6.2 Replication
Fundamentals of Viral Replication and Metabolism
Metabolic Inactivity: Viruses do not possess their own metabolic processes or metabolic pathways. Consequently, they cannot replicate or reproduce independently.
Host Dependency: To construct new viral particles (virions), a virus must invade a host cell and utilize its raw materials, energetic machinery, and organellar infrastructure, including:
Amino acids
Nucleotides
Enzymes
Organelles
Terminology: The overall process of hijacking host machinery to create new virions is termed viral replication or the viral life cycle.
Clinical Significance: Understanding the individual biochemical and physiological steps of viral replication is essential for medical science. Pharmacologically interrupting any step along the replication pathway can halt viral propagation, thereby preventing, inhibiting, or resolving infection and viral spread within a patient.
Viral Tropism and Attachment Mechanisms
Attachment Definition: Attachment is the initial phase of infection where a virion anchors itself to the host cell's outer membrane.
Structural Mediators of Attachment:
Enveloped Viruses: Utilize specialized glycoprotein structures called spike proteins embedded in their outer membrane to recognize and bind to specific target proteins or receptors on the host plasma membrane. If the virion does not encounter its specific target receptor, attachment and subsequent infection cannot occur.
Naked (Non-Enveloped) Viruses: Lack a lipid envelope; instead, specialized surface proteins on the capsomeres—the protein subunits that assemble to form the protective capsid shell—bind directly to host cell surface structures.
Viral Tropism: Tropism refers to the specific affinity and targeting capability of a virus for particular host species, tissues, or cell types based on receptor availability.
Narrow Tropism: The virus can only attach to and infect a highly specific cell type or tissue within the host organism.
Example: A hepatotropic virus specifically targets liver tissue.
Broad Tropism: The virus can recognize target receptors found across multiple distinct cell types or tissues within the host.
Example: Ebola virus exhibits broad tropism, infecting diverse tissues throughout the host body.
Mechanisms of Viral Entry and Uncoating
Mechanisms of Entry in Animal Viruses: Animal viruses gain entry into the host cell cytoplasm through two primary pathways:
Membrane Fusion:
Exclusively used by enveloped viruses.
The virion attaches to target proteins on the host plasma membrane via its spike proteins.
The viral envelope (which originated from a host cell lipid membrane) fuses directly with the host cell's plasma membrane.
Fusion allows the internal viral core to pass directly into the host cytoplasm.
Endocytosis:
Utilized by both enveloped viruses and naked (non-enveloped) viruses.
The virion binds to specific target receptors on the host cell plasma membrane.
Receptor binding signals the host cell to engulf the virion, invaginating the membrane to enclose the entire virion inside an endocytic vesicle.
Direct Entry (Bacteriophages): A third distinct mode of entry called direct entry is utilized exclusively by bacteriophages—viruses that specifically infect prokaryotic bacterial cells.
Uncoating:
Uncoating is the enzymatic breakdown or degradation of the protective viral protein capsid, resulting in the liberation of viral nucleic acids (genome) and viral enzymes into the host cell.
Cytoplasmic Degradation: In membrane fusion or standard cytoplasmic delivery, host cytoplasmic enzymes degrade the capsid proteins.
Vesicular Uncoating: When entry occurs via endocytosis, enzymes present inside the endocytic vesicle degrade the capsid shell to release its genomic contents into the cytoplasm.
Nuclear Uncoating Exception: Certain DNA viruses do not undergo uncoating within the cytoplasm or endocytic vesicles. Instead, their intact or partially intact capsids are transported to the host cell nucleus, uncoating only after entering or reaching the nuclear membrane so that viral DNA can be directly transcribed and translated.
The Six-Step Replication Cycle of Animal Viruses
Attachment: The virion attaches to the surface of the host cell by specific binding between viral proteins (capsid capsomeres in naked viruses, or spike proteins in enveloped viruses) and host cell membrane target proteins/receptors.
Entry: The virion or its genomic contents pass into the interior cytoplasm of the host cell via membrane fusion or endocytosis.
Uncoating: Host or viral enzymes strip away the protein capsid, releasing the viral genome and associated functional proteins into the cytoplasm or nucleus.
Replication (Synthesis): The host cell machinery is hijacked to replicate the viral genome, transcribe viral genes into messenger RNA (), and translate into functional and structural viral proteins.
Assembly: Newly synthesized viral genomes, essential enzymes, and capsomere proteins spontaneously or enzymatically package together to form new, intact structural virions.
Release: Fully assembled virions exit the host cell to infect neighboring cells, occurring via budding or host cell lysis.
Intracellular Localization of Genome Replication
DNA Viruses: Translocate their DNA genome into the host cell nucleus. Host or viral machinery replicates the DNA and transcribes it into messenger RNA (). The then travels out into the host cytoplasm, where host ribosomes translate it into structural and functional viral proteins.
RNA Viruses: Replicate their genome and translate viral proteins directly within the host cell cytoplasm without entering the nucleus.
Retroviruses: Unique RNA viruses (such as HIV) that perform essential replicative steps inside the host cell nucleus using specialized enzymes.
Virion Release Mechanisms: Budding vs. Lysis
Budding:
Mechanism typical of enveloped viruses.
Prior to exit, viral envelope proteins are manufactured and inserted into specific regions of the host cell's plasma membrane.
The newly assembled capsid pushes outward against these modified membrane regions.
As the virion buds off, it wraps itself in the host cell's plasma membrane, picking up host lipid bilayer components and embedded host cell surface proteins along with the viral envelope proteins.
Example: Human Immunodeficiency Virus (HIV) budding from an infected white blood cell.
Immunological Advantage: By coating itself in host plasma membrane and host proteins, the newly formed enveloped virion mimics host tissue, allowing it to evade detection and destruction by the host immune system.
Lysis:
Mechanism typical of naked (non-enveloped) viruses.
Virions cause the host cell membrane to burst or lyse, killing the host cell.
Because the cell lyses, released naked virions do not acquire an outer lipid membrane or host membrane proteins.
Clinical Patterns of Viral Infection
Acute Infections:
The virus immediately initiates active replication upon entering the host cell.
Characterized by rapid onset of symptoms and fast disease progression.
Resolved rapidly as the host immune system clears the viral pathogen completely from the body.
Example: The common cold.
Persistent Infections:
Infections that are not rapidly cleared by the immune system, persisting in the host for months, years, or a lifetime.
Rely on altered replication strategies that allow the virus to remain in the body long-term.
Divided into latent persistent infections and chronic persistent infections.
Dynamics of Latent and Chronic Persistent Infections
Latent Persistent Infections:
Viral replication spikes initially during the primary infection and then drops significantly.
The host immune system cannot clear the virus from the body, allowing it to remain permanently present.
Exhibits prolonged periods of negligible or low virion replication known as quiet periods.
Chronic Persistent Infections:
Following initial infection, viral replication remains at a low baseline level for extended periods.
Viral load can stay low enough that the host experiences no noticeable symptoms and remains unaware of the active infection.
Transmission Hazard: Hosts remain actively infectious to others during low-replication quiet periods.
Over time, virion production can ramp up significantly, leading to severe cell damage and clinical disease progression.
Retroviral Integration, Proviruses, and Oncogenic Pathogens
Human Immunodeficiency Virus (HIV):
The causative agent of Acquired Immunodeficiency Syndrome (AIDS).
Follows a chronic persistent infection model.
Replication Cycle Details:
Attachment, entry, and uncoating occur in white blood cells.
As a retrovirus, HIV releases its single-stranded RNA along with a specialized enzyme called reverse transcriptase.
Reverse transcriptase converts the viral RNA into viral DNA.
Additional retroviral enzymes integrate the newly generated viral DNA directly into the host cell's nuclear DNA chromosome.
Integrated viral DNA within the host genome is called a provirus.
Proviral Latency and Progression:
Depending on the integration site within the host genome, a provirus can remain dormant or silent, producing minimal or no virions.
This dormancy maintains low viral replication, corresponding to the long quiet period on a chronic persistent infection graph.
Eventually (often years after initial infection), the provirus reactivates, initiating active transcription, genome replication, virion assembly, and viral release.
Reactivation leads to widespread destruction of host T lymphocytes (T cells), crippling host immune function and leading to AIDS.
Human Papillomavirus (HPV):
A persistent viral pathogen that establishes itself as a provirus within host cell DNA.
Proviral persistence can disrupt host cell growth regulation, directly causing malignant transformations.
HPV infection is a direct cause of cervical cancer as well as head and neck cancers.