Basic Properties of Viruses

Structure and Genetic Material

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.

• Viral Genome: 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

Envelope Structure and Host Specificity

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.

• Some viruses are absorbed into the host by endocytosis.

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 assembled 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

Structure and Replication

Animal virus infection 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.

Types of Viral Genomes and Replication Strategies

Double-Stranded DNA and RNA Viruses

Viruses can have different types of genomes, which determine their replication strategies.

• Double-stranded DNA virus: Enters the nucleus to replicate, often during S phase of the cell cycle.

• Double-stranded RNA virus: Enters the cytosol and uses viral enzymes to replicate its genome.

Example: Diagram showing transcription of viral DNA by RNA polymerase.

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; can be immediately translated.

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

• Retrovirus (+ssRNA): Uses reverse transcriptase to transcribe its genome into dsDNA, which integrates into the host genome.

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 mad cow disease and Creutzfeldt-Jakob disease.

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

Summary Table: Types of Viral Genomes

Key Equations and Concepts

• Central Dogma of Molecular Biology:

• Reverse Transcription (Retroviruses):

• Viral Replication:

Additional info: These notes cover the biology and chemistry of viruses and other infectious agents, which are relevant to biochemistry and molecular biology but may be included in General Chemistry courses as an introduction to biological macromolecules and their chemical properties.