Viruses
KNOWT GENERATED NOTES
Nature of Viruses
All viruses share a basic structure:
Nucleic acid core (either DNA or RNA) surrounded by a protein coat (capsid).
Viruses lack cytoplasm and other cellular structures, making them acellular.
Viral Structure
Capsid: Nearly all viruses have a protective protein sheath.
Made up of repeated individual proteins (capsomers).
Some viruses include specialized enzymes (e.g., reverse transcriptase).
Envelope: Many animal viruses possess an envelope derived from the host cell membrane, containing viral proteins.
Virus Size and Shape
Viruses exhibit significant diversity in size and shape.
Complex viruses often have unique symmetries (e.g., binal symmetry).
Viral Hosts
Viruses are obligate intracellular parasites, needing a host for replication.
Host Range: Specific types of organisms infected by each virus.
Attachment to specific cell receptors determines host range.
Tissue Tropism: Within a host, viruses may infect specific tissues (e.g., Rhabdovirus infects neurons).
Viral Replication Process
Virus acts as a set of instructions, tricking the host cell into producing more viruses.
Often leads to damage of the host cell.
Viruses cannot reproduce outside their host; outside they exist as inactive virions.
Lacks own ribosomes and enzymes; relies on host cell's mechanisms (transcription and translation).
Types of Viral Genomes
Viral genomes vary in:
Type of nucleic acid (DNA or RNA).
Number of strands (single-stranded or double-stranded).
Most RNA viruses are single-stranded and replicate in the host's cytoplasm, leading to higher mutation rates.
Retroviruses (like HIV): Single-stranded RNA that reverse-transcribes into double-stranded DNA using reverse transcriptase.
Important Human Viral Diseases Table
SARS
Pathogen: SARS-CoV, (+) single-stranded RNA, primarily spread via respiratory droplets.
Symptoms: Severe acute respiratory syndrome, ~3% mortality.
COVID-19
Pathogen: SARS-CoV-2, (+) single-stranded RNA, first identified in December 2019.
Symptoms: Fever, cough, fatigue, loss of taste or smell, etc.
Rabies
Pathogen: Rhabdovirus, (−) single-stranded RNA, transmitted through animal bites.
Fatal if untreated.
Virus Classification
Classification methods include:
Taxonomy by the International Committee on Taxonomy of Viruses (ICTV): Order, Family, Subfamily, Genus.
Based on the diseases they cause.
By the hosts they infect.
Baltimore Classification: Based on genome structure and expression relations.
Metagenomics and Viral Diversity
Advances in metagenomics allow the study of ecological roles of viruses.
Allows for studying viruses in diverse environments, revealing massive genetic diversity (e.g., seawater has 10x more viruses than bacteria).
Bacteriophage (Bacterial Viruses)
Infect bacteria and vary significantly; commonly referred to as phages.
Two main reproductive cycles:
Lytic Cycle: Virus attaches, penetrates, synthesizes components, assembles, and releases (lysates the cell).
Lysogenic Cycle: Viral nucleic acid integrates into host genome, allowing it to be replicated with the host's DNA during cell division.
Phage Conversion
During lysogenic cycles, some viral genes alter the host phenotype.
Example: Cholera toxin gene introduced into Vibrio cholerae converts harmless bacteria into pathogenic forms.
GOOGLE GEMINI GENERATED NOTES
I. The Nature of Viruses
Viruses are not cells; they lack cytoplasm.
A virus's basic structure consists of a nucleic acid core surrounded by a protein sheath called a capsid.
The nucleic acid core can be either DNA or RNA, which can be single-stranded or double-stranded, and linear or circular. RNA viruses may be segmented or not.
The capsid is composed of repeating units of one or a few proteins.
Some viruses have specialized enzymes within the nucleic acid core, such as reverse transcriptase.
Many animal viruses have an envelope derived from the host cell membrane and contain viral proteins.
Viruses vary in size and shape.
Viruses are obligate intracellular parasites, meaning they can only replicate inside living cells.
Viruses infect every kind of organism and have a limited host range, attaching to cells via specific receptors.
Inside a host, viruses may exhibit tissue tropism, infecting only certain tissues (e.g., rabies virus infects neurons).
Viral infection involves the virus hijacking the host cell's machinery to produce more viruses.
The viral genome "tricks" the host cell into making viruses, often damaging the cell in the process.
Outside of cells, viruses are metabolically inert and exist as virions.
Viral replication involves the virus hijacking the cell's transcription and translation machinery to express viral genes in a specific sequence (early, intermediate, and late genes), ultimately leading to the assembly and release of new viruses.
II. Viral Diversity
Viruses are classified based on various criteria, including taxonomy, the diseases they cause, the hosts they infect, and their genome expression.
The International Committee on Taxonomy of Viruses (ICTV) uses a hierarchical system (order, family, subfamily, and genus) for viral taxonomy.
Classification by disease or host has limitations because some viruses don't cause disease, some cause different diseases, and some infect different organisms.
Baltimore classification is a widely applicable system that categorizes viruses based on the relationship between their genome structure and expression.
Viral genomes vary greatly in nucleic acid type (DNA or RNA) and strand number (single- or double-stranded).
Most RNA viruses are single-stranded.
Retroviruses have a single-stranded RNA genome that is reverse-transcribed into double-stranded DNA using the enzyme reverse transcriptase (e.g., HIV).
Most DNA viruses are double-stranded and replicate in the host cell's nucleus (e.g., smallpox, herpes simplex).
Table 26.1 lists important human viral diseases, including:
Chickenpox, Hepatitis B, Herpes, Mononucleosis (caused by double-stranded DNA viruses)
Ebola, Influenza, Measles, Rabies (caused by single-stranded RNA viruses)
AIDS, Cervical and penile cancer, Zika virus disease, West Nile fever, SARS, COVID-19 (caused by single-stranded RNA viruses)
RNA viruses, such as those causing influenza, measles, and the common cold, replicate in the host cell's cytoplasm.
Replication of RNA viruses is error-prone, leading to high mutation rates, which makes it difficult to target them with the immune system, vaccines, and drugs.
Metagenomics has become a valuable tool for studying viral ecology and evolution, revealing the vast diversity of viruses, especially in prokaryotes and protists.
Metagenomics involves isolating DNA from environmental samples, amplifying and sequencing it, and comparing it to databases to identify new viruses.
Seawater contains a huge number of viruses, significantly contributing to genetic diversity.
III. Bacteriophage: Bacterial Viruses
Bacteriophages are viruses that infect bacteria.
They are diverse but share the common trait of using bacteria as their hosts.
E. coli*-infecting viruses (T series phages) are well-studied.
Bacteriophages have two reproductive cycles: lytic and lysogenic.
The lytic cycle involves:
Attachment of the virus to the bacterial cell wall.
Penetration or injection of the viral genome into the host cell.
Synthesis of viral components by taking over the host's replication and protein synthesis machinery.
Assembly of viral components.
Release of new virus particles through lysis (breaking down the host cell) or budding.
The lysogenic cycle is a latent phase where the virus doesn't immediately kill the host cell.
The viral nucleic acid integrates into the host cell's genome, becoming a prophage.
The prophage is replicated along with the host DNA as the host cell divides.
Temperate or lysogenic phages can undergo this cycle.
The cell containing a prophage is called a lysogen.
Under stress conditions, the prophage can be induced to exit the host chromosome and initiate the lytic cycle.
Phage conversion occurs when integrated viral genes in the lysogenic cycle alter the phenotype of the host bacterium.
For example, a lysogenic phage introduces a gene for cholera toxin into Vibrio cholerae, converting harmless bacteria into a disease-causing form