Viruses: Structure and Classification

Basic Structure of Viruses

Viruses are microscopic infectious agents that require a host cell to replicate. They are significantly smaller than cells and consist of genetic material encased in a protein shell.

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

• Capsomere: A subunit of the capsid.

• 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 structures.

Example: Bacteriophage structure with a DNA core and protein capsid.

Viral Envelope and Host Range

Some viruses possess an additional lipid membrane 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 interactions between viral surface proteins and host cell receptors.

• Viral envelope: Accessory structure common in animal viruses, often derived from the host cell's membrane.

• Viruses identify host cells via surface proteins that attach to specific receptors on the host.

Example: Enveloped virus with glycoproteins, genome, capsid, and coat.

Viral Infection and Replication

Viral Entry and Genome Injection

Viral infection begins when the virus binds to the host cell, and the viral genome enters the cell. The method of entry varies among viruses:

• Some viruses, like bacteriophages, inject their genome into the host.

• Some viruses are absorbed into the host by endocytosis.

• Some viruses 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 from its own genes. The host provides nucleotides, enzymes, ribosomes, tRNA, amino acids, and ATP for viral replication.

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

Example: Diagram of viral replication inside a host cell, showing attachment, entry, synthesis, assembly, and release.

Viral Life Cycles

Lytic and Lysogenic Cycles

Viruses, especially bacteriophages, can follow two main replication cycles:

• Lytic cycle: Viral replication that culminates in the death of the host cell.

• Virulent phage: A phage that replicates only by the lytic cycle.

• Many bacteria have restriction enzymes that degrade viral DNA as a defense.

• Lysogenic cycle: Viral genome replicates without killing the host cell.

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

• Prophage: Viral DNA integrated into the bacterial chromosome.

Example: Diagram comparing the lytic and lysogenic cycles.

Animal Viruses and Retroviruses

Animal Virus Replication

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

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

Retroviruses

Retroviruses are RNA viruses that reverse transcribe their genes into the host's DNA chromosome using the enzyme reverse transcriptase.

• Reverse transcriptase: The enzyme that catalyzes RNA to DNA transcription.

• The viral DNA is integrated into the host genome.

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

Viral Genomes and Replication Strategies

DNA and RNA Viruses

Viruses can have various types of genomes, which determine their replication strategies:

• Double-stranded DNA viruses: Enter the nucleus to be replicated, often during S phase of the cell cycle.

• Double-stranded RNA viruses: Enter the cytosol and use viral enzymes to replicate their genome.

Viruses can infect a wide 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; genome enters the cell and is immediately translated into proteins.

• Negative sense RNA virus: Genome contains the complementary sequences to those coding for viral proteins; viral RNA polymerase must accompany the genome to transcribe RNA.

• Retrovirus (+ssRNA): Viral reverse transcriptase enters the cell with the genome and transcribes the dsDNA version of the genome.

Example: Baltimore classification of viruses based on genome type (dsDNA, ssDNA, dsRNA, +ssRNA, -ssRNA, etc.).

Other Infectious Agents

Viroids

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

Example: Diagram of viroid RNA structure.

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, leading to disease.

Example: Diagram showing conversion of normal PrPC protein to the infectious PrPSc form and its accumulation.

Summary Table: Types of Infectious Agents

Key Formulas and Concepts

• Baltimore Classification: Viruses are classified based on their genome type and replication method.

• Reverse Transcription:

• Central Dogma (for viruses): (with exceptions in retroviruses)

Additional info: This content is more relevant to introductory biology or microbiology than to general chemistry, but understanding the chemical nature of nucleic acids and proteins is foundational for biochemistry and molecular biology topics that may be covered in advanced chemistry courses.