6.1 Viruses
3 min read•june 18, 2024
Viruses are fascinating microorganisms that blur the line between living and non-living. These tiny parasites hijack host cells to replicate, causing a wide range of diseases. From the common cold to deadly pandemics, viruses shape our world in profound ways.
Understanding viral structure and life cycles is crucial for developing treatments and vaccines. By exploring how viruses enter cells, replicate, and spread, we can better combat these microscopic invaders and protect public health.
Viral Characteristics and Structure
Characteristics of viral pathogens
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- Non-living obligate intracellular parasites lack metabolic enzymes and ribosomes for protein synthesis rely on host cell machinery for replication ()
- Consist of nucleic acid (DNA or RNA) surrounded by a protein coat () some have an additional lipid derived from host cell membrane (HIV)
- Highly specific to host cell types due to receptor-mediated entry allows targeting of specific tissues or organs ()
- Capable of causing disease by disrupting host cell functions and inducing cell leading to tissue damage and symptoms ()
- Exhibit , which determines the specific cell types or tissues a virus can infect and replicate in
Structure of viral genomes
- Viral genomes can be DNA or RNA
- DNA viruses: double-stranded (dsDNA) or single-stranded (ssDNA) ()
- RNA viruses: double-stranded (dsRNA) or single-stranded (ssRNA) ()
- ssRNA can be positive-sense (+ssRNA) or negative-sense (-ssRNA)
- Genome size varies from a few thousand to hundreds of thousands of nucleotides smaller genomes often associated with higher mutation rates (influenza virus)
- Viral genomes can be linear or circular circular genomes more compact and stable ()
- Some viral genomes are segmented consisting of multiple nucleic acid molecules allows for genetic reassortment and increased diversity (influenza virus)
Viral Life Cycles and Host Interactions
Stages in viral life cycles
- Attachment: Viral surface proteins bind to specific receptors on the host cell (HIV protein binds to )
- Entry: Virus or viral genome enters the host cell through or membrane fusion (influenza virus enters via receptor-mediated endocytosis)
- Replication: Viral genome is replicated using host cell machinery
- DNA viruses typically replicate in the nucleus ()
- RNA viruses typically replicate in the cytoplasm ()
- Some RNA viruses undergo to produce DNA from their RNA genome
- Assembly: Viral proteins and nucleic acid are assembled into new virions (hepatitis B virus assembly in cytoplasm)
- Release: Mature virions are released from the host cell through lysis or budding (influenza virus budding from host cell membrane)
Bacteriophages vs plant and animal viruses
- Bacteriophages (phages) infect bacteria
- Lytic cycle: Phage replicates and lyses the host cell ()
- Lysogenic cycle: Phage genome integrates into the host genome and replicates with it ()
- Plant viruses often transmitted by insect vectors or mechanical damage cause symptoms such as leaf mottling stunting and reduced crop yields ()
- Animal viruses can cause a wide range of diseases from mild to severe or fatal transmission can occur through various routes such as respiratory droplets bodily fluids or vectors ()
Viruses as obligate intracellular parasites
- Viruses lack the necessary components for independent replication and metabolism
- They depend on host cell machinery for:
- Protein synthesis using host ribosomes and enzymes (influenza virus)
- Nucleic acid replication using host polymerases and nucleotides (hepatitis C virus)
- Energy production using host cell ATP and metabolic pathways (HIV)
- Viral replication often leads to host cell lysis or disruption of normal cellular functions causing disease symptoms (poliovirus)
- Some viruses can establish latent infections integrating their genome into the host genome without active replication allows long-term persistence in the host (herpes simplex virus)
Viral Evolution and Transmission
- occurs through gradual accumulation of mutations in viral surface proteins, allowing viruses to evade host immune responses
- refers to the transmission of viruses from animals to humans, which can lead to the emergence of new human pathogens
- are modified viruses used in gene therapy and vaccine development to deliver genetic material to target cells
Key Terms to Review (74)
Acellular: Acellular entities are those that do not consist of cells, such as viruses. They require a host cell to replicate and cannot carry out metabolic processes on their own.
Adenovirus: Adenoviruses are a group of non-enveloped, double-stranded DNA viruses that can infect a variety of host cells, including those in the human gastrointestinal tract. These viruses are known for their ability to cause a range of illnesses, from common colds to more severe respiratory and gastrointestinal diseases.
Antibiotics: Antibiotics are chemical substances used to kill or inhibit the growth of bacteria. They are a crucial tool in fighting bacterial infections in both humans and animals.
Antigenic Drift: Antigenic drift is a gradual, continuous process of genetic changes that occur in the surface proteins of viruses, particularly influenza viruses, leading to the emergence of new viral strains. This evolutionary mechanism allows the virus to evade the host's immune system and cause recurring infections.
Bacteriophage: A bacteriophage is a virus that infects and replicates within bacteria. They are composed of proteins that encapsulate a DNA or RNA genome.
Bacteriophage preparation: Bacteriophage preparation is the process of isolating, purifying, and amplifying bacteriophages for research or therapeutic use. Bacteriophages are viruses that specifically infect bacteria.
Baltimore Classification: The Baltimore classification is a system used to categorize viruses based on their genetic material and method of replication. It provides a standardized framework for understanding the fundamental characteristics of different types of viruses.
Baltimore classification system: The Baltimore classification system categorizes viruses based on their type of genome and their method of replication. It includes seven classes, each defined by whether the viral genome is DNA or RNA, single-stranded or double-stranded, and how it replicates.
Beijerinck: Martinus Beijerinck was a Dutch microbiologist who is considered one of the founders of virology. He is renowned for his discovery of viruses as a new infectious agent distinct from bacteria.
Biological vector: A biological vector is an organism that transmits pathogens from one host to another, where the pathogen undergoes part of its life cycle within the vector. Common examples include mosquitoes transmitting malaria and ticks spreading Lyme disease.
Capsid: A capsid is the protein shell of a virus that encases its genetic material. It plays a crucial role in protecting viral nucleic acids and aiding in their delivery into host cells.
Capsid: The capsid is the protein shell that encloses the genetic material of a virus. It is a crucial structural component that protects the viral genome and facilitates the virus's ability to infect host cells during the viral life cycle.
Capsomere: Capsomeres are the protein subunits that make up the capsid, which is the protective outer shell of a virus. They play a crucial role in the assembly and structural integrity of viruses.
CD4 Receptor: The CD4 receptor is a glycoprotein found on the surface of certain immune cells, particularly T helper cells. It plays a crucial role in the recognition and binding of the human immunodeficiency virus (HIV), the causative agent of acquired immunodeficiency syndrome (AIDS), during the viral infection process.
Chamberland: Chamberland is a type of porcelain filter invented by Charles Chamberland in 1884. It is used to remove bacteria from liquids, making it crucial for studying viruses and controlling microbial growth.
Chamberland filters: Chamberland filters are porcelain filters used to remove bacteria from liquids by passing the liquid through pores small enough to trap bacterial cells. These filters were pivotal in the discovery of viruses as entities smaller than bacteria.
Complex: In microbiology, 'complex' refers to a structural form of certain viruses that is neither purely helical nor purely icosahedral, often involving additional structures such as tail fibers or an envelope. These complex viruses can have intricate architectures designed for specific functions like host cell attachment and genome delivery.
Ebola Virus: The Ebola virus is a highly contagious and often fatal viral infection that primarily affects humans and other primates. It is a member of the Filoviridae family, known for its unique filamentous shape and ability to cause severe hemorrhagic fever. The Ebola virus is a significant public health concern due to its high mortality rate and the potential for rapid spread in the absence of effective treatment or vaccine.
Endocytosis: Endocytosis is the process by which cells bring materials into their interior by engulfing them with their cell membrane. It is a crucial mechanism for the uptake of nutrients, signaling molecules, and even pathogens, and is a key step in the viral life cycle.
Envelope: The envelope is a protective, lipid-containing structure that surrounds the capsid of certain viruses, including influenza, HIV, and coronaviruses. It is derived from the host cell's membrane and plays a crucial role in the virus's ability to infect and spread to new host cells.
Enveloped virus: An enveloped virus is a type of virus that has an outer lipid bilayer membrane derived from the host cell. This envelope contains viral glycoproteins essential for infection.
Fomites: Fomites are inanimate objects that can carry infectious agents and facilitate their transmission. Examples include doorknobs, utensils, and medical equipment.
Giant viral species: Giant viral species are unusually large viruses that can be as big as or bigger than small bacteria. They have complex genomes and can encode more proteins than typical smaller viruses.
Gp120: gp120 is a glycoprotein found on the surface of the human immunodeficiency virus (HIV), the causative agent of acquired immunodeficiency syndrome (AIDS). It plays a crucial role in the virus's ability to infect and enter human cells, making it a key virulence factor in the pathogenesis of HIV/AIDS.
H1N1 influenza: H1N1 influenza is a subtype of the influenza A virus known for causing respiratory infections in humans. It was responsible for the 2009 global pandemic, often referred to as 'swine flu.'
H2N2: H2N2 is a subtype of the Influenza A virus known for causing pandemics, including the Asian Flu in 1957. It is characterized by its hemagglutinin (H2) and neuraminidase (N2) surface proteins.
Helical: Helical refers to the spiral shape of some viral capsids, which are composed of protein subunits that form a cylindrical structure. This shape allows for efficient packaging of the viral genome.
Helical virus: A helical virus is a type of virus whose capsid (protein shell) is shaped in a rod-like, spiral structure. This shape allows the viral nucleic acid to be tightly wound inside the protein coat.
Hepatitis B Virus: The hepatitis B virus (HBV) is a highly contagious DNA virus that primarily infects the liver, causing inflammation and potential long-term liver damage. It is a significant global health concern, as chronic HBV infection can lead to cirrhosis and liver cancer if left untreated.
Herpes Simplex Virus: Herpes simplex virus (HSV) is a highly contagious virus that can cause a variety of infections, primarily affecting the skin, mucous membranes, and nervous system. It is a member of the Herpesviridae family and is a common cause of viral infections in humans, with two main types: HSV-1 and HSV-2.
Host range: Host range refers to the spectrum of host cells that a virus can infect. It is determined by specific interactions between viral proteins and host cell receptors.
Human Papillomavirus: Human papillomavirus (HPV) is a group of viruses that can infect the skin and mucous membranes, leading to a variety of diseases, including warts, precancerous lesions, and certain types of cancer. HPV is a significant topic in the context of viruses, the viral life cycle, the anatomy and microbiota of the urogenital tract, and viral infections of the reproductive system.
Icosahedral: Icosahedral is a geometric shape characterized by 20 equilateral triangular faces. Many viruses adopt this shape for their capsid, providing structural stability and efficient packaging of genetic material.
Immunofluorescent staining: Immunofluorescent staining is a technique that uses antibodies labeled with fluorescent dyes to detect specific antigens in biological samples. It is commonly used to visualize the presence and location of viruses within cells.
Influenza B virus: Influenza B virus is one of the primary types of viruses responsible for causing seasonal flu in humans. Unlike Influenza A, it primarily affects humans and is less likely to cause pandemics.
Influenza Virus: The influenza virus is a type of RNA virus that causes the contagious respiratory illness known as the flu. It is a member of the Orthomyxoviridae family and is a significant public health concern due to its ability to cause seasonal epidemics and occasional global pandemics.
International Classification of Diseases (ICD): The International Classification of Diseases (ICD) is a global health information standard for mortality and morbidity statistics. It is used by healthcare providers to classify diseases, including acellular pathogens like viruses.
International Committee on Taxonomy of Viruses (ICTV): The International Committee on Taxonomy of Viruses (ICTV) is responsible for the classification and naming of viruses. ICTV aims to standardize viral taxonomy to facilitate global communication and research.
International Union of Microbiological Societies: The International Union of Microbiological Societies (IUMS) is a global scientific organization dedicated to promoting the understanding and advancement of microbiology. It coordinates international collaboration and standardization in microbiological research.
Ivanovski: Ivanovski was a pioneering scientist in microbiology who discovered that infectious agents smaller than bacteria could cause disease. He is often credited with laying the groundwork for the field of virology.
Lambda Phage: Lambda phage is a type of bacteriophage, a virus that infects and replicates within bacterial cells. It is a well-studied model organism that has provided important insights into viral life cycles, genetic engineering, and the interactions between viruses and their bacterial hosts.
Latency: Latency refers to the period of time between the initial infection or exposure to a virus and the appearance of symptoms or the ability to detect the virus. It is a crucial concept in understanding the dynamics and characteristics of viral infections.
Lysis: Lysis is the process of cell rupture or disintegration, resulting in the release of the cell's contents. This term is particularly relevant in the context of viruses and their interaction with host cells during the viral infection cycle.
Measles Virus: The measles virus is a highly contagious RNA virus that belongs to the Morbillivirus genus within the Paramyxoviridae family. It is the causative agent of the infectious disease known as measles, which can lead to serious complications if left untreated.
Mechanical vector: A mechanical vector is an organism that transmits pathogens from one host to another without being infected itself. It often involves the physical transfer of infectious agents on the vector's body surface.
Multidrug resistant bacteria: Multidrug resistant bacteria are bacteria that have developed resistance to multiple antibiotics, making them difficult to treat. They pose a significant challenge in clinical settings and public health due to limited treatment options.
Naked virus: A naked virus is a virus that lacks an outer lipid envelope and consists only of a nucleocapsid. These viruses are typically more resistant to environmental conditions compared to enveloped viruses.
Nonenveloped virus: A nonenveloped virus is a type of virus that lacks a lipid bilayer surrounding its capsid. These viruses are typically more resistant to environmental factors and disinfectants.
Pasteur: Louis Pasteur was a pioneering French microbiologist and chemist renowned for his discoveries in the principles of vaccination, microbial fermentation, and pasteurization. His work laid the foundation for modern microbiology and helped disprove the theory of spontaneous generation.
Phage therapy: Phage therapy is the therapeutic use of bacteriophages to treat pathogenic bacterial infections. It leverages viruses that specifically target and destroy bacteria.
Poliovirus: Poliovirus is a highly contagious virus that primarily infects the gastrointestinal tract and can lead to the debilitating disease polio. It is a member of the Picornaviridae family and is the causative agent of the viral infection known as poliomyelitis.
Polyhedral: Polyhedral refers to a geometric shape with flat faces and straight edges, commonly seen in the structure of certain viruses. These shapes often have 20 equilateral triangular faces, known as icosahedrons.
Reverse transcriptase-polymerase chain reaction (RT-PCR): Reverse transcriptase-polymerase chain reaction (RT-PCR) is a laboratory technique used to detect and quantify RNA by converting it into complementary DNA (cDNA) and then amplifying specific DNA targets. It is commonly used in virology to diagnose and study RNA viruses.
Reverse Transcription: Reverse transcription is the process by which a retrovirus, such as HIV, converts its single-stranded RNA genome into a double-stranded DNA molecule that can be integrated into the host cell's genome. This allows the viral genetic material to be replicated and expressed along with the host cell's own genes.
Reverse zoonoses: Reverse zoonoses, also known as anthroponosis, occurs when diseases are transmitted from humans to animals. This phenomenon is particularly significant in the study of viruses that can infect multiple species.
Rhinovirus: Rhinoviruses are a group of small, non-enveloped RNA viruses that are the primary causative agents of the common cold. They are closely associated with infections of the upper respiratory tract and play a significant role in viral infections discussed in the context of the anatomy and normal microbiota of the respiratory tract, as well as viral infections of the respiratory tract.
Sheath: A sheath is a protective protein layer surrounding the nucleic acid of some viruses. It often plays a role in the injection of the viral genome into host cells.
Spike: Spikes are glycoprotein projections found on the surface of some viruses. They play a crucial role in the virus's ability to attach and enter host cells.
Stanley: Stanley is known for his pioneering work in crystallizing viruses, specifically the tobacco mosaic virus. His work laid the foundation for understanding the molecular structure of viruses.
T4 Phage: T4 phage is a type of bacteriophage, a virus that infects and replicates within bacterial cells. It is a lytic phage, meaning it lyses or breaks open the host cell to release new viral particles. T4 phage is a widely studied model organism for understanding viral life cycles and host-virus interactions.
Tail fiber: Tail fibers are protein structures that extend from the baseplate of a bacteriophage and play a crucial role in recognizing and binding to specific receptors on the surface of bacterial cells. They are essential for the initial stages of viral infection.
Tail pins: Tail pins are structural components of certain bacteriophages that help the virus attach to and penetrate the host bacterial cell wall. They play a crucial role in the infection process by facilitating the injection of viral DNA into the host.
Tobacco mosaic disease: Tobacco mosaic disease is a viral infection that affects plants, particularly tobacco and other members of the Solanaceae family. The disease is caused by Tobacco mosaic virus (TMV), which leads to distinctive mosaic-like patterns on the leaves.
Tobacco mosaic virus: Tobacco mosaic virus (TMV) is a non-enveloped, rod-shaped virus that infects plants, particularly tobacco and other members of the Solanaceae family. It is known for causing mosaic-like mottling on the leaves of infected plants.
Tobacco Mosaic Virus: Tobacco mosaic virus (TMV) is a plant virus that infects a wide range of plant species, particularly tobacco plants, and is one of the most well-studied and significant viruses in the field of virology. It is an icosahedral, positive-sense, single-stranded RNA virus that serves as a model organism for understanding viral structure, replication, and host-pathogen interactions.
Twort: Twort is a bacteriophage discovered by Frederick Twort in 1915. It infects and lyses bacterial cells.
Vector: A vector is an organism that transmits pathogens from one host to another, facilitating the spread of diseases. Common vectors include mosquitoes, ticks, and fleas.
Viral envelope: A viral envelope is a lipid bilayer that surrounds the capsid of some viruses, derived from portions of the host cell membranes. It often contains viral glycoproteins essential for infecting host cells.
Viral Tropism: Viral tropism refers to the specific affinity a virus has for certain host cells or tissues, which determines the sites of viral infection and replication. This concept is crucial in understanding the pathogenesis and tissue tropism of different viruses, as well as their potential for causing disease in various organs and systems.
Viral Vectors: Viral vectors are modified viruses that are used as vehicles to deliver genetic material, such as DNA or RNA, into target cells. These engineered viruses are designed to infect cells and introduce the desired genetic information without causing the original viral disease.
Virion: A virion is a complete virus particle that consists of an RNA or DNA core with a protein coat and, in some cases, other layers. It is the extracellular form of a virus that can infect host cells.
Virion: A virion is the complete, extracellular, infectious form of a virus. It consists of the viral genome, either DNA or RNA, enclosed within a protective protein coat called a capsid, and in some cases, an additional outer envelope derived from the host cell membrane.
Zoonoses: Zoonoses are infectious diseases that are transmitted between animals and humans. These diseases can be caused by viruses, bacteria, parasites, or fungi.
Zoonosis: Zoonosis refers to any infectious disease that can be transmitted from animals to humans. These diseases are caused by various pathogens, including parasites, viruses, bacteria, and fungi, that can cross the species barrier and infect human hosts.