Characteristics of Viruses

Definition and General Properties

Viruses are minuscule, acellular infectious agents that contain either DNA or RNA as their genetic material. They are responsible for a wide range of diseases in humans, animals, plants, and even bacteria. Viruses are unique in that they are not considered living organisms by all scientists, as they lack cellular structure and independent metabolism.

• Extracellular State (Virion): The virus exists as a complete particle, with a protein coat (capsid) surrounding its nucleic acid. Some virions possess a phospholipid envelope derived from the host cell membrane.

• Intracellular State: Once inside a host cell, the capsid is removed, and the virus exists as naked nucleic acid.

Are Viruses Alive?

The debate over whether viruses are alive centers on their lack of cellular structure and metabolism. Some scientists view them as complex pathogenic chemicals, while others consider them the simplest forms of life due to their ability to invade cells, control host machinery, and replicate.

Genetic Material of Viruses

Nature and Diversity of Viral Genomes

Viruses exhibit greater diversity in their genetic material than cellular organisms. Their genomes may be composed of DNA or RNA, but never both. This diversity is a primary criterion for virus classification.

• Types of Viral Genomes: Double-stranded DNA (dsDNA), single-stranded DNA (ssDNA), double-stranded RNA (dsRNA), or single-stranded RNA (ssRNA).

• Classification: The type of nucleic acid, presence of an envelope, shape, and size are used to classify viruses.

Hosts of Viruses

Host Specificity

Most viruses are highly specific, infecting only particular cells of a specific host species. Some, known as generalists, can infect multiple cell types or species.

Size and Morphology of Viruses

Relative Size of Viruses

Viruses are much smaller than most cells and organelles. Their size typically ranges from 20 nm to 300 nm, making them visible only with electron microscopy.

Capsid Morphology and Viral Shapes

The capsid is a protein shell that protects the viral genome and facilitates attachment to host cells. Viruses exhibit three basic shapes:

• Helical: Rod-shaped, with the genome coiled inside a helical capsid.

• Polyhedral: Many-sided, often icosahedral.

• Complex: Structures that do not fit into the other categories, such as bacteriophages.

The Viral Envelope

Some viruses possess an envelope derived from the host cell membrane during replication or release. This envelope contains viral proteins and glycoproteins that play a crucial role in host recognition and attachment.

Classification of Viruses

Taxonomic Criteria

Viruses are classified based on:

• Type of nucleic acid (DNA or RNA, single or double-stranded)

• Presence or absence of an envelope

• Shape (helical, polyhedral, complex)

• Size

Viral genera are grouped into families, but relationships among viruses are not as well understood as in cellular organisms.

Viral Replication

Entry into Host Cells

Viruses enter host cells through several mechanisms:

• Direct Penetration: The viral genome is injected directly into the host cell (e.g., poliovirus).

• Membrane Fusion: The viral envelope fuses with the host membrane, releasing the capsid (e.g., HIV, measles, SARS-CoV-2).

• Endocytosis: The host cell engulfs the virus in a vesicle (e.g., adenovirus, herpesvirus).

Lytic Replication Cycle

The lytic cycle is a replication process that results in the destruction (lysis) of the host cell. It consists of five stages:

1. Attachment

2. Entry

3. Synthesis

4. Assembly

5. Release

Lysogenic Replication Cycle

Some bacteriophages undergo lysogeny, where the viral genome integrates into the host chromosome and replicates along with it. The host cell survives and divides, passing the viral DNA to daughter cells. Environmental triggers can induce the lytic cycle.

Replication of Animal Viruses

Animal viruses follow similar replication steps as bacteriophages but differ due to the presence of envelopes, the eukaryotic nature of host cells, and the absence of a cell wall. Attachment is mediated by glycoprotein spikes or other molecules. DNA viruses often replicate in the nucleus, while RNA viruses replicate in the cytoplasm.

Latency in Animal Viruses

Some animal viruses can remain dormant (latent) within host cells for years. Latent viruses may or may not integrate into the host genome. If integrated, the provirus state is permanent.

The Role of Viruses in Cancer

Oncogenic Viruses

Some viruses can induce cancer by disrupting normal cell cycle regulation. They may insert oncogenes or disrupt tumor suppressor genes, leading to uncontrolled cell division (neoplasia). Viruses are implicated in 20–25% of human cancers, including Burkitt’s lymphoma, Hodgkin’s disease, Kaposi’s sarcoma, cervical cancer, and liver cancer.

Culturing Viruses in the Laboratory

Methods of Culturing Viruses

• In Mature Organisms: Viruses can be grown in bacteria, plants, or animals.

• In Embryonated Chicken Eggs: Eggs provide a sterile, nutrient-rich environment for viral growth and are commonly used for vaccine production.

• In Cell (Tissue) Culture: Cells isolated from organisms are grown in vitro. Two types of cultures are used: diploid cell cultures (from normal cells) and continuous cell cultures (from tumor cells).

Other Parasitic Particles: Viroids and Prions

Viroids

Viroids are extremely small, circular pieces of RNA that are infectious and pathogenic in plants. Unlike viruses, viroids lack a protein capsid.

Prions

Prions are infectious proteins that cause fatal neurodegenerative diseases. They induce abnormal folding of normal cellular proteins, leading to brain damage characterized by spongiform changes. Prion diseases are resistant to standard decontamination methods and can only be destroyed by incineration or autoclaving in 1 M NaOH.

Additional info: Prion diseases include Creutzfeldt-Jakob disease, mad cow disease (bovine spongiform encephalopathy), and scrapie in sheep.