Detailed Notes on Viruses and Prions
Section 7.1: Introduction to Viruses
Learning Outcomes:
Identify terms better than 'alive' or 'dead' for viruses.
List characteristics that distinguish viruses from cellular life.
Introduction to Viruses
Viruses are extremely abundant and can infect all types of cells, including:
Bacteria
Algae
Fungi
Protozoa
Plants
Animals
The Nature of Viruses
Key Questions Regarding Viruses:
Are they living organisms?
What was their role in the evolution of life?
What are their biological characteristics?
How do such small particles cause disease?
What is their connection to cancer?
Infective Biological Entities
Active vs. Inactive:
Viruses are better described as active or inactive rather than alive or dead.
Considered non-living because they cannot multiply independently.
Obligate Intracellular Parasites:
Require a host cell to multiply, directing the host’s machinery to produce new viruses.
Properties of Viruses
Characteristics:
Non-cellular
Obligate intracellular parasites
Active only inside host cells
Basic structure:
Protein shell (capsid) surrounding nucleic acid core
Size ranging from 20 nm to 1,500 nm
Contain either DNA or RNA (not both)
Can be:
Double-stranded DNA
Single-stranded DNA
Single-stranded RNA
Double-stranded RNA
Specific surface molecules for host cell attachment
Lack the enzymes and machinery for metabolism and protein synthesis.
Size Comparison of Viruses
Sizes of various viruses and bacteria compared:
E. coli: 2 μm
Streptococcus: 1 μm
Mimivirus: 1 μm
Influenza virus: 100 nm
Rabies virus: 125 nm
Polio virus: 30 nm
Virus Particle Structure
Essential Structure:
Nucleic acid core (DNA or RNA)
External protein coat (capsid)
Some viruses have enzymes.
Viral Components
Capsid and Nucleocapsid:
Capsid: protective shell
Nucleocapsid: capsid + nucleic acid
Envelope:
May be derived from the host cell's membrane, not present in all viruses.
Spikes:
Projections that aid in host cell docking.
Generalized Structure of Viruses
Virion:
Fully formed virus capable of infecting a host.
Icosahedral Capsids
Characteristics:
Formed in the shape of an icosahedron (20-sided figure).
Vary in the number of capsomeres (e.g., poliovirus has 32).
Complex Capsids
Complex Structure:
Found in bacteriophages, characterized by asymmetrical shapes and multiple proteins.
The Viral Envelope
Composed of the host's membrane system, enveloped viruses utilize spikes for attachment.
Nucleic Acids: At the Core of a Virus
Genome:
The complete set of nucleic acids in a virus (either DNA or RNA).
Viral gene quantity is generally smaller than in cellular organisms.
Important Human Virus Families—DNA Viruses
Examples:
Herpesviridae (e.g., HSV-1, HSV-2) - Causes herpes.
Papillomaviridae - Human papillomavirus causes warts.
Important Human Virus Families—RNA Viruses
Examples:
Picornaviridae (Poliovirus) causes poliomyelitis.
Retroviridae (HIV) causes AIDS.
Multiplication Cycles in Animal Viruses
Phases of Life Cycle:
Adsorption
Penetration
Uncoating
Synthesis
Assembly
Release
Adsorption and Host Range
Viruses attach to specific host molecules on cell membranes, with ranges varying from specific (Hepatitis B) to broad (Rabies).
Penetration and Uncoating of Animal Viruses
Method of Entry:
Endocytosis: engulfed in a vesicle; uncoating releases the virus into the cytoplasm.
Synthesis: Genome Replication
Viral nucleic acid takes control over host cell machinery, differing between RNA and DNA viruses:
RNA viruses replicate in the cytoplasm
DNA viruses replicate in the nucleus.
Release of Mature Viruses
Mechanism:
Enveloped viruses exit by budding, while others may cause lysis in the host cell.
Viruses and Cancer
Oncogenic viruses account for approximately 13% of human cancers.
Viruses may carry genes that cause cancer or induce cell growth loss regulation.
Bacteriophages
Viruses that infect bacteria, important in studies of viral pathology.
Techniques in Cultivating and Identifying Animal Viruses
Methods:
In vivo (lab animals) and in vitro (cell cultures) cultivation.
Importance of Viral Study in Health
Viral infections are common causes of acute illness; some have high mortality rates like Rabies and Ebola.
Other Noncellular Infectious Agents: Prions and Viroids
Prions:
Protein misfolding leading to transmissible diseases (e.g., CJD).
Viroids:
Plant pathogens made of RNA without a capsid, significant for agriculture.
Conclusion
Understanding viruses requires knowledge of their structure, reproduction, and impact on health, paving the way for effective treatments and preventive measures against viral infections.
quiz
A virus is a tiny particle
Viruses are known to infect all organisms
The Nucleic acid of a virus is either DNA or RNA
The general steps in a viral multiplication cycle are absorption, penetration, synthesis, assembly, and release
A prophage is a stage in the development of an/an a bacterial virus
In general, RNA viruses multiply in the cell cytoplasm and DNA viruses multiply in the nucleus
Viruses cannot be cultivated in/an blood agar
Clear patches in cell culture that indicate sites of virus infection are called plaques
In Lysogeny, viral DNA is inserted into the host chromosome, - True
A viral Capsid is composed of subunits called virions- False
The envelope of an animal virus is derived from the peptidoglycan - False
The nucleic acid of animal viruses enters the cell through a process call translocation -False
Viruses that persist in the host cell and cause recurrent disease are called latent- True.