Viruses: Structure and Classification

Introduction to Viruses

Viruses are microscopic infectious agents that are significantly smaller than cells. They require host cells to replicate and act as vessels for genetic material. Viruses infect a wide range of organisms, including bacteria, plants, and animals.

• Capsid: The protein coat covering the viral genome, which may take various forms.

• Capsomere: A subunit of the capsid.

• Genome: Viruses may contain double-stranded DNA, single-stranded DNA, double-stranded RNA, or single-stranded RNA.

• Bacteriophages: Viruses that infect bacteria and often have complex capsids.

Example: Bacteriophage structure showing DNA enclosed in a protein capsid.

Viral Envelope and Host Range

Some animal viruses possess an additional structure called the viral envelope, derived from the host cell membrane. The host range refers to the collection of hosts that a virus can infect, determined by specific surface proteins that recognize receptors on host cells.

• Viral envelope: Accessory structure common in animal viruses, often derived from host cell membranes.

• Host range: Determined by viral surface proteins and host cell receptors.

Example: Animal virus with glycoproteins embedded in the envelope, surrounding the capsid and genome.

Viral Infection and Replication

Entry and Infection Mechanisms

Viral infection begins when the virus binds to the host cell and its genome enters the cell. Entry mechanisms vary among viruses:

• Bacteriophages inject their genome into the host cell.

• Some viruses are absorbed by endocytosis.

• Others fuse their membranes with the host's membrane.

Example: Bacteriophage injecting viral DNA into a bacterial cell.

Viral Replication

Once inside, the virus hijacks the host's replicative machinery to produce viral components. The host provides nucleotides, enzymes, ribosomes, tRNA, amino acids, and ATP for viral replication.

• Nucleic acids and capsomeres are produced and assemble into new viruses.

Example: Diagram showing viral RNA replication and protein synthesis within a host cell.

Viral Life Cycles: Lytic and Lysogenic

Lytic Cycle

The lytic cycle involves phage replication that results in the death of the host cell.

• The phage inserts its DNA, degrades host DNA, and synthesizes viral components.

• Virulent phage: Replicates only by the lytic cycle.

• Bacteria may have restriction enzymes that degrade viral DNA.

Lysogenic Cycle

The lysogenic cycle allows the viral genome to replicate without killing the host.

• The phage DNA integrates into the host chromosome as a prophage.

• Temperate phage: Capable of replicating through both lytic and lysogenic cycles.

Example: Diagram comparing the lytic and lysogenic cycles, showing integration and replication of viral DNA.

Animal Viruses and Retroviruses

Animal Virus Structure and Replication

Animal viruses often have viral envelopes and RNA genomes. Replication involves entry via cell surface protein-receptor recognition.

• Viral RNA serves as a template for synthesis and replication by viral RNA polymerases.

Retroviruses and Reverse Transcriptase

Retroviruses contain RNA genomes and use reverse transcriptase to transcribe their genes into the host's DNA chromosome.

• Reverse transcriptase: Enzyme that catalyzes RNA to DNA transcription.

• Viral DNA integrates into the host genome.

Example: HIV replication cycle showing reverse transcription and integration into host DNA.

Viral Genomes: DNA and RNA Viruses

Double-Stranded DNA Viruses

These viruses enter the nucleus to replicate, often during the S phase of the cell cycle. They infect a wide array of organisms except plant viruses.

Double-Stranded RNA Viruses

These viruses enter the cytosol and use viral enzymes to replicate their genome. They infect a variety of organisms, including fungi, plants, vertebrates, bacteria, and insects.

Positive and Negative Sense RNA Viruses

• Positive sense RNA virus: Genome contains the same sequences needed to produce viral proteins; can be immediately translated.

• Negative sense RNA virus: Genome contains complementary sequences; must be transcribed by viral RNA polymerase before translation.

Retrovirus (+ssRNA)

Retroviruses use reverse transcriptase to transcribe their RNA genome into a DNA version, which integrates into the host genome.

Example: Diagram showing the classification of viral genomes (dsDNA, ssDNA, dsRNA, +ssRNA, -ssRNA, ssRNA-RT, dsDNA-RT).

Other Infectious Agents: Viroids and Prions

Viroids

Viroids are the smallest known pathogens, consisting of short, circular, single-stranded RNA. They mostly infect plants and disrupt growth. Viroids do not encode proteins but replicate using host enzymes.

Prions

Prions are infectious self-propagating proteins that cause brain diseases in animals. Prions can fold in multiple ways, some of which are transmissible to other proteins.

Example: Diagram showing prion conversion and accumulation in neural tissue.

Summary Table: Types of Infectious Agents

Additional info: These notes cover the biology and biochemistry of viruses and other infectious agents, which are relevant to molecular biology and biochemistry but not directly to General Chemistry. However, understanding viral structure and replication involves chemical principles such as macromolecular structure, enzyme function, and nucleic acid chemistry.