Viruses: Structure and Classification

Basic Properties of Viruses

Viruses are microscopic infectious agents that require a host cell to replicate. They are significantly smaller than cells and act as vessels for genetic material.

• 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 contain complex capsids.

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

Viral Envelope and Host Range

Some viruses possess an additional structure called the viral envelope, which is derived from the host cell membrane and contains viral glycoproteins.

• Viral envelope: Accessory structure common in animal viruses, often derived from the membrane of host cells.

• Host range: The collection of hosts that the virus can enter and infect, determined by specific surface proteins that attach to receptors on the host cell.

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

Viral Infection and Replication

Entry and Genome Injection

Viral infection begins when the virus binds to the host cell, and the viral genome enters the cell.

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

• Other 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.

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

Example: Diagram showing viral RNA replication and assembly in a host cell.

Viral Life Cycles

Lytic Cycle

The lytic cycle is a viral replication process that culminates in the death of the host cell.

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

• 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

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

• The phage DNA is integrated into the bacterial chromosome as a prophage.

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

Example: Diagram comparing the lytic and lysogenic cycles in bacteriophages.

Animal Viruses and Retroviruses

Animal Virus Structure and 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 and Reverse Transcriptase

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

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

• Viral DNA is integrated 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

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

• They infect a wide array of organisms, except plant viruses and some smallpox viruses.

Double-Stranded RNA Viruses

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

• They infect a wide variety of organisms, including fungi, plants, vertebrates, bacteria, and insects.

Positive and Negative Sense RNA Viruses

RNA viruses are classified by the sense of their RNA genome.

• Positive sense RNA virus: Genome contains the same sequences needed to produce viral proteins; genome enters the cell and is immediately translated.

• Negative sense RNA virus: Genome contains 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: Diagram showing the classification of viral genomes (dsDNA, ssDNA, dsRNA, +ssRNA, -ssRNA, etc.).

Other Infectious Agents

Viroids

Viroids are the smallest known pathogens, consisting of short, circular, single-stranded RNA.

• Viroids are mostly plant pathogens and tend to disrupt growth.

• Viroids do not encode proteins but replicate in the host using host enzymes.

Example: Structure of a viroid RNA molecule.

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.

• Prion diseases include Creutzfeldt-Jakob disease and mad cow disease.

Example: Diagram showing conversion of normal PrPC protein to infectious PrPSc prion form.

Summary Table: Classification of Viral Genomes

Key Equations and Concepts

• Central Dogma of Molecular Biology (as applied to viruses):

• Reverse Transcription (Retroviruses):

• Viral Replication (Generalized):

Additional info: These notes are relevant for introductory biology and biochemistry courses, and may be referenced in General Chemistry when discussing biomolecules, molecular genetics, or chemical properties of nucleic acids and proteins.