micro chapter 5

Chapter 5: Viruses

Lesson 5.1: General Structure and Classification

  • Methods of Classifying Viruses:

    • Two commonly used methods:

    • Morphological characteristics

    • Nucleic acid type

  • Definitions and Descriptions:

    • Capsid: The protein coat of a virus that encases the genetic material.

    • Capsomeres: The individual protein subunits that make up the capsid.

    • Nucleocapsid: The complex of the viral nucleic acid and the capsid, serving to protect the viral genome.

    • Virion: A complete, infectious virus particle consisting of nucleic acid, capsid, and optional envelope.

    • Envelope: A lipid membrane surrounding some viruses, derived from the host cell's membrane, containing viral glycoproteins.

  • Types of Viral Genomes:

    • Viral genomes can be classified as:

    • DNA

    • RNA

    • Types of nucleic acids:

    • Double-stranded (ds) or single-stranded (ss)

    • Can be linear or circular.

General Structure of Viruses

  • Viruses: Microscopic particles that infect host cells to replicate.

    • Composed of either DNA or RNA as genome.

    • Cannot reproduce independently; are obligate intracellular parasites.

    • Infect both eukaryotic and prokaryotic cells.

    • Bacteriophage: A type of virus that specifically infects bacteria.

Classification of Viruses

  • Challenge in Classification:

    • Taxonomic classification is complicated due to diversity.

  • Classification Criteria:

    • Viruses classified based on:

    • Morphology

    • Nucleic acid type

    • Mode of replication

    • Host organism

    • Associated diseases

  • International Classification:

    • Order: -virales

    • Family: -viridae

    • Subfamily: -virinae

    • Genus: -virus

    • Species: -virus

Baltimore Classification System

  • Overview:

    • Categorizes viruses into one of seven groups based on the relationship between genomic nucleic acid and messenger RNA:

    • Group I: dsDNA

    • Group II: ssDNA

    • Group III: dsRNA

    • Group IV: ssRNA+

    • Group V: ssRNA-

    • Group VI: ssRNA-RT

    • Group VII: dsDNA-RT

Morphology of Viruses

  • Size Variation:

    • Viruses range from approximately 20 nm (e.g., parvovirus) to up to 450 nm (e.g., poxvirus).

    • Most viruses cannot be visualized with light microscopy except for some larger viruses like mimivirus (750 nm) found in amoebas, which has about 1000 genes on double-stranded circular DNA.

  • Capsid Structure:

    • Composed of proteins encoded by viral genes; provides structural integrity and serves as basis for morphological classification.

    • Protomers aggregate to form capsomeres, which then assemble into the capsid.

  • Nucleoproteins:

    • Proteins that associate with nucleic acid; when capsid proteins complex with nucleic acid, they form the nucleocapsid.

  • Spikes:

    • Long projections extending from the nucleocapsid.

  • Virion:

    • The fully assembled and infectious virus particle.

Classification by Morphology

  • Helical Viruses:

    • Features rod-shaped capsomeres, encapsulating either ssRNA or ssDNA.

    • Can be naked (e.g., tobacco mosaic virus) or enveloped (e.g., influenza virus).

  • Icosahedral Viruses:

    • Form a three-dimensional geometric shape with 12 corners, 20 triangular faces, and 30 edges.

    • The arrangement of capsomeres varies among different viruses.

    • Examples include: Herpesviridae, Adenoviridae, Papovaviridae, and Parvoviridae.

Enveloped and Complex Viruses

  • Enveloped Viruses:

    • Consist of a viral envelope that surrounds the nucleocapsid.

    • Envelope is typically acquired by budding from a host membrane, which may include viral glycoproteins and some host membrane proteins replaced by viral proteins.

  • Complex Viruses:

    • Bacteriophages:

    • Comprise a head or capsid containing nucleic acid, variable in size and shape, made up of one or more proteins.

    • Include a tail—a hollow tube surrounded by a contractile sheath—with a base plate and fibers for attachment.

    • Poxviruses:

    • Lack a regular capsid structure.

    • Feature a nucleoid surrounded by a membrane with two lateral bodies present.

Genome Types

  • General Types:

    • All viruses possess either DNA or RNA, classified as:

    • dsDNA

    • ssDNA

    • dsRNA

    • ssRNA

    • Genetic material may be linear or circular, usually found in one form.

    • Exception: Cytomegalovirus (genus of herpesviruses) contains both DNA and RNA capacities.

  • DNA Viruses:

    • Most belong to Group I or II of the Baltimore classification.

    • Replicate using DNA-dependent DNA polymerase.

  • RNA Viruses:

    • Utilize RNA as genetic material and do not undergo DNA replication.

    • Typically group II, IV, or V in the Baltimore classification.

  • Reverse Transcribing Viruses:

    • Contain RNA genomes and replicate through a DNA intermediate.

    • Classified under group VI and VII in the Baltimore system.

Lesson 5.2: Viral Multiplication, Effects on Host Cells, and Subviral Agents

  • Overview of Viral Multiplication:

    • Viruses are acellular and lack the capacity for replication through mitosis or binary fission; they rely on the host cell's machinery to produce copies.

    • Outside of host cells, viruses behave as inert particles.

Multiplication of Bacteriophages

  • Phage Typing:

    • Bacteriophages (phages) are utilized in diagnostic laboratories to identify pathogenic bacteria due to their specificity to particular bacterial strains.

  • Steps in Phage Multiplication:

    1. Adsorption:

    • Phage attaches to its bacterial host, mediated by tail fibers.

    • Phage receptors on bacterial surface include proteins on cell walls, pili, and flagella.

    1. Penetration:

    • After binding, phage injects nucleic acid into the bacterium through contraction of the tail structure.

    • Usually, only the nucleic acid enters the cell, leaving the phage outside.

    1. Replication:

    • Host cell metabolism shifts towards the expression of viral nucleic acid.

    • Production of viral components occurs.

    1. Assembly:

    • Viral components are assembled into new phage particles.

    1. Maturation:

    • Finalization of viral assembly into mature virions.

    1. Release:

    • New phages cause cell lysis, releasing newly formed particles into the environment.

Types of Bacteriophages

  • Lytic or Virulent Phages:

    • Multiply and destroy the bacterium post-multiplication, causing lysis.

    • Phage Life Cycle Stages:

    • Eclipse period: No infectious blood particles observed.

    • Intracellular accumulation phase: Assembly occurs.

    • Lysis and release phase: New phages are released from the destroyed host.

  • Lysogenic or Temperate Phages:

    • Phage integrates its DNA into the host genome as a prophage, persisting in this dormant state.

    • Prophage can subsequently lead to lysis or continue to coexist within the host cell division.

Multiplication of Animal Viruses

  • Comparison to Bacteriophage Multiplication:

    • Similar stages in the multiplication cycle:

    • Adsorption

    • Penetration

    • Uncoating

    • Replication

    • Assembly

    • Release

    • The duration of the viral cycle can vary significantly between different viruses.

Viral Multiplication Steps in Detail

  • Adsorption:

    • Virus binds to a susceptible host via specific attachment sites, known as receptors, which may be located on either capsids or envelopes.

    • Host species can be limited in range as viruses can only infect species with available receptors.

  • Penetration:

    • The virus enters the cytoplasm of the host cell either through endocytosis or membrane fusion, where the nucleocapsid enters the cell.

  • Uncoating:

    • Viral nucleic acid is released from the viral capsid.

    • If the virus enters the host cell via endocytosis, it is enclosed in a vesicle, which is dissolved by cellular enzymes, releasing the viral nucleic acid.

  • Replication:

    • DNA Viruses: Viral DNA enters the host cell's nucleus for transcription into mRNA, which is translated to produce early viral proteins necessary for further replication.

    • RNA Viruses: Must provide their own polymerase for transcription; retroviruses have unique replication mechanisms.

    • Translation of mRNA into viral proteins occurs on host ribosomes, contributing to the final assembly.

  • Assembly and Release:

    • Mature viral particles are constructed from accumulating viral components.

    • Release Mechanisms:

    • Budding (exocytosis)

    • Lysis of the host cell

Types of Viral Infections

  • Classification of Viral Infections:

    • Abortive: Infections without production of viral particles.

    • Lytic or Cytocidal: Viral infections that cause host cell death.

    • Persistent Infections: Infections that are not lytic but can lead to chronic or latent states.

    • Chronic persistent: Ongoing replication without host cell death.

    • Latent: Limited synthesis with long asymptomatic periods; examples—cold sores and shingles.

    • Slow infections: Long incubation periods leading to eventual disease; damage may take years.

    • Transforming infections: Viral nucleic acids may integrate into host cell genomes, leading to oncogenic transformations.

Host Cell Damage from Viral Infections

  • Morphological Effects:

    • Cytopathic changes induced by viral infections include altered cell shape, surface detachment, lysis, membrane fusion, altered permeability, and inclusion body formation.

  • Physiological Effects:

    • Incorporation of viral proteins alters the behavior and characteristics of host cell membranes, affecting ion movement and cellular activity.

  • Biochemical Effects:

    • Viral presence can inhibit or alter the synthesis of host macromolecules, leading to dysfunction.

  • Genotoxic Effects:

    • Genotoxicity may result in host DNA damage and mutations, which can initiate processes leading to cancer.

Major Groups of Viruses in Vertebrates

  • Characteristics:

    • Most viruses cause disease upon breaching protective host barriers and are typically specific to certain tissue or cell types.

  • Viral Genomes:

    • Consists of either DNA or RNA, which can lead to various disease outcomes.

DNA Viruses

  • Overview:

    • Various DNA viruses with associated diseases:

    • Adenoviruses:

    • 57 adenoviral serotypes identified in humans, resistant to environmental agents, with implications in gene therapy.

    • Associated with respiratory illness, gastrointestinal disease, conjunctivitis, and cystitis.

    • Hepadnaviruses:

    • Cause hepatitis, notably hepatitis B, capable of producing acute or chronic diseases.

    • Herpesviruses:

    • Examples include Herpes simplex 1 and 2; Varicella-zoster virus.

    • Papillomaviruses and Polyomaviruses:

    • Associated with warts and potential cervical carcinomas.

    • Parvoviruses:

    • Only B19 causes disease in humans; others affect animals.

    • Poxviruses:

    • The largest viruses known, including those causing smallpox.

RNA Viruses

  • Overview:

    • Diverse families with significant impacts on human health:

    • Bunyaviridae:

    • Transmitted by arthropods.

    • Coronaviruses:

    • Responsible for various respiratory and enteric diseases including SARS.

    • Orthomyxoviruses:

    • Include influenza A, B, and C viruses, with A being the most virulent to humans.

    • Paramyxoviruses:

    • Includes Morbillivirus (measles) and Nipah virus among others.

    • Picornaviruses:

    • A large group including enteroviruses, rhinoviruses, and various hepatoviruses, acknowledged as the smallest RNA viruses.

    • Rhabdoviruses:

    • Infect a broad spectrum of hosts; the rabies virus is a well-known member.

    • Reoviruses:

    • Not specifically disease-associated but includes rotavirus, the leading cause of gastroenteritis in children.

    • Retroviruses:

    • Groups including Oncovirinae, Lentivirinae (HIV) among others that replicate through reverse transcription.

    • Togaviruses:

    • Includes alphaviruses and rubella virus.

    • Flaviviruses:

    • Includes viruses causing yellow fever, dengue fever, and hepatitis C.

Subviral Agents

  • Viroids:

    • Consist of circular RNA molecules that cause diseases in plants without requiring helper viruses.

  • Virusoids:

    • RNA molecules that require the presence of a helper virus, often causing plant infections.

    • Example: Hepatitis D virus necessitates simultaneous infection with hepatitis B virus.

  • Prions:

    • Infectious particles derived from abnormally folded proteins, lacking nucleic acids.

    • Associated diseases known as transmissible spongiform encephalopathies (e.g., mad cow disease, kuru).