Reading Essentials for Biology Glencoe (794133), страница 47
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Lysis, the bursting of acell, is one way viruses can be released. Exocytosis, the activetransport process by which materials are expelled from a cell, isthe second way. Both ways release new viruses from the host cell.6. Analyze Why is the presence of the enzymereverse transcriptase asign of infection by aretrovirus?________________________________________________________________________________________________Many viruses are RNA viruses, meaning that RNA is theironly nucleic acid.
The human immunodeficiency virus (HIV)that causes AIDS is an example of an RNA virus. The RNAvirus with the most complex replication cycle is the retrovirus(reh tro VY rus). Retroviruses have an enzyme that changes theirRNA into DNA.How does that happen? Once inside a host cell, the retrovirusmakes DNA from its RNA. To do this, it uses reverse transcriptase (trans KRIHP tayz), an enzyme it carries inside its capsid.This enzyme helps produce DNA from viral RNA. The viralDNA is integrated into the host cell’s chromosome and becomesa provirus.
If reverse transcriptase is found in a person, it isevidence for infection by a retrovirus.What is HIV?Once it gets into a human host, HIV infects white blood cells.Because the viral genetic material is a provirus, new viruses areproduced slowly.An HIV-infected person might experience no AIDS symptomsfor a long time. Most people with an HIV infection eventually getAIDS because more and more white blood cells are infected and196Chapter 18READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.What is a retrovirus?NameDateClassSection18.1Viruses, continuedproduce new viruses over time. Infected host cells function untilthe proviruses enter a lytic cycle and kill their host cells. Whiteblood cells are part of the body’s disease-fighting system.
Whenthey are lost, a body cannot protect itself from organisms thatcause disease. This is a symptom of AIDS.Cancer and VirusesSome viruses have been linked to cancer in humans and animals. The viruses disrupt the normal growth and division of cellsin a host. This causes abnormal growth and can create tumors.Examples of viruses that have been linked to cancer include thehepatitis B virus, which has been shown to play a role in causingliver cancer.Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.What are prions and viroids?Researchers have recently found particles that behave somewhat like viruses and cause infectious diseases.
Prions are madeup of proteins but have no genetic material. They are thought tocause other proteins to malfunction. Prions are responsible formany animal diseases, such as mad cow disease and its humanequivalent.Viroids are a single circular strand of RNA with no proteincoat. They have been found to cause infectious diseases inseveral plants.What is a plant virus?7. Identify Which of thefollowing is an exampleof a viroid? (Circle yourchoice.)a. the virus that causesmad cow diseaseb. a plant diseasec. tobacco mosaic virusMore than 400 viruses infect a variety of plants. These virusescause diseases that stunt growth and reduce production in theirhost plants.
Plant viruses enter and infect a host plant throughwounds or insect bites. However, not all viral plant diseases areharmful or fatal. Some mosaic viruses cause beautiful patterns ofcolor in the flowers of plants.Origin of VirusesBecause they are relatively simple in structure, you might thinkthat viruses represent an early form of life. This is probably nottrue. Since viruses need host cells to replicate, scientists suggestthat viruses might have originated from their host cells.
Some scientists suggest that viruses may be nucleic acids that break freefrom their host cells while still being able to replicate parasiticallywithin the host cells.READING ESSENTIALSChapter 18197NameDateClassSection18.1Viruses, continued◗ After You ReadMini Glossarybacteriophage (bak TIHR ee uh fayj): also calledphages, viruses that infect bacteriaprion: a virus-like infectious agent composed ofonly protein, with no genetic materialcapsid: outer coat of protein that surrounds avirus’s inner core of nucleic acid; arrangement of capsid proteins gives a virus its shapeprovirus: viral DNA that is integrated into a hostcell’s chromosome and replicated each timethe host cell replicateshost cell: living cell in which a virus replicatesretrovirus (reh tro VY rus): the RNA virus withthe most complex replication stylelysogenic cycle: viral replication cycle in which avirus’s nucleic acid is integrated into a hostcell’s chromosome; a provirus is formed andreplicated each time the host cell reproduces; the host cell is not killed until thelytic cycle is activatedlytic (LIH tic) cycle: viral replication cycle in whicha virus takes over a host cell’s genetic material and uses the host cell’s structures andenergy to replicate until the host cell bursts,killing itreverse transcriptase (trans KRIHP tayz): enzymecarried in the capsid of a retrovirus thathelps produce viral DNA from viral RNAviroid: a virus-like infectious agent that iscomposed of only a single, circular strandof RNAvirus: a disease-causing, nonliving particle madeup of nucleic acids enclosed in a proteincoat; inside living cells called host cells1.
Read the key terms and their definitions in the Mini Glossary above. Then use the spacebelow to write a sentence for each of the following terms: provirus and virus.________________________________________________________________________________________________________________________________________________________________________2. Choose one of the question headings in the Read to Learn section.
Write the question in thespace below. Then write your answer to that question on the lines that follow.Question:Answer:________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________198Chapter 18READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.____________________________________________________________________________________NameDateClassSection18.1Viruses, continued3.
Use the pyramid diagram below to help you review what you have read. Arrange the steps inthe replication process of the retrovirus in the correct order by placing the letters below nextto the numbers on the pyramid.a. DNA is made from viral RNAb. Virus enters cellc.
Host cell and provirus reproduce; new virus formsd. Viral DNA (provirus) integrates into host cell’s chromosomes1.2.3.Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.4.Visit the Glencoe Science Web site at science.glencoe.comto find your biology book and learn more about viruses.READING ESSENTIALSChapter 18199NameDateClassSection18.2 Archaebacteria and Eubacteria◗ Before You ReadSC.F.1.4.2 The student knows that body structures are uniquely designed and adapted fortheir function. SC.F.2.4.1 The student understands the mechanisms of asexual and sexualreproduction and knows the different genetic advantages and disadvantages of asexualand sexual reproduction.What do you think about when you hear the word bacteria? You probably think about germs orsomething that is bad for you.
Did you know that some bacteria are actually helpful? For example,some bacteria help with human digestion, while other bacteria help produce cheese, yogurt, andsourdough bread. In this section, you will learn about types of bacteria and what they do. Before youread this section, highlight the headings in the Read to Learn section. These are the topics you willbe learning about.◗ Read to LearnIdentify MainIdeas Skim the reading andhighlight the main idea ofeach paragraph.Bacteria are prokaryotes.
Prokaryotes are unicellular organismsthat do not have a nucleus or membrane-bound organelles. Bacteriaare classified in two kingdoms—archaebacteria and eubacteria.There are many differences between archaebacteria and eubacteria. For example, their cell walls and the lipids in their plasmamembranes differ. In addition, the structure and function of thegenes of archaebacteria are more similar to those of eukaryotesthan those of eubacteria.While they are so different, many scientists propose thatarchebacteria and eubacteria came from a common ancestor several billion years ago.What are archaebacteria?1.
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