assess the position and sequence of the promoter and enhancer for this gene.

Correct

Chapter 19 Pre-Test Question 1 Part A The nucleic acid of a virus particle is enclosed in a protein coat. What is it called?

Hint 1. Review the structure of a virus. ANSWER: envelope capsid nuclear envelope nucleoid genome

Correct The protein shell that encloses a viral genome is a capsid.

Viral Replication Viruses are nonliving intracellular parasites that infect plants, animals, and bacteria. Viruses use host cell components to make copies of themselves. Often, but not always, the process of viral replication kills the host cell.

Part A - Virus structure Sort the items according to whether they may be found only in free virus particles, only in uninfected host cells, or in both viruses and host cells. Drag each item to the appropriate bin.

Hint 1. Viruses and host cells Viruses are obligate intracellular parasites: They can reproduce within a host cell but cannot reproduce on their own. Although a few viruses can infect a variety of species, many viruses are able to infect only one particular species. Host specificity typically results from the virus’s recognition of certain proteins or glycoproteins on the surface of host cells. Once inside a cell, the virus takes over the cell, reprogramming it to manufacture viral proteins and duplicate viral nucleic acids. The host cell does this using its own components (nucleosides, ribosomes, tRNA, etc.) and usually dies after releasing a new generation of viruses, which can go on to infect other cells.

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Hint 2. What are the parts of a virus? Drag the terms on the left to the appropriate blanks on the right to complete the sentences. ANSWER:

ANSWER:

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Correct Viruses are infectious particles that contain genetic material (either DNA or RNA) surrounded by a capsid (protein shell). Some viruses also have a membranous envelope that contains glycoproteins.

Part B - Reproductive cycles of a bacteriophage Classify each phrase as applying to the lytic cycle, the lysogenic cycle, or both types of reproductive cycles of phages. Drag the descriptions into the appropriate bins.

Hint 1. Bacteriophages Bacteriophages (literally "bacteria-eaters") are viruses that infect bacteria. Bacteriophages contain double-stranded DNA and can duplicate themselves by two alternative mechanisms: the lytic cycle and the lysogenic cycle.

Hint 2. Lytic cycle In the lytic cycle, the bacteriophage binds to the surface of the bacterium. The virus then injects its own DNA into the cell, and the cell's DNA is broken up (hydrolyzed). The viral DNA directs the replication of viral DNA and the synthesis of viral proteins, which self-assemble into new viruses. Finally, the virus triggers destruction of the bacterial cell wall, which causes the cell to lyse, releasing new viruses that can go on to infect other cells.

Hint 3. Lysogenic cycle Unlike the lytic cycle, the lysogenic cycle does not lyse the host cell. After the bacteriophage attacks the cell, the viral DNA is integrated into the bacterial chromosome and is duplicated with each cell division. In this manner, a large number of infected bacteria form. Occasionally, an infected bacterium in the lysogenic cycle can enter the lytic cycle, in which viral particles are produced and the cell is eventually ruptured.

Hint 4. What are the differences between the lytic and lysogenic cycles? Label the diagram of the lytic and lysogenic cycles. Drag the labels to their appropriate locations on the diagram of the cycle. ANSWER:

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ANSWER:

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Correct Bacteriophages, which contain double-stranded DNA, can reproduce by two alternative mechanisms: the lytic cycle and the lysogenic cycle. In the lytic cycle, virus parts are made, new viruses are assembled, and the cell is lysed, releasing the newly assembled viruses. In the lysogenic cycle, the viral DNA incorporates into the bacterial DNA and is passed on to daughter bacterial cells when the infected bacteria reproduce normally. Occasionally, an infected bacterium in the lysogenic cycle can enter the lytic cycle, in which viral particles are produced and the cell is eventually ruptured.

Part C - Virus properties Which statements about viruses are true? Select the four statements that are true.

Hint 1. Enveloped viruses and nonenveloped viruses In an enveloped virus, the capsid is surrounded by a membrane called the envelope. If the envelope fuses with a host cell’s plasma membrane, the capsid and viral genome can enter the cell. When an enveloped virus emerges from an infected cell, it takes a small amount of cell membrane with it (in the form of the viral envelope). In a nonenveloped virus, the capsid does not have a surrounding membrane.

Hint 2. Reproductive cycle of a retrovirus A retrovirus is an enveloped virus that carries single-stranded RNA. In the host cell, a viral enzyme called reverse transcriptase synthesizes a DNA strand complementary to the viral RNA. The newly formed DNA goes on to make copies of the virus.

Hint 3. HIV Human immunodeficiency virus (HIV) is a retrovirus that contains two copies of its RNA genome and two copies of reverse transcriptase in a capsid. The viral envelope facilitates binding to a white blood cell. After the virus enters the host cell, the capsid releases the RNA and reverse transcriptase.

Hint 4. What is the diversity of viral genetic material? What genetic material has not been found in any virus? ANSWER: single-stranded DNA double-stranded DNA single-stranded RNA double-stranded RNA a combination of DNA and RNA (single- or double-stranded)

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Gene Regulation & Viruses ANSWER: A retrovirus contains RNA. All viral genomes contain both DNA and RNA. HIV contains two identical strands of DNA. All RNA-containing viruses are retroviruses. The capsid enters the host cell if the virus is enveloped. HIV contains reverse transcriptase. Enveloped viruses bud from the host cell.

Correct Viruses are classified into two major groups: nonenveloped and enveloped. Nonenveloped viruses (a) typically exit the host cell by bursting through the plasma membrane. Enveloped viruses (b) generally escape from the host cell by budding. Because of this, the viral envelope is often derived from the host cell's plasma membrane.

Viruses contain either single-stranded DNA, double-stranded DNA, single-stranded RNA, or double-stranded RNA. Human immunodeficiency virus (HIV) is a retrovirus that contains two copies of its RNA genome and two copies of reverse transcriptase. The HIV viral envelope facilitates binding to a white blood cell. After HIV enters the cell, the reverse transcriptase synthesizes viral DNA from viral RNA.

Activity: Phage Lysogenic and Lytic Cycles

Click here to complete this activity. Then answer the questions.

Part A session.masteringbiology.com/myct/assignmentPrintView?assignmentID=1937217

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Gene Regulation & Viruses Click on the diagram to start the animation. The arrow is indicating _____.

ANSWER: host cell DNA a capsid an envelope a Golgi apparatus a prophage

Correct Viral DNA incorporated into host DNA is referred to as a "prophage."

Part B Click on the diagram to start the animation. This animation is illustrating an early stage of _____.

ANSWER: the lytic cycle the lysogenic cycle transcription translation lysis

Correct Integration of viral DNA into host DNA is an early stage of the lysogenic cycle.

Part C In the lysogenic cycle _____. ANSWER: host DNA is destroyed and viral DNA is replicated a bacterium replicates without passing viral DNA to its daughter cells viral DNA is destroyed and host DNA is replicated a bacterium divides once before the lytic cycle is initiated viral DNA is replicated along with host DNA

Correct Viral DNA, incorporated into host DNA as a prophage, is replicated along with host DNA.

Part D Click on the diagram to start the animation. Which of these is the best description of this animation?

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ANSWER: The lysogenic cycle has begun. The lytic cycle is making a transition to the lysogenic cycle. The bacterium has successfully fought off the viral infection. The lysogenic cycle is making a transition to the lytic cycle. The host cell has begun to reproduce.

Correct Phage DNA has exited the bacterial chromosome as a prelude to taking over the host's metabolic machinery.

Part E Cycle A is the _____ cycle and cycle B is the _____ cycle.

ANSWER: lytic ... lysogenic lytic ... phage lysogenic ... phage lysogenic ... lytic phage ... lysogenic

Correct The lytic cycle concludes with rupture of the host cell. The lysogenic cycle involves replication of viral DNA along with replication of the host.

Activity: Retrovirus (HIV) Reproductive Cycle

Click here to complete this activity. Then answer the questions.

Part A The genetic material of HIV consists of _____.

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Gene Regulation & Viruses ANSWER: single-stranded DNA single-stranded RNA double-stranded DNA double-stranded RNA none of the above

Correct The genetic material of HIV consists of two molecules of single-stranded RNA.

Part B Which of these binds to receptor molecules on the host cell membrane?

ANSWER: A B C D E

Correct Glycoproteins on the viral envelope recognize and bind to receptors on the host cell.

Part C What is the function of reverse transcriptase? ANSWER: It catalyzes the formation of a polypeptide from an RNA template. It catalyzes the formation of DNA from a polypeptide template. It catalyzes the formation of RNA from a polypeptide template. It catalyzes the formation of RNA from a DNA template. It catalyzes the formation of DNA from an RNA template.

Correct This is the function of reverse transcriptase.

Part D What is the source of a viral envelope? ANSWER:

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Gene Regulation & Viruses host cell DNA prophages provirus host cell membrane viral glycoproteins

Correct The viral envelope is derived from host cell membrane.

Part E Which of these is reverse transcriptase?

ANSWER: A B C D E

Correct This is reverse transcriptase.

Part F Which of these is the viral genome?

ANSWER: A B C D E

Correct HIV is an RNA virus.

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Gene Regulation & Viruses

Part G To view the animation, click here. Then click on the image to start the animation. Click on the diagram to start the animation. What enzyme is responsible for the process seen here? ANSWER: RNA polymerase DNA polymerase reverse transcriptase endonuclease DNA ligase

Correct Reverse transcriptase catalyzes the formation of DNA from an RNA template.

Part H Double-stranded viral DNA is incorporated into a host cell as a _____. ANSWER: promoter provirus transposon lac homeoboxes

Correct "Provirus" is the name given to double-stranded viral DNA that has been incorporated into a host cell's genome.

Misconception Question 94

Part A Identify all correct statements about how viroids differ from viruses. Select all that apply. ANSWER: Unlike viruses, viroids cause plant disease. Unlike viruses, viroids do not encode proteins. Unlike viruses, the genetic material of a viroid is protein.

Correct Unlike viruses, viroids do not encode proteins but can replicate in host plant cells, apparently using host cell enzymes. Read about viroids.

Misconception Question 95

Part A How do prions differ from viruses? Select all that apply. ANSWER:

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Gene Regulation & Viruses Unlike viruses, prions do not include any nucleic acids. Unlike a virus, a prion is a single molecule. Unlike viruses, prions are infectious proteins.

Correct Read about prions. Score Summary: Your score on this assignment is 99.6%. You received 23.91 out of a possible total of 24 points.

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