Viral Life Cycles, Replication Strategies (2B) - MCAT Biological and Biochemical Foundations of Living Systems

Attachment (adsorption) to host receptors. Specific binding to host receptors initiates infection by facilitating viral entry into susceptible cells.

Entry, uncoating, replication, assembly, release. This sequence follows attachment, enabling genome delivery, replication using host systems, virion formation, and exit from the cell.

Removal of the capsid to release the viral genome. This process exposes the viral nucleic acid, allowing access to host machinery for transcription and replication.

Lytic kills quickly; lysogenic integrates and can remain latent. Lytic cycles lead to rapid replication and host cell death, while lysogenic cycles involve genome integration for potential dormancy.

Viral genome integrated into a eukaryotic host genome. This integration allows latent infection in eukaryotic cells, as seen in retroviruses like HIV.

Reverse transcriptase (RNA-dependent DNA polymerase). This enzyme synthesizes complementary DNA from the viral RNA template, a key step in retroviral replication.

Insertion of viral DNA into the host genome. Integrase catalyzes the covalent insertion of viral cDNA into the host chromosome, forming a stable provirus.

Class I: double-stranded DNA (dsDNA). Baltimore Class I viruses, like herpesviruses, use dsDNA genomes that can be transcribed by host polymerases.

Class II: single-stranded DNA (ssDNA). Baltimore Class II viruses, such as parvoviruses, replicate via ssDNA that is converted to dsDNA in the host nucleus.

Class III: double-stranded RNA (dsRNA). Baltimore Class III viruses, like reoviruses, use dsRNA genomes requiring viral polymerase for mRNA synthesis.

Class IV: positive-sense single-stranded RNA (+ssRNA). Baltimore Class IV viruses, such as picornaviruses, have +ssRNA genomes that function directly as mRNA.

Class V: negative-sense single-stranded RNA (-ssRNA). Baltimore Class V viruses, like orthomyxoviruses, require viral RdRp to transcribe -ssRNA into mRNA.

Class VI: +ssRNA with reverse transcription (retroviruses). Baltimore Class VI viruses, including retroviruses, use reverse transcription to convert +ssRNA to DNA for integration.

Class VII: dsDNA with reverse transcription (pararetroviruses). Baltimore Class VII viruses, such as hepadnaviruses, employ reverse transcription during replication of their dsDNA genomes.

+ssRNA is mRNA; -ssRNA must be transcribed first. Positive-sense RNA can be directly translated by host ribosomes, whereas negative-sense requires prior transcription to mRNA.

-ssRNA and dsRNA viruses. Host cells lack RNA-dependent RNA polymerase, so these viruses must package it to transcribe their genomes upon entry.

Host ribosomes translate the genome directly into protein. The +ssRNA genome acts as mRNA, enabling immediate synthesis of viral proteins by host translational machinery.

Budding through a host membrane. Budding allows acquisition of the lipid envelope from host membranes while often preserving host cell integrity.

Host cell lysis. Accumulation of virions inside the cell leads to membrane rupture, releasing nonenveloped viruses into the environment.

Their lipid envelope is disrupted by detergents and desiccation. The lipid bilayer is susceptible to environmental factors that disrupt membrane integrity, inactivating the virus.

It cannot initiate infection; it must package RNA-dependent RNA polymerase. Negative-sense ssRNA requires transcription to mRNA by RdRp, which must be virion-packaged since hosts lack this enzyme.

This is a lysogenic/latent strategy (provirus or prophage). This integration enables latent persistence and replication with host DNA without causing immediate cell death.

It must use host ribosomes and metabolism to replicate. Viruses lack their own translational and metabolic machinery, necessitating host cell resources for genome replication and protein synthesis.

Capsid proteins or envelope glycoprotein spikes. These proteins recognize and interact with host cell receptors, conferring specificity for cell types and species.