Explain the Similarities and Differences Between Viruses and Living

Module 2B –Viruses– Viruses Objective # 9

Viruses are not complete living organisms. They are smaller and simpler in structure than even the Explain the similarities and simplest prokaryotic cells. differences between viruses However, because they have some and living organisms. characteristics of life, they are important subjects of research for biologists.

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Objective 9 Objective # 10 Like living organisms, viruses contain Describe the structure of a genetic instructions. However, they lack the machinery needed to carry out typical virus. Be sure to these instructions. discuss the following: On their own, they are inert chemicals and cannot perform any life functions. nucleic acid core, protein However, if they enter a living host coat or capsid, envelope, cell, they can use the cell’s machinery to replicate. viralviral--specificspecific enzymes.

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Objective 10 Objective 10 A virus consists of a nucleic acid core All viruses contain at least 2 parts: surrounded by a protective coat. 1) Nucleic acid core or Genome Here is a photo of the Influenza virus: ¾ May be DNA or RNA but not both ¾ Usually only 1 or 2 molecules ¾ DNA or RNA may be linear or circular, and either single-single-strandedstranded or double-double-strandedstranded ¾ Functions as the genetic material 5 6 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

1 Objective 10 Objective 10 2) Protein coat or Capsid Viruses with helical and icosahedral capsids:

¾ Surrounds and protects the nucleic RNA acid core Capsid DNA ¾ Composed of one or a few proteins repeated many times Structure ¾ Shape is usually helicalor icosahedralicosahedral.. Capsid An icosahedron may look spherical on lower power, but is actually composed Virus Plant virus (TMV) Animal virus (Adenovirus) Viral shape Helical capsid Icosahedral capsid of 20 equilateral triangular facets: 7 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 8

Objective 10 Head DNA Complex ¾ Some viruses have a complex capsid Capsid (protein sheath) capsid of a with binal (two- (two-fold)fold) symmetry that is TT--eveneven phage Collar neither purely icosahedral nor helical consists of an Tail ¾ For examppple, the capsid of T-T-eveneven Whiskers icosahedral bacteriophages consists of a head that head attached is an elongated icosahedron attached to a helical tail to a tail that is helical. Tail fibersand a with tail fibers base plate are also present: and base plate Base plate Tail fiber

Objective 10 Objective 10 In addition to a genome and capsid, some viruses also have: 4) Viral- Viral-specificspecific enzymes 3) Envelope ¾ Enzymes the virus needs, but that ¾ Surrou nds th e caps id are not supplied by the host cell ¾ ¾ Rich in lipids, proteins, and glycolipids Coded by the viral genome ¾ ¾ Derived from the host’s cell Stored inside the capsid membrane, but also contains proteins coded by viral genes 11 12

2 Envelope protein Viral Diversity

Envelope VIRUS VIRUS Flavivirus VIRUS Poliovirus VIRUS Adenovirus (causes VIRUS Influenza 30 nm yellow BACTERIUM HIV 75 nm Streptococcus 100 nm fever) VIRUS 110 nm 22 nm 1 µm Rabies VIRUS 125 nm Herpes simplex Capsid 150 nm VIRUS Poxvirus PROTEIN 250 nm VIRUS Hemoglobin T2 bacteriophage 15 nm 65 nm

BACTERIUM Viral-specific EUKARYOTE E. coli Yeast cell 2 mm long Enzyme 7 mm long RNA Enveloped virus Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13 14

Objective # 11 Objective 11 Head Bacteriophages, DNA called phages Capsid Explain what a bacteriophage (protein sheath) for short, are is, and be able to name and Collar viruses that Tail describe the 2 types of infect bacteria. Whiskers reproductive cycles found in bacteriophages.

Base plate Tail fiber

Objective 11 Two types of reproductive cycles are found in bacteriophages: 1) Lytic cycle: ¾ Phage exists free in the host cell’s cytoplasm. ¾ Phage replicates and the new phages are released by lysis(bursting) of the host cell.

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3 Objective 11 Lytic Cycle Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Attachment: virus attaching Bacterial to cell wall chromosome Bacteriophage Penetration: 2) Lysogenic cycle: viral DNA injected ¾ Phage DNA is integrated into the into cell Life Cycles

host cell’s DNA. Release: lysis of cell Lysogenic Cycle ¾ Integration of genome PPageDNcahage DNA can re ma in inact actveadive and Synthesis: ldleads to proph age protein and Propagation of nucleic acid be replicated along with the host prophage along with DNA for many generations. host genome

¾ Under certain conditions, phage will Cell stress Reproduction of enter lytic cycle and lyse the host lysogenic bacteria Assembly: involves Induction: prophage exits the spontaneous assembly of bacterial chromosome, viral genes cell. 19 capsid and enzyme to insert DNA are expressed

Objective # 12 Objective 12

One group of animal viruses are the Describe the life cycle of a retrovirusesretroviruses.. representative animal virus, such Retroviruses are enveloped viruses that as HIV. have an RNA genome. Once inside a host cell, they use thethe enzyme reverse transcriptase to make a DNA copy of their RNA genome. HIV is an example of a retrovirus.

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