Viruses: Structure, Function, and Impact in Biology

Viruses

Introduction to Viruses

Viruses are unique biological entities that function as obligate, intracellular parasites. They require a host cell to replicate and cannot carry out life processes independently. Outside the host, viruses exist as infectious particles known as virions.

• Virus: An obligate, intracellular parasite that depends on a host cell for replication.

• Host cell: The cell that provides biosynthetic machinery for viral replication.

• Virion: The infectious form of a virus outside the host cell.

• Viruses possess a genome, adapt to their environment, and evolve over time.

Size and Scale of Viruses

Viruses are much smaller than both eukaryotic and bacterial cells, typically ranging from 20 to 300 nanometers in diameter.

• Eukaryotic cell (e.g., human red blood cell): ~7 μm

• Bacterial cell (e.g., E. coli): ~2 μm

• Virus particles (e.g., HIV): ~0.1 μm

Viruses versus Organisms

Viruses differ fundamentally from cellular organisms in several key characteristics:

Characteristic	Viruses	Organisms
Hereditary material	DNA or RNA; can be single or double stranded	DNA; always double stranded
Plasma membrane present?	No	Yes
Can carry out transcription independently?	No	Yes
Can carry out translation independently?	No	Yes
Metabolic capabilities	Virtually none	Extensive (synthesis of ATP, vitamins, lipids, etc.)

Why Do Biologists Study Viruses?

Understanding viruses is essential for developing treatments and preventive measures against viral diseases.

• Vaccines: Activate the immune system against specific viral targets.

• Antiviral drugs: Directly interfere with viral replication.

Viruses Shape the Evolution of Organisms

Viruses play a significant role in the evolution and genetic diversity of organisms.

• Promote evolution of barriers and defenses against infection.

• Facilitate lateral gene transfer by introducing foreign genes into cellular genomes and shuttling genes between organisms.

• Approximately 8% of the human genome consists of viral remnants.

• Some viral genes are essential for normal development, such as those involved in placenta formation.

Human Organs and Systems Parasitized by Viruses

Viruses can infect a wide range of human organs and systems, including the respiratory, digestive, nervous, and immune systems.

Viruses Cause Disease

Viruses are responsible for some of the most devastating epidemics and pandemics in human history.

• Epidemic: Disease that rapidly infects many individuals over a wide area.

• Pandemic: Worldwide epidemic (e.g., Spanish flu of 1918–1919).

The Impact of Viral Epidemics on Human Populations

Viral epidemics can cause significant population declines and have shaped human history through widespread disease outbreaks.

Viruses and Disease: How Does HIV Cause AIDS?

HIV is a virus that primarily affects young adults and has killed millions worldwide. It targets and destroys helper T cells of the immune system, leading to AIDS.

• Loss of helper T cells impairs the immune response.

• Replacement of T cells cannot keep up with destruction, resulting in immune deficiency.

Studying Viruses

How Do Biologists Study Viruses?

Biologists use several approaches to study viruses, including:

• Analyzing the structure of the virion

• Examining the nature of the genetic material

• Investigating the mechanism of viral replication

Structure of the Virion

Viruses vary in size, shape, and whether they possess an envelope.

• Naked viruses: Enclosed only by a protein shell called a capsid.

• Enveloped viruses: Enclosed by both a capsid and one or more membranous envelopes.

Nature of the Genetic Material

Viral genomes are highly diverse and can be classified by several features:

• DNA or RNA

• Linear or circular

• Single molecule or segmented

• Single-stranded or double-stranded

Single-Stranded RNA Genomes

• Positive-sense: Genome sequence matches mRNA required for protein synthesis.

• Negative-sense: Genome sequence is complementary to viral mRNA.

Mechanism by Which the Virus Replicates

Viruses use various strategies to express their genetic material, all converging on the translation of mRNA.

• RNA replicase: Synthesizes RNA from an RNA template.

• Reverse transcriptase: Used by retroviruses to transcribe RNA to DNA, producing complementary DNA (cDNA) and double-stranded DNA (dsDNA) that integrates into the host genome.

Modes of Infection

• Replicative growth: Rapid production of virions, often killing the host cell.

• Dormant infection: Virion production is suspended, allowing coexistence with the host.

Viral Replicative Growth in Bacteria (Lytic Cycle)

The lytic cycle involves the following steps:

1. Attachment and entry into the host cell

2. Transcription and production of viral proteins

3. Replication of viral genome

4. Assembly of new virions

5. Exit from the host cell (often by bursting)

6. Transmission to new hosts

Lysogeny

Some bacteriophages can switch between the lytic cycle and lysogeny, a dormant state where the viral genome integrates into the host genome and replicates along with it.

Phases of Replicative Growth

• Attachment and entry

• Genome transcription and protein production

• Genome replication

• Assembly of new virions

• Exit from infected cell

• Transmission to new host

How Do Viruses Enter a Cell?

Viruses enter host cells by binding to specific receptors on the cell surface. Plant viruses may be inserted directly when the cell wall is disrupted.

• Binding to cell wall or plasma membrane receptor

• Uncoating of the viral genome

Enveloped Viruses and Fusion

Enveloped viruses bind to host membranes and uncoat via fusion, often involving specific host proteins such as CD4 (for HIV).

How Do Viruses Produce Proteins?

Viruses rely on the host cell's translational machinery to produce proteins from viral mRNAs.

• Envelope protein mRNAs: Translated on rough ER ribosomes

• Non-envelope protein mRNAs: Translated by free cytosolic ribosomes

How Do Viruses Copy Their Genomes?

Viruses use host or viral enzymes to replicate their genetic material.

• DNA viruses: Often use host-cell DNA polymerase

• RNA viruses: Supply their own RNA replicase

How Are New Virions Assembled?

Viral genomes are packaged into capsids, and enveloped viruses use the host endomembrane system for assembly.

Viruses Leave Infected Cells by Budding or Bursting

Viruses exit host cells either by budding (enveloped viruses) or by bursting (naked viruses), releasing new virions.

Transmission to New Hosts

Successful viruses are those that can replicate within a host and be transmitted to new hosts, ensuring their long-term survival.

Emerging Viruses and Genome Reassortment

Some viruses, such as influenza, can generate new strains through genome reassortment, especially when multiple virions infect the same cell.

• Influenza has a segmented RNA genome (eight segments).

• Genome segments can be shuffled, producing progeny with mixed genetic material.

• Annual updates to influenza vaccines are necessary due to frequent genetic changes.

Baltimore Classification of Viruses

Viruses are classified into Baltimore classes based on their genome type and replication strategy (DNA viruses, RNA viruses, and reverse-transcribing viruses).

Example:

• Class I: Double-stranded DNA viruses

• Class IV: Positive-sense single-stranded RNA viruses

• Class VI: Reverse-transcribing RNA viruses (e.g., HIV)

Additional info: The Baltimore classification system is widely used in virology to organize viruses by their genetic material and replication mechanisms.