Viruses: Structure, Classification, and Life Cycles in Genetics

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Virus Structure and Morphology

Bacteriophage Structure

Bacteriophages are viruses that infect bacteria, exhibiting complex structures with distinct head and tail regions. The head contains the viral genome, while the tail facilitates attachment and genome injection into the host.

Capsid: Protein shell protecting the viral genome.
Tail fibers: Structures used for host recognition and attachment.
Genome: Can be DNA or RNA, single- or double-stranded.
Example: Phage T4 and Lambda are well-studied bacteriophages.

Bacteriophage structure and labeled parts

Virus Morphology and Symmetry

Viruses display a variety of shapes and symmetries, including icosahedral, helical, and complex forms. Symmetry is determined by the arrangement of capsid proteins.

Icosahedral symmetry: 20-sided structure, common in many animal viruses.
Helical symmetry: Rod-shaped viruses, such as tobacco mosaic virus.
Complex symmetry: Seen in bacteriophages with both icosahedral heads and helical tails.

Icosahedral virus symmetry and structure

Virus Life Cycles

Phage Lambda Life Cycle

Phage Lambda can undergo both lytic and lysogenic cycles. The lytic cycle results in host cell lysis and release of new virions, while the lysogenic cycle involves integration of the phage genome into the host DNA.

Lytic cycle: Viral replication leads to cell destruction.
Lysogenic cycle: Viral genome integrates and replicates with host DNA, remaining dormant until induction.
Decision between cycles: Controlled by regulatory proteins and environmental signals.

Phage lambda life cycle and regulatory steps

Rolling Circle Replication and Headful Packaging

Many bacteriophages replicate their genomes via rolling circle replication, producing long concatemers of DNA. Packaging into capsids often follows a headful mechanism, where DNA is cleaved and packed until the capsid is full.

Rolling circle replication: Produces multiple genome copies in a continuous strand.
Headful packaging: DNA is cut and packed into capsids, with each capsid receiving a 'headful' of DNA.

Headful packaging mechanism in bacteriophage DNA

Virus Classification

ICTV Classification Criteria

The International Committee on Taxonomy of Viruses (ICTV) classifies viruses based on several key criteria:

Genome composition: DNA or RNA, single- or double-stranded.
Capsid symmetry: Icosahedral, helical, or complex.
Envelope presence: Enveloped or non-enveloped.
Virion size: Measured in nanometers.
Host range: Specificity for certain organisms.

Baltimore Classification System

The Baltimore system groups viruses based on genome type and replication strategy, defining seven fundamental groups:

Group I: Double-stranded DNA viruses
Group II: Single-stranded DNA viruses
Group III: Double-stranded RNA viruses
Group IV: (+) sense single-stranded RNA viruses
Group V: (–) sense single-stranded RNA viruses
Group VI: Retroviruses (RNA reverse-transcribing viruses)
Group VII: Pararetroviruses (DNA reverse-transcribing viruses)

Baltimore virus classification groups and replication pathways

Viral Genomes

Genome Diversity

Viral genomes vary widely in size, structure, and composition, influencing their replication and infection strategies.

DNA or RNA: Viruses may have either type of nucleic acid.
Single- or double-stranded: Both forms are found among viruses.
Linear, circular, or segmented: Genome structure affects replication and packaging.

DNA virus genome types and structures RNA virus genome types and structures

Viroids and Prions

Viroids

Viroids are infectious agents composed solely of a short strand of circular, single-stranded RNA. They lack a protein coat and infect plants, causing various diseases.

No protein coat: Unlike viruses, viroids are naked RNA.
Plant pathogens: Cause diseases such as potato spindle tuber.

Viroid RNA structure

Prions

Prions are infectious proteins that cause neurodegenerative diseases by inducing abnormal folding of normal cellular proteins.

Protein-only: No nucleic acid component.
Diseases: Includes Creutzfeldt-Jakob disease and mad cow disease.
Abnormal conformation: Prions convert normal proteins into misfolded forms.

Normal and abnormal prion protein conformations

Ecological Roles of Viruses

Viruses in Ecosystems

Viruses play crucial roles in ecological systems, influencing population dynamics, nutrient cycling, and evolution.

Regulation of populations: Viruses control host populations, such as bacteria and algae.
Nutrient cycling: Viral lysis releases nutrients back into the environment.
Evolutionary impact: Viruses drive genetic diversity through horizontal gene transfer.

Ecological roles of viruses in algal blooms

Host Range and Viral Disease

Host Range

The host range of a virus refers to the spectrum of species it can infect. Some viruses are highly specific, while others infect multiple species.

Specificity: HIV infects only humans; West Nile virus infects birds and mammals.
Chronic vs. acute infections: Chronic infections are more common than acute diseases.
Antiviral drugs: Fewer available compared to antibiotics for bacteria.

Animal and Plant Viruses

Complexity of Eukaryotic Virus Life Cycles

Animal and plant viruses face challenges similar to bacteriophages but must navigate the more complex structure of eukaryotic cells. This results in greater diversity in replication cycles.

Host attachment: Viruses must recognize and bind to specific cell receptors.
Genome entry: Mechanisms include membrane fusion and endocytosis.
Gene expression: Strategies vary depending on genome type.
Virion assembly and release: May involve budding or cell lysis.

Tissue Tropism

Tissue tropism refers to the preference of animal viruses for specific tissues or cell types within a host.

Example: Influenza virus targets respiratory epithelium; hepatitis viruses target liver cells.

Are Viruses Alive?

Debate on Viral Life

Virologists debate whether viruses are alive, as they lack cellular organization and metabolism but possess heredity and the ability to evolve.

Noncellular: Viruses are not cells.
Dependence: Require host machinery for reproduction.
Evolution: Capable of evolving and adapting.

Summary Table: Baltimore Virus Classification

Group	Genome Type	Replication Strategy	Example Virus
I	dsDNA	Uses host or viral DNA polymerase	Herpesvirus
II	ssDNA	DNA polymerase generates complementary strand	Parvovirus
III	dsRNA	RNA-dependent RNA polymerase	Reovirus
IV	(+) ssRNA	Genome serves as mRNA	Poliovirus
V	(–) ssRNA	RNA-dependent RNA polymerase	Influenza virus
VI	ssRNA (RT)	Reverse transcription to DNA	HIV
VII	dsDNA (RT)	Reverse transcription	Hepadnavirus

Key Equations and Concepts

Rolling Circle Replication:
Virus Classification:

Additional info: Academic context was added to clarify virus structure, classification, and life cycles, as well as the ecological and evolutionary roles of viruses.