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Biology 2e

21.2 Virus Infections and Hosts

Biology 2e21.2 Virus Infections and Hosts
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  1. Preface
  2. The Chemistry of Life
    1. 1 The Study of Life
      1. Introduction
      2. 1.1 The Science of Biology
      3. 1.2 Themes and Concepts of Biology
      4. Key Terms
      5. Chapter Summary
      6. Visual Connection Questions
      7. Review Questions
      8. Critical Thinking Questions
    2. 2 The Chemical Foundation of Life
      1. Introduction
      2. 2.1 Atoms, Isotopes, Ions, and Molecules: The Building Blocks
      3. 2.2 Water
      4. 2.3 Carbon
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
    3. 3 Biological Macromolecules
      1. Introduction
      2. 3.1 Synthesis of Biological Macromolecules
      3. 3.2 Carbohydrates
      4. 3.3 Lipids
      5. 3.4 Proteins
      6. 3.5 Nucleic Acids
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
  3. The Cell
    1. 4 Cell Structure
      1. Introduction
      2. 4.1 Studying Cells
      3. 4.2 Prokaryotic Cells
      4. 4.3 Eukaryotic Cells
      5. 4.4 The Endomembrane System and Proteins
      6. 4.5 The Cytoskeleton
      7. 4.6 Connections between Cells and Cellular Activities
      8. Key Terms
      9. Chapter Summary
      10. Visual Connection Questions
      11. Review Questions
      12. Critical Thinking Questions
    2. 5 Structure and Function of Plasma Membranes
      1. Introduction
      2. 5.1 Components and Structure
      3. 5.2 Passive Transport
      4. 5.3 Active Transport
      5. 5.4 Bulk Transport
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    3. 6 Metabolism
      1. Introduction
      2. 6.1 Energy and Metabolism
      3. 6.2 Potential, Kinetic, Free, and Activation Energy
      4. 6.3 The Laws of Thermodynamics
      5. 6.4 ATP: Adenosine Triphosphate
      6. 6.5 Enzymes
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    4. 7 Cellular Respiration
      1. Introduction
      2. 7.1 Energy in Living Systems
      3. 7.2 Glycolysis
      4. 7.3 Oxidation of Pyruvate and the Citric Acid Cycle
      5. 7.4 Oxidative Phosphorylation
      6. 7.5 Metabolism without Oxygen
      7. 7.6 Connections of Carbohydrate, Protein, and Lipid Metabolic Pathways
      8. 7.7 Regulation of Cellular Respiration
      9. Key Terms
      10. Chapter Summary
      11. Visual Connection Questions
      12. Review Questions
      13. Critical Thinking Questions
    5. 8 Photosynthesis
      1. Introduction
      2. 8.1 Overview of Photosynthesis
      3. 8.2 The Light-Dependent Reactions of Photosynthesis
      4. 8.3 Using Light Energy to Make Organic Molecules
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
    6. 9 Cell Communication
      1. Introduction
      2. 9.1 Signaling Molecules and Cellular Receptors
      3. 9.2 Propagation of the Signal
      4. 9.3 Response to the Signal
      5. 9.4 Signaling in Single-Celled Organisms
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    7. 10 Cell Reproduction
      1. Introduction
      2. 10.1 Cell Division
      3. 10.2 The Cell Cycle
      4. 10.3 Control of the Cell Cycle
      5. 10.4 Cancer and the Cell Cycle
      6. 10.5 Prokaryotic Cell Division
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
  4. Genetics
    1. 11 Meiosis and Sexual Reproduction
      1. Introduction
      2. 11.1 The Process of Meiosis
      3. 11.2 Sexual Reproduction
      4. Key Terms
      5. Chapter Summary
      6. Visual Connection Questions
      7. Review Questions
      8. Critical Thinking Questions
    2. 12 Mendel's Experiments and Heredity
      1. Introduction
      2. 12.1 Mendel’s Experiments and the Laws of Probability
      3. 12.2 Characteristics and Traits
      4. 12.3 Laws of Inheritance
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
    3. 13 Modern Understandings of Inheritance
      1. Introduction
      2. 13.1 Chromosomal Theory and Genetic Linkage
      3. 13.2 Chromosomal Basis of Inherited Disorders
      4. Key Terms
      5. Chapter Summary
      6. Visual Connection Questions
      7. Review Questions
      8. Critical Thinking Questions
    4. 14 DNA Structure and Function
      1. Introduction
      2. 14.1 Historical Basis of Modern Understanding
      3. 14.2 DNA Structure and Sequencing
      4. 14.3 Basics of DNA Replication
      5. 14.4 DNA Replication in Prokaryotes
      6. 14.5 DNA Replication in Eukaryotes
      7. 14.6 DNA Repair
      8. Key Terms
      9. Chapter Summary
      10. Visual Connection Questions
      11. Review Questions
      12. Critical Thinking Questions
    5. 15 Genes and Proteins
      1. Introduction
      2. 15.1 The Genetic Code
      3. 15.2 Prokaryotic Transcription
      4. 15.3 Eukaryotic Transcription
      5. 15.4 RNA Processing in Eukaryotes
      6. 15.5 Ribosomes and Protein Synthesis
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    6. 16 Gene Expression
      1. Introduction
      2. 16.1 Regulation of Gene Expression
      3. 16.2 Prokaryotic Gene Regulation
      4. 16.3 Eukaryotic Epigenetic Gene Regulation
      5. 16.4 Eukaryotic Transcription Gene Regulation
      6. 16.5 Eukaryotic Post-transcriptional Gene Regulation
      7. 16.6 Eukaryotic Translational and Post-translational Gene Regulation
      8. 16.7 Cancer and Gene Regulation
      9. Key Terms
      10. Chapter Summary
      11. Visual Connection Questions
      12. Review Questions
      13. Critical Thinking Questions
    7. 17 Biotechnology and Genomics
      1. Introduction
      2. 17.1 Biotechnology
      3. 17.2 Mapping Genomes
      4. 17.3 Whole-Genome Sequencing
      5. 17.4 Applying Genomics
      6. 17.5 Genomics and Proteomics
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
  5. Evolutionary Processes
    1. 18 Evolution and the Origin of Species
      1. Introduction
      2. 18.1 Understanding Evolution
      3. 18.2 Formation of New Species
      4. 18.3 Reconnection and Speciation Rates
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
    2. 19 The Evolution of Populations
      1. Introduction
      2. 19.1 Population Evolution
      3. 19.2 Population Genetics
      4. 19.3 Adaptive Evolution
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
    3. 20 Phylogenies and the History of Life
      1. Introduction
      2. 20.1 Organizing Life on Earth
      3. 20.2 Determining Evolutionary Relationships
      4. 20.3 Perspectives on the Phylogenetic Tree
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
  6. Biological Diversity
    1. 21 Viruses
      1. Introduction
      2. 21.1 Viral Evolution, Morphology, and Classification
      3. 21.2 Virus Infections and Hosts
      4. 21.3 Prevention and Treatment of Viral Infections
      5. 21.4 Other Acellular Entities: Prions and Viroids
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    2. 22 Prokaryotes: Bacteria and Archaea
      1. Introduction
      2. 22.1 Prokaryotic Diversity
      3. 22.2 Structure of Prokaryotes: Bacteria and Archaea
      4. 22.3 Prokaryotic Metabolism
      5. 22.4 Bacterial Diseases in Humans
      6. 22.5 Beneficial Prokaryotes
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    3. 23 Protists
      1. Introduction
      2. 23.1 Eukaryotic Origins
      3. 23.2 Characteristics of Protists
      4. 23.3 Groups of Protists
      5. 23.4 Ecology of Protists
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    4. 24 Fungi
      1. Introduction
      2. 24.1 Characteristics of Fungi
      3. 24.2 Classifications of Fungi
      4. 24.3 Ecology of Fungi
      5. 24.4 Fungal Parasites and Pathogens
      6. 24.5 Importance of Fungi in Human Life
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    5. 25 Seedless Plants
      1. Introduction
      2. 25.1 Early Plant Life
      3. 25.2 Green Algae: Precursors of Land Plants
      4. 25.3 Bryophytes
      5. 25.4 Seedless Vascular Plants
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    6. 26 Seed Plants
      1. Introduction
      2. 26.1 Evolution of Seed Plants
      3. 26.2 Gymnosperms
      4. 26.3 Angiosperms
      5. 26.4 The Role of Seed Plants
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    7. 27 Introduction to Animal Diversity
      1. Introduction
      2. 27.1 Features of the Animal Kingdom
      3. 27.2 Features Used to Classify Animals
      4. 27.3 Animal Phylogeny
      5. 27.4 The Evolutionary History of the Animal Kingdom
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    8. 28 Invertebrates
      1. Introduction
      2. 28.1 Phylum Porifera
      3. 28.2 Phylum Cnidaria
      4. 28.3 Superphylum Lophotrochozoa: Flatworms, Rotifers, and Nemerteans
      5. 28.4 Superphylum Lophotrochozoa: Molluscs and Annelids
      6. 28.5 Superphylum Ecdysozoa: Nematodes and Tardigrades
      7. 28.6 Superphylum Ecdysozoa: Arthropods
      8. 28.7 Superphylum Deuterostomia
      9. Key Terms
      10. Chapter Summary
      11. Visual Connection Questions
      12. Review Questions
      13. Critical Thinking Questions
    9. 29 Vertebrates
      1. Introduction
      2. 29.1 Chordates
      3. 29.2 Fishes
      4. 29.3 Amphibians
      5. 29.4 Reptiles
      6. 29.5 Birds
      7. 29.6 Mammals
      8. 29.7 The Evolution of Primates
      9. Key Terms
      10. Chapter Summary
      11. Visual Connection Questions
      12. Review Questions
      13. Critical Thinking Questions
  7. Plant Structure and Function
    1. 30 Plant Form and Physiology
      1. Introduction
      2. 30.1 The Plant Body
      3. 30.2 Stems
      4. 30.3 Roots
      5. 30.4 Leaves
      6. 30.5 Transport of Water and Solutes in Plants
      7. 30.6 Plant Sensory Systems and Responses
      8. Key Terms
      9. Chapter Summary
      10. Visual Connection Questions
      11. Review Questions
      12. Critical Thinking Questions
    2. 31 Soil and Plant Nutrition
      1. Introduction
      2. 31.1 Nutritional Requirements of Plants
      3. 31.2 The Soil
      4. 31.3 Nutritional Adaptations of Plants
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
    3. 32 Plant Reproduction
      1. Introduction
      2. 32.1 Reproductive Development and Structure
      3. 32.2 Pollination and Fertilization
      4. 32.3 Asexual Reproduction
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
  8. Animal Structure and Function
    1. 33 The Animal Body: Basic Form and Function
      1. Introduction
      2. 33.1 Animal Form and Function
      3. 33.2 Animal Primary Tissues
      4. 33.3 Homeostasis
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
    2. 34 Animal Nutrition and the Digestive System
      1. Introduction
      2. 34.1 Digestive Systems
      3. 34.2 Nutrition and Energy Production
      4. 34.3 Digestive System Processes
      5. 34.4 Digestive System Regulation
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    3. 35 The Nervous System
      1. Introduction
      2. 35.1 Neurons and Glial Cells
      3. 35.2 How Neurons Communicate
      4. 35.3 The Central Nervous System
      5. 35.4 The Peripheral Nervous System
      6. 35.5 Nervous System Disorders
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    4. 36 Sensory Systems
      1. Introduction
      2. 36.1 Sensory Processes
      3. 36.2 Somatosensation
      4. 36.3 Taste and Smell
      5. 36.4 Hearing and Vestibular Sensation
      6. 36.5 Vision
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    5. 37 The Endocrine System
      1. Introduction
      2. 37.1 Types of Hormones
      3. 37.2 How Hormones Work
      4. 37.3 Regulation of Body Processes
      5. 37.4 Regulation of Hormone Production
      6. 37.5 Endocrine Glands
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    6. 38 The Musculoskeletal System
      1. Introduction
      2. 38.1 Types of Skeletal Systems
      3. 38.2 Bone
      4. 38.3 Joints and Skeletal Movement
      5. 38.4 Muscle Contraction and Locomotion
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    7. 39 The Respiratory System
      1. Introduction
      2. 39.1 Systems of Gas Exchange
      3. 39.2 Gas Exchange across Respiratory Surfaces
      4. 39.3 Breathing
      5. 39.4 Transport of Gases in Human Bodily Fluids
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    8. 40 The Circulatory System
      1. Introduction
      2. 40.1 Overview of the Circulatory System
      3. 40.2 Components of the Blood
      4. 40.3 Mammalian Heart and Blood Vessels
      5. 40.4 Blood Flow and Blood Pressure Regulation
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    9. 41 Osmotic Regulation and Excretion
      1. Introduction
      2. 41.1 Osmoregulation and Osmotic Balance
      3. 41.2 The Kidneys and Osmoregulatory Organs
      4. 41.3 Excretion Systems
      5. 41.4 Nitrogenous Wastes
      6. 41.5 Hormonal Control of Osmoregulatory Functions
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    10. 42 The Immune System
      1. Introduction
      2. 42.1 Innate Immune Response
      3. 42.2 Adaptive Immune Response
      4. 42.3 Antibodies
      5. 42.4 Disruptions in the Immune System
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    11. 43 Animal Reproduction and Development
      1. Introduction
      2. 43.1 Reproduction Methods
      3. 43.2 Fertilization
      4. 43.3 Human Reproductive Anatomy and Gametogenesis
      5. 43.4 Hormonal Control of Human Reproduction
      6. 43.5 Human Pregnancy and Birth
      7. 43.6 Fertilization and Early Embryonic Development
      8. 43.7 Organogenesis and Vertebrate Formation
      9. Key Terms
      10. Chapter Summary
      11. Visual Connection Questions
      12. Review Questions
      13. Critical Thinking Questions
  9. Ecology
    1. 44 Ecology and the Biosphere
      1. Introduction
      2. 44.1 The Scope of Ecology
      3. 44.2 Biogeography
      4. 44.3 Terrestrial Biomes
      5. 44.4 Aquatic Biomes
      6. 44.5 Climate and the Effects of Global Climate Change
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    2. 45 Population and Community Ecology
      1. Introduction
      2. 45.1 Population Demography
      3. 45.2 Life Histories and Natural Selection
      4. 45.3 Environmental Limits to Population Growth
      5. 45.4 Population Dynamics and Regulation
      6. 45.5 Human Population Growth
      7. 45.6 Community Ecology
      8. 45.7 Behavioral Biology: Proximate and Ultimate Causes of Behavior
      9. Key Terms
      10. Chapter Summary
      11. Visual Connection Questions
      12. Review Questions
      13. Critical Thinking Questions
    3. 46 Ecosystems
      1. Introduction
      2. 46.1 Ecology of Ecosystems
      3. 46.2 Energy Flow through Ecosystems
      4. 46.3 Biogeochemical Cycles
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
    4. 47 Conservation Biology and Biodiversity
      1. Introduction
      2. 47.1 The Biodiversity Crisis
      3. 47.2 The Importance of Biodiversity to Human Life
      4. 47.3 Threats to Biodiversity
      5. 47.4 Preserving Biodiversity
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
  10. A | The Periodic Table of Elements
  11. B | Geological Time
  12. C | Measurements and the Metric System
  13. Index
By the end of this section, you will be able to do the following:
  • List the steps of replication and explain what occurs at each step
  • Describe the lytic and lysogenic cycles of virus replication
  • Explain the transmission of plant and animal viruses
  • Discuss some of the diseases caused by plant and animal viruses
  • Discuss the economic impact of plant and animal viruses

Viruses are obligate, intracellular parasites. A virus must first recognize and attach to a specific living cell prior to entering it. After penetration, the invading virus must copy its genome and manufacture its own proteins. Finally, the progeny virions must escape the host cell so that they can infect other cells. Viruses can infect only certain species of hosts and only certain cells within that host. Specific host cells that a virus must occupy and use to replicate are called permissive. In most cases, the molecular basis for this specificity is due to a particular surface molecule known as the viral receptor on the host cell surface. A specific viral receptor is required for the virus to attach. In addition, differences in metabolism and host-cell immune responses (based on differential gene expression) are a likely factor in determining which cells a virus may target for replication.

Steps of Virus Infections

A virus must use its host-cell processes to replicate. The viral replication cycle can produce dramatic biochemical and structural changes in the host cell, which may cause cell damage. These changes, called cytopathic effects, can change cell functions or even destroy the cell. Some infected cells, such as those infected by the common cold virus known as rhinovirus, die through lysis (bursting) or apoptosis (programmed cell death or “cell suicide”), releasing all progeny virions at once. The symptoms of viral diseases result both from such cell damage caused by the virus and from the immune response to the virus, which attempts to control and eliminate the virus from the body.

Many animal viruses, such as HIV (human immunodeficiency virus), leave the infected cells of the immune system by a process known as budding, where virions leave the cell individually. During the budding process, the cell does not undergo lysis and is not immediately killed. However, the damage to the cells that the virus infects may make it impossible for the cells to function normally, even though the cells remain alive for a period of time. Most productive viral infections follow similar steps in the virus replication cycle: attachment, penetration, uncoating, replication, assembly, and release (Figure 21.8).

Attachment

A virus attaches to a specific receptor site on the host cell membrane through attachment proteins in the capsid or via glycoproteins embedded in the viral envelope. The specificity of this interaction determines the host—and the cells within the host—that can be infected by a particular virus. This can be illustrated by thinking of several keys and several locks, where each key will fit only one specific lock.

Link to Learning

This video explains how influenza attacks the body.

Entry

Viruses may enter a host cell either with or without the viral capsid. The nucleic acid of bacteriophages enters the host cell “naked,” leaving the capsid outside the cell. Plant and animal viruses can enter through endocytosis (as you may recall, the cell membrane surrounds and engulfs the entire virus). Some enveloped viruses enter the cell when the viral envelope fuses directly with the cell membrane. Once inside the cell, the viral capsid degrades, and then the viral nucleic acid is released and becomes available for replication and transcription.

Replication and Assembly

The replication mechanism depends on the viral genome. DNA viruses usually use host-cell proteins and enzymes to replicate the viral DNA and to transcribe viral mRNA, which is then used to direct viral protein synthesis. RNA viruses usually use the RNA core as a template for synthesis of viral genomic RNA and mRNA. The viral mRNA directs the host cell to synthesize viral enzymes and capsid proteins, and assemble new virions.

Of course, there are exceptions to this pattern. If a host cell does not provide the enzymes necessary for viral replication, viral genes supply the information to direct synthesis of the missing proteins. Retroviruses, such as HIV (group VI of the Baltimore classification scheme), have an RNA genome that must be reverse transcribed into DNA, which then is incorporated into the host cell genome. To convert RNA into DNA, retroviruses must contain genes that encode the virus-specific enzyme reverse transcriptase that transcribes an RNA template to DNA. Reverse transcription never occurs in uninfected host cells—the enzyme reverse transcriptase is only derived from the expression of viral genes within the infected host cells. The fact that HIV produces some of its own enzymes not found in the host has allowed researchers to develop drugs that inhibit these enzymes without affecting the host’s metabolism.

This approach has led to the development of a variety of drugs used to treat HIV and has been effective at reducing the number of infectious virions (copies of viral RNA) in the blood to non-detectable levels in many HIV-infected individuals.

Egress

The last stage of viral replication is the release of the new virions produced in the host organism, where they are able to infect adjacent cells and repeat the replication cycle. As you’ve learned, some viruses are released when the host cell dies, and other viruses can leave infected cells by budding through the membrane without directly killing the cell.

Visual Connection

The illustration shows the steps of an influenza virus infection. In step 1, influenza virus becomes attached to a target epithelial cell. In step 2, the cell engulfs the virus by endocytosis, and the virus becomes encased in the cell's plasma membrane. In step 3, the membrane dissolves, and the viral contents are released into the cytoplasm. Viral m R N A enters the nucleus, where it is replicated by viral R N A polymerase. In step 4, viral m R N A exits to the cytoplasm, where it is used to make viral proteins. In step 5, new viral particles are released into the extracellular fluid. The cell, which is not killed in the process, continues to make new virus.
Figure 21.8 The influenza reproductive cycle. In influenza virus infection, glycoproteins on the capsid attach to a host epithelial cell. Following this, the virus is engulfed. RNA and proteins are then made and assembled into new virions.

Influenza virus is packaged in a viral envelope that fuses with the plasma membrane. This way, the virus can exit the host cell without killing it. What advantage does the virus gain by keeping the host cell alive?

Link to Learning

Watch a video on viruses, identifying structures, modes of transmission, replication, and more.

Different Hosts and Their Viruses

As you’ve learned, viruses often infect very specific hosts, as well as specific cells within the host. This feature of a virus makes it specific to one or a few species of life on Earth. On the other hand, so many different types of viruses exist on Earth that nearly every living organism has its own set of viruses trying to infect its cells. Even prokaryotes, the smallest and simplest of cells, may be attacked by specific types of viruses. In the following section, we will look at some of the features of viral infection of prokaryotic cells. As we have learned, viruses that infect bacteria are called bacteriophages (Figure 21.9). Archaea have their own similar viruses.

Bacteriophages

The micrograph shows hexagonal bacteriophage capsids attached to a host bacterial cell by slender stalks.
Figure 21.9 Bacteriophages attached to a host cell (transmission electron micrograph). In bacteriophage with tails, like the one shown here, the tails serve as a passageway for transmission of the phage genome. (credit: modification of work by Dr. Graham Beards; scale-bar data from Matt Russell)

Most bacteriophages are dsDNA viruses, which use host enzymes for DNA replication and RNA transcription. Phage particles must bind to specific surface receptors and actively insert the genome into the host cell. (The complex tail structures seen in many bacteriophages are actively involved in getting the viral genome across the prokaryotic cell wall.) When infection of a cell by a bacteriophage results in the production of new virions, the infection is said to be productive. If the virions are released by bursting the cell, the virus replicates by means of a lytic cycle (Figure 21.10). An example of a lytic bacteriophage is T4, which infects Escherichia coli found in the human intestinal tract. Sometimes, however, a virus can remain within the cell without being released. For example, when a temperate bacteriophage infects a bacterial cell, it replicates by means of a lysogenic cycle (Figure 21.10), and the viral genome is incorporated into the genome of the host cell. When the phage DNA is incorporated into the host-cell genome, it is called a prophage. An example of a lysogenic bacteriophage is the λ (lambda) virus, which also infects the E. coli bacterium. Viruses that infect plant or animal cells may sometimes undergo infections where they are not producing virions for long periods. An example is the animal herpesviruses, including herpes simplex viruses, the cause of oral and genital herpes in humans. In a process called latency, these viruses can exist in nervous tissue for long periods of time without producing new virions, only to leave latency periodically and cause lesions in the skin where the virus replicates. Even though there are similarities between lysogeny and latency, the term lysogenic cycle is usually reserved to describe bacteriophages. Latency will be described in more detail in the next section.

Visual Connection

The bacteriophage lytic cycle begins when the phage attaches via a slender stalk to the host cell. Linear DNA from the viral head is injected into the host cell. The phage DNA circularizes, remaining separate from the host DNA. The phage DNA replicates, and new phage proteins are made. New phage particles are assembled. The cell lyses, releasing the phage. The bacteriophage lysogenic cycle begins the same way as the lytic cycle, with phage infecting a host cell. However, the phage DNA becomes incorporated into the host genome. The cell divides, and phage DNA is passed on to daughter cells. Under stressful conditions, the phage DNA is excised from the bacterial chromosome and enters the lytic cycle.
Figure 21.10 A temperate bacteriophage has both lytic and lysogenic cycles. In the lytic cycle, the phage replicates and lyses the host cell. In the lysogenic cycle, phage DNA is incorporated into the host genome, where it is passed on to subsequent generations. Environmental stressors such as starvation or exposure to toxic chemicals may cause the prophage to excise and enter the lytic cycle.

Which of the following statements is false?

  1. In the lytic cycle, new phages are produced and released into the environment.
  2. In the lysogenic cycle, phage DNA is incorporated into the host genome.
  3. An environmental stressor can cause the phage to initiate the lysogenic cycle.
  4. Cell lysis only occurs in the lytic cycle.

Plant Viruses

Most plant viruses, like the tobacco mosaic virus, have single-stranded (+) RNA genomes. However, there are also plant viruses in most other virus categories. Unlike bacteriophages, plant viruses do not have active mechanisms for delivering the viral genome across the protective cell wall. For a plant virus to enter a new host plant, some type of mechanical damage must occur. This damage is often caused by weather, insects, animals, fire, or human activities like farming or landscaping. Movement from cell to cell within a plant can be facilitated by viral modification of plasmodesmata (cytoplasmic threads that pass from one plant cell to the next). Additionally, plant offspring may inherit viral diseases from parent plants. Plant viruses can be transmitted by a variety of vectors, through contact with an infected plant’s sap, by living organisms such as insects and nematodes, and through pollen. The transfer of a virus from one plant to another is known as horizontal transmission, whereas the inheritance of a virus from a parent is called vertical transmission.

Symptoms of viral diseases vary according to the virus and its host (Table 21.4). One common symptom is hyperplasia, the abnormal proliferation of cells that causes the appearance of plant tumors known as galls. Other viruses induce hypoplasia, or decreased cell growth, in the leaves of plants, causing thin, yellow areas to appear. Still other viruses affect the plant by directly killing plant cells, a process known as cell necrosis. Other symptoms of plant viruses include malformed leaves; black streaks on the stems of the plants; altered growth of stems, leaves, or fruits; and ring spots, which are circular or linear areas of discoloration found in a leaf.

Some Common Symptoms of Plant Viral Diseases
Symptom Appears as
Hyperplasia Galls (tumors)
Hypoplasia Thinned, yellow splotches on leaves
Cell necrosis Dead, blackened stems, leaves, or fruit
Abnormal growth patterns Malformed stems, leaves, or fruit
Discoloration Yellow, red, or black lines, or rings in stems, leaves, or fruit
Table 21.4

Plant viruses can seriously disrupt crop growth and development, significantly affecting our food supply. They are responsible for poor crop quality and quantity globally, and can bring about huge economic losses annually. Others viruses may damage plants used in landscaping. Some viruses that infect agricultural food plants include the name of the plant they infect, such as tomato spotted wilt virus, bean common mosaic virus, and cucumber mosaic virus. In plants used for landscaping, two of the most common viruses are peony ring spot and rose mosaic virus. There are far too many plant viruses to discuss each in detail, but symptoms of bean common mosaic virus result in lowered bean production and stunted, unproductive plants. In the ornamental rose, the rose mosaic disease causes wavy yellow lines and colored splotches on the leaves of the plant.

Animal Viruses

Animal viruses, unlike the viruses of plants and bacteria, do not have to penetrate a cell wall to gain access to the host cell. The virus may even induce the host cell to cooperate in the infection process. Non-enveloped or “naked” animal viruses may enter cells in two different ways. As a protein in the viral capsid binds to its receptor on the host cell, the virus may be taken inside the cell via a vesicle during the normal cell process of receptor-mediated endocytosis. An alternative method of cell penetration used by non-enveloped viruses is for capsid proteins to undergo shape changes after binding to the receptor, creating channels in the host cell membrane. The viral genome is then “injected” into the host cell through these channels in a manner analogous to that used by many bacteriophages.

Enveloped viruses also have two ways of entering cells after binding to their receptors: receptor-mediated endocytosis, or fusion. Many enveloped viruses enter the cell by receptor-mediated endocytosis in a fashion similar to that seen in some non-enveloped viruses. On the other hand, fusion only occurs with enveloped virions. These viruses, which include HIV among others, use special fusion proteins in their envelopes to cause the envelope to fuse with the plasma membrane of the cell, thus releasing the genome and capsid of the virus into the cell cytoplasm.

After making their proteins and copying their genomes, animal viruses complete the assembly of new virions and exit the cell. As we have already discussed using the example the influenza virus, enveloped animal viruses may bud from the cell membrane as they assemble themselves, taking a piece of the cell’s plasma membrane in the process. On the other hand, non-enveloped viral progeny, such as rhinoviruses, accumulate in infected cells until there is a signal for lysis or apoptosis, and all virions are released together.

As you will learn in the next module, animal viruses are associated with a variety of human diseases. Some of them follow the classic pattern of acute disease, where symptoms get increasingly worse for a short period followed by the elimination of the virus from the body by the immune system and eventual recovery from the infection. Examples of acute viral diseases are the common cold and influenza. Other viruses cause long-term chronic infections, such as the virus causing hepatitis C, whereas others, like herpes simplex virus, only cause intermittent symptoms. Still other viruses, such as human herpesviruses 6 and 7, which in some cases can cause the minor childhood disease roseola, often successfully cause productive infections without causing any symptoms at all in the host, and thus we say these patients have an asymptomatic infection.

In hepatitis C infections, the virus grows and reproduces in liver cells, causing low levels of liver damage. The damage is so low that infected individuals are often unaware that they are infected, and many infections are detected only by routine blood work on patients with risk factors such as intravenous drug use. On the other hand, since many of the symptoms of viral diseases are caused by immune responses, a lack of symptoms is an indication of a weak immune response to the virus. This allows the virus to escape elimination by the immune system and persist in individuals for years, all the while producing low levels of progeny virions in what is known as a chronic viral disease. Chronic infection of the liver by this virus leads to a much greater chance of developing liver cancer, sometimes as much as 30 years after the initial infection.

As already discussed, herpes simplex virus can remain in a state of latency in nervous tissue for months, even years. As the virus “hides” in the tissue and makes few if any viral proteins, there is nothing for the immune response to act against, and immunity to the virus slowly declines. Under certain conditions, including various types of physical and psychological stress, the latent herpes simplex virus may be reactivated and undergo a lytic replication cycle in the skin, causing the lesions associated with the disease. Once virions are produced in the skin and viral proteins are synthesized, the immune response is again stimulated and resolves the skin lesions in a few days or weeks by destroying viruses in the skin. As a result of this type of replicative cycle, appearances of cold sores and genital herpes outbreaks only occur intermittently, even though the viruses remain in the nervous tissue for life. Latent infections are common with other herpesviruses as well, including the varicella-zoster virus that causes chickenpox. After having a chickenpox infection in childhood, the varicella-zoster virus can remain latent for many years and reactivate in adults to cause the painful condition known as “shingles” (Figure 21.11).

Part a shows a micrograph of the varicella zoster virus, which has an icosahedral capsid surrounded by an irregularly shaped envelope. Part b shows a red, bumpy shingles rash on a person's face.
Figure 21.11 A latent virus infection. (a) Varicella-zoster, the virus that causes chickenpox, has an enveloped icosahedral capsid visible in this transmission electron micrograph. Its double-stranded DNA genome becomes incorporated in the host DNA and can reactivate after latency in the form of (b) shingles, often exhibiting a rash. (credit a: modification of work by Dr. Erskine Palmer, B. G. Martin, CDC; credit b: modification of work by “rosmary”/Flickr; scale-bar data from Matt Russell)

Some animal-infecting viruses, including the hepatitis C virus discussed above, are known as oncogenic viruses: They have the ability to cause cancer. These viruses interfere with the normal regulation of the host cell cycle either by introducing genes that stimulate unregulated cell growth (oncogenes) or by interfering with the expression of genes that inhibit cell growth. Oncogenic viruses can be either DNA or RNA viruses. Cancers known to be associated with viral infections include cervical cancer, caused by human papillomavirus (HPV) (Figure 21.12), liver cancer caused by hepatitis B virus, T-cell leukemia, and several types of lymphoma.

The micrograph shows an icosahedral virus with glycoproteins protruding from its capsid.
Figure 21.12 HPV, or human papillomavirus, has a naked icosahedral capsid visible in this transmission electron micrograph and a double-stranded DNA genome that is incorporated into the host DNA. The virus, which is sexually transmitted, is oncogenic and can lead to cervical cancer. (credit: modification of work by NCI, NIH; scale-bar data from Matt Russell)

Link to Learning

This video shows the life cycle of a virus.

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