CHAPTER 12 THE STRUCTURE AND INFECTION CYCLE OF VIRUSES © Eye of Science / Science Photo Library

OVERVIEW

Viruses Viruses: are noncellular or Acellular infectious agents Virology: study of viruses Virologists: scientists who study viruses

General features of Viruses Viruses are infectious agents with both living and nonliving characteristics. 1. Living characteristics of viruses a. They reproduce at a fantastic rate, but only in living host cells. b. They can mutate.

…General features of Viruses 2. Nonliving characteristics of viruses They are acellular, that is, they contain no cytoplasm or cellular organelles. They carry out no metabolism on their own and must replicate using the host cell's metabolic machinery.

…General features of Viruses Virus particles contains either DNA or RNA (not both) Nucleic Acid is surrounded or coated by a protein shell (capsid) Some viruses possess a membrane-like envelope surrounding the particle

…General Properties of viruses Consists of 1 molecule of DNA or RNA enclosed in coat of protein May have additional layers Cannot reproduce independent of living cells nor carry out cell division as procaryotes and eucaryotes do An intact viral particle is called a virion.

The Size and Morphology of Selected Viruses Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Generalized Structure of Viruses Viral components Nucleic acids Capsid Envelope

…Generalized Structure of Viruses

The Structure of Viruses Virion size range is ~10-400 nm All virions contain a nucleocapsid which is composed of nucleic acid (DNA or RNA) and a protein coat (capsid) Some viruses consist only of a nucleocapsid, others have additional components Envelopes virions having envelopes = enveloped viruses virions lacking envelopes = naked viruses

VIRAL ENVELOPES Many viruses that infect humans and other animals are enveloped. Envelopes form when viral glycoproteins and oligosaccharides associate with the plasma membrane of the host cell. All envelopes have a phospholipid bilayer.

VIRAL ENVELOPES Envelopes vary in: Size Morphology Complexity Composition

ENVELOPE GLYCOPROTEINS They are firmly embedded in the envelope bilayer. This is facilitated by domains of host membrane proteins called spanners. They can form spikes or other structures on the outside of the virion. These can be used to attach to a host cell.

Capsids Capsids are large macromolecular structures which serve as protein coat of virus Protect viral genetic material and aid in its transfer between host cells Made of protein subunits called protomers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

GENOMIC PACKAGING Genome packaging has an important role in the infection. Viral genomes are packaged in one of three ways: Directly in the capsid-inner side of the protein coat Enclosed in special proteins-nucleic acid binding protein Enclosed in proteins from the host cell

VIRUS CLASSIFICATION

Helical Capsids-Tobacco Mosaic Virus TMV are shaped like hollow tubes with protein walls Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Influenza Virus – an Enveloped Virus with a Helical Nucleocapsid Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

HELICAL VIRUSES

ICOSAHEDRAL VIRUSES Their shape is derived from 20 triangular faces that make up the capsid. The capsid ‘has’ 12 points of symmetry.

Icosahedral capsids

THE INFECTION CYCLE The infection cycle was first worked out in bacteriophages (bacterial viruses). Animal virus infections can be either lytic or lysogenic.

LYTIC VERSUS LYSOGENIC INFECTION In a lytic infection, the host cells fills with virions and bursts. The result is cell death. Lysogenic infections are also known as latent infections. The viral genome becomes incorporated into the host cell’s DNA. It can remain this way for an extended period. The host cell lives.

LYTIC VERSUS LYSOGENIC INFECTION

LYTIC INFECTION For animal viruses, there are six steps in lytic infection: Attachment Penetration Uncoating Biosynthesis Maturation Release

Attachment Specific surface structures on host to which viruses attach Receptor sites: Specific surface structures on host to which viruses attach Specific for each virus Can be proteins, lipopolysaccharides, techoic acids, etc. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

TYPES OF RECEPTOR BINDING Non-enveloped viruses Binding takes place between viral capsid and receptor. Enveloped viruses Binding takes place between viral envelope proteins and receptor.

Entry into the Host Most bacterial viruses (bacteriophages) inject their nucleic acid into host Eucaryotic viruses usually enter the cytoplasm with the genome still enclosed. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Bacteriophages

Viruslike Agents Prions

Fusion with Host Membrane Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Assignment Homework: Important group of RNA viruses

Viral Diseases (Next chapter) A. Influenza B. Diseases Caused by the Herpes Family C. Viral Hepatitis D. Human Immunodeficiency Virus E. Miscellaneous Viral Diseases

Bibliography Microbiology, A clinical Approach -Danielle Moszyk-Strelkauskas-Garland Science 2010. http://en.wikipedia.org/wiki/Scientific_method https://files.kennesaw.edu/faculty/jhendrix/bio 2261/home.html http://www.cdc.gov/cmv/