Gen Viro Chapter 2: General Virology – Viral Replication & Multiplication

Viral Replication

The biological process by which a virus hijacks host cell metabolic machinery to produce new viral particles.

Viral Attachment (Adsorption)

The initial infection phase where viral surface ligands (spikes/glycoproteins) bind specifically to host cell surface receptors.

Viral Ligands

Surface proteins or glycoproteins on the outer viral membrane/capsid that recognize and fit onto specific host cell receptors.

HIV Viral Ligand

gp120, a surface glycoprotein that specifically binds to the host primary CD4 receptor.

CD4 Receptor

The primary host cell surface receptor on T-helper lymphocytes targeted and bound by HIV gp120.

CCR5 Coreceptor

The essential secondary chemokine coreceptor on human T-cells required alongside CD4 for HIV entry.

Retrovirus Target Cell

T-helper lymphocytes expressing the CD4 antigen.

Viral Penetration

The process by which the viral particle or genome crosses the host plasma membrane into the cytoplasm.

Viropexis

A classic term describing viral penetration into the host cell via receptor-mediated endocytosis / vesicle engulfment.

Bacteriophage Penetration Mode

Direct injection of viral DNA through the bacterial cell wall while the protein capsid remains extracellular.

Retrovirus Penetration Mode

Injection/entry of viral RNA genetic material into the host cell cytoplasm.

Enveloped Virus Penetration Mechanism

The virion lipid envelope fuses directly with the host plasma membrane, allowing entry of the nucleocapsid.

Non-enveloped Virus Penetration Mechanism

Receptor-mediated endocytosis (pinocytosis) or molecular translocation across the host plasma membrane.

Translocation (Viral Entry)

A penetration mechanism where a naked virus directly crosses the plasma membrane into the cytoplasm.

Uncoating

The physical emptying or freeing of the viral genome from its protective capsid into the host cytoplasm or nucleus.

RNA Virus Replication Site

Most RNA viruses replicate in the host cell cytoplasm.

DNA Virus Replication Site

Most DNA viruses replicate in the host cell nucleus.

Poxvirus Replication Site Exception

Poxvirus is a DNA virus that replicates entirely in the cytoplasm rather than the nucleus.

Viral Multiplication

The overall process by which a virus hijacks a living host cell to produce multiple progeny copies.

Multiplication Stage 1: Adsorption

Attachment of viral surface spikes/ligands to specific host receptors.

Multiplication Stage 2: Penetration

The intact virus is engulfed into an intracellular vesicle or fuses with the cell membrane.

Multiplication Stage 3: Uncoating

Freeing of the viral genetic material into the host cell cytoplasm.

Multiplication Stage 4: Synthesis

Replication of the viral genome and synthesis of viral structural and non-structural proteins.

Multiplication Stage 5: Assembly

Newly synthesized spike proteins are inserted into host membranes and capsids assemble around genomes.

Multiplication Stage 6: Release

Newly assembled progeny virions exit the host cell to infect neighboring cells.

Positive-Sense (+) ssRNA

Viral RNA with the identical nucleotide sequence as host mRNA, allowing direct translation into proteins by host ribosomes.

Negative-Sense (-) ssRNA

Viral RNA complementary to mRNA that cannot be read directly and must first be transcribed into (+) mRNA.

Infectivity of (+) ssRNA Alone

Purified (+) ssRNA alone is infectious because host ribosomes can translate it immediately without virion enzymes.

Infectivity of (-) ssRNA Alone

Purified (-) ssRNA alone is non-infectious because host cells lack the transcriptase enzyme needed to copy it.

Positive-Sense ssRNA Viruses

Togaviruses and Picornaviruses.

Negative-Sense ssRNA Viruses

Arenaviruses, Bunyaviruses, Orthomyxoviruses, Paramyxoviruses, and Rhabdoviruses.

Virion Transcriptase

An RNA-dependent RNA polymerase packaged inside (-) ssRNA virions required to transcribe (-) RNA into (+) mRNA upon uncoating.

Replication Cycle of (+) ssRNA

(+) ssRNA translates proteins -> synthesizes complementary (-) template -> (-) template synthesizes multiple (+) progeny strands.

Replication Cycle of (-) ssRNA

(-) ssRNA is transcribed into (+) mRNA by virion transcriptase -> translated to proteins and templates new (-) genomes.

Retrovirus Replication Cycle

(+) ssRNA is reverse transcribed into cDNA by virion Reverse Transcriptase -> cDNA integrates into cellular DNA -> integrated provirus is transcribed and translated.

Reverse Transcriptase

A virion-carried RNA-dependent DNA polymerase that synthesizes complementary DNA (cDNA) from a viral RNA template.

Complementary DNA (cDNA)

Double-stranded DNA synthesized from a viral RNA template by reverse transcriptase.

Provirus

Retroviral cDNA that has integrated into the host cell chromosomal DNA.

Reovirus Multiplication (dsRNA)

dsRNA genome is transcribed by virion RNA polymerase; (+) mRNA translates proteins and pairs with (-) strands to assemble dsRNA virions.

Parvovirus Multiplication (ssDNA)

Replicates in the nucleus; uses host enzymes to synthesize complementary DNA forming dsDNA, which is then transcribed to mRNA and genomic ssDNA.

Papovavirus Replication Mechanism

dsDNA transported to the nucleus; uses host cell DNA polymerases for replication and cellular RNA polymerases for mRNA.

Herpesvirus & Adenovirus Replication Mechanism

dsDNA enters the nucleus; genome replication utilizes virally coded polymerases.

Poxvirus Multiplication Mechanism

Occurs in cytoplasm; virion is uncoated by cellular enzymes and early mRNA products; replicates DNA using virion-carried cytoplasmic enzymes.

Cytolytic Release

Release mechanism typical of naked viruses where host cell lysis ruptures the membrane and sacrifices the host cell.

Cytolytic Virus Examples

Togavirus, Paramyxovirus, and Rhabdovirus.

Cytopathic (Budding) Release

Release mechanism of enveloped viruses where virions acquire their envelope by budding through host membranes without immediate host cell lysis.

Enveloped Negative-Strand Release

Enveloped negative-strand RNA viruses (e.g., Retroviruses, Arenaviruses) exit the host cell via budding.

Poxvirus Envelopment Mechanism

Acquires an intracellular envelope from the Golgi complex and can acquire a second envelope at the plasma membrane via budding.

Poxvirus Non-enveloped Release

Released without an envelope by mechanical disruption/lysis of the host cell.

Poxvirus Virion Infectivity

Both enveloped and non-enveloped forms of Poxvirus are infectious and undergo intracellular/intranuclear multiplication.