Reading Essentials for Biology Glencoe (794133), страница 46
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Read the key terms and definitions in the Mini Glossary above. On the lines below, explainhow the terms cladistics, cladogram, and phylogeny are related.________________________________________________________________________________________________________________________________________________________________________2. On the lines below, list the six kingdoms of organisms. Next to each kingdom, write one factabout that kingdom.________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________Visit the Glencoe Science Web site at science.glencoe.com to findyour biology book and learn more about the six kingdoms.192Chapter 17READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.eubacteria (yew bak TEER ee uh): group ofprokaryotes with strong cell walls and avariety of structures; may be autotrophs(chemosynthetic or photosynthetic) orheterotrophsNameDateClassSection18.1 VirusesSC.F.2.4.2 The student knows that every cell contains a “blueprint” coded in DNA molecules that specify how proteinsare assembled to regulate cells.
Also covers SC.F.1.4.2◗ Before You ReadHas your doctor ever told you that your sore throat was caused by a virus? Have you received a shotto help your body defend itself against the flu? What are some of the things you think about whenyou hear the word virus? On the lines below, write some examples of health problems that arecaused by viruses.______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________◗ Read to LearnCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.What is a virus?Viruses are particles that are not alive.
They cause diseasesand infections. Viruses are made up of nucleic acids, either DNAor RNA, surrounded by a protein coat. They are smaller than thetiniest bacterium. Most biologists agree that viruses are not alivebecause they don’t grow, develop, or carry out respiration.All viruses replicate, or make copies of themselves. However,viruses need the help of living cells to copy themselves. In orderto copy itself, a virus must enter a living cell. The cell in whicha virus replicates is called the host cell.What is the structure of a virus?Viruses differ in size and shape. Some viruses contain as fewas four genes. Other viruses have hundreds of genes. Everyvirus has an inner core of nucleic acid. This core contains thevirus’s genetic material.
Some viruses contain RNA, some contain DNA. The nucleic acid contains the instructions formaking copies of the virus.The outer protein coat surrounding the virus is called a capsid.The arrangement of proteins in the capsid gives the virus itsshape. The protein arrangement also determines what cell can beinfected and how the virus infects the cell. Some large virusescontain another layer called an envelope.Identify IdeasCircle each part of the virusin the diagram below as youread about it.CapsidNucleicacidEnvelope1. What is the cell in whicha virus reproduces called?________________________________________________READING ESSENTIALSChapter 18193NameDateClassSection18.1Viruses, continuedHow does a virus attach to a host cell?2.
Analyze Another layeraround some large virusesis called the (Circle yourchoice.)a. capsid.b. envelope.Before a virus can replicate, it has to enter a host cell. In orderto do this, it must first attach itself to the host cell. Every virushas a specially shaped device called an attachment protein. Becauseof the specific shape, each virus can only attach to a few kinds ofcells. The attachment process is like two pieces of a jigsaw puzzlefitting together.A virus that infects a bacterium is called a bacteriophage(bak TIHR ee uh fayj), or phage for short.
A protein located inthe tail fibers of the bacteriophage recognizes and attaches to itsbacterial host cell.Do viruses infect more than one species?Many viruses are species specific. They infect only certainspecies. For example, smallpox infects only humans. Other virusesare not species specific.
The virus that causes the flu infects bothhumans and animals. Viruses that are not species specific aremore difficult for scientists to eliminate.3. Although there are twoways that viruses get intoa cell, both ways result inthe virus releasing itsnucleic acid into the cell.(Circle your choice.)a. Trueb.
FalseOnce a virus attaches itself to the host cell, the virus enters thecell and takes over its metabolism. Only then can the virus replicate itself. There are two ways that viruses get into the cells. First,the virus can inject its nucleic acid into the host cell. When thistakes place, the capsid of the virus stays attached to the outside ofthe host cell. An enveloped virus enters a host cell in another way.After attachment, the plasma membrane of the host cell surroundsthe virus. This produces a virus-filled vacuole inside the host cell’scytoplasm. Recall that a vacuole is a membrane-bound compartment used for temporary storage of materials within a cell.
Oncethe vacuole is in the cytoplasm, the virus bursts out of the vacuoleand releases its nucleic acid into the cell.What is a lytic cycle?Once the virus is inside the host cell, the virus’s genes areexpressed. The substances that are produced take over the hostcell’s genetic material.194Chapter 18READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Viral Replication CyclesNameDateClassSection18.1Viruses, continuedWhen viruses takeover the cell, the cellstops producing thematerials it needs tolive. It uses its ownenzymes and energyto make new viruses.The new viruses thenburst from the hostcell and the host celldies.
The new virusescan then infect andkill other host cells.This process is calleda lytic (LIH tik)cycle. A lytic cycle isillustrated at right.BacteriophageBacterial DNANucleicacidBacterialhost cellAAttachmentBEntryThe bacteriophageinjects its nucleic acidinto the bacterial cell.ELysis and ReleaseThe host cell breaks open andreleases new virus particles.CCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.What happensduring alysogenic cycle?DAssemblyNew virusparticles areassembled.Not all viruses kill the cells they infect. Some viruses gothrough a lysogenic cycle.
A lysogenic cycle is a replication cyclein which the virus’s nucleic acid is integrated into the chromosome of the host cell.The cycle begins in the same way as a lytic cycle. The virusattaches to the host cell and its nucleic acid enters the cell.Instead of taking over the genetic material of the host, the viralDNA is included in the host cell’s chromosome. Once that happens, the viral DNA is called a provirus. The cell continues tocarry out its own metabolic activity, but every time the host cellreproduces, the provirus is replicated as well. This means thatevery cell that comes from an infected host cell has a copy ofthe provirus.The lysogenic cycle can last for many years. At any time, though,the provirus can be activated and enter a lytic cycle.
When thathappens, the virus will replicate and kill the host cell.ReplicationThe host’smetabolic machinery makesviral nucleic acidand proteins.4. What is the cycle calledduring which the virusdoes not destroy the hostcell?________________________________________________5. What is the name forviral DNA that is integrated into the hostcell’s chromosome?________________________________________________READING ESSENTIALSChapter 18195NameDateClassSection18.1Viruses, continuedWhat are some of the diseases causedby proviruses?Have you ever had a cold sore? Cold sores are caused by theherpes simplex I virus.
Reoccurring cold sores are an example ofthe lysogenic process. Even though a cold sore heals, the virusremains in your cells as a provirus. When the provirus enters alytic cycle, another cold sore erupts. No one knows for sure whatcauses a provirus to be activated. Some scientists think that physical stress, such as a sunburn, and emotional stress, such as anxiety,play a role.Many viruses have lysogenic cycles. For example, the virusesthat cause hepatitis B and chicken pox are lysogenic viruses.How are viruses released?Viruses can be released in two ways.
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