Viral Replication: Lytic vs Lysogenic Cycles | MCAT Biology

Transduction is a bacteriophage-mediated gene transfer mechanism in bacteria where phages accidentally transfer bacterial DNA fragments between cells; generalized transduction transfers any random DNA fragment during the lytic cycle when phage packaging mistakenly includes bacterial DNA, while specialized transduction transfers specific DNA fragments near the integration site during prophage excision from the bacterial chromosome, both processes enabling horizontal gene transfer that provides bacteria with new genetic material for antibiotic resistance, metabolic capabilities, and environmental adaptation.

Bacterial transduction is a horizontal gene transfer mechanism where bacteriophages (viruses that infect bacteria) transfer bacterial genes between cells. It occurs through two mechanisms: generalized transduction during the lytic cycle, where random bacterial DNA fragments are accidentally packaged into viral particles and transferred to new hosts; and specialized transduction during the lysogenic cycle, where improper excision of the viral genome carries specific bacterial DNA sequences near the integration site. A key distinction from transformation is that transduction always transfers double-stranded DNA, whereas transformation involves single-stranded DNA uptake.

Generalized transduction involves lytic phages packaging random bacterial DNA fragments during viral particle assembly, allowing any gene segment to be transferred between bacteria; specialized transduction occurs when lysogenic phages accidentally package bacterial DNA adjacent to their integrated viral genome during the switch from lysogenic to lytic cycle, transferring specific bacterial gene fragments along with viral DNA.

Transduction transfers bacterial DNA via bacteriophages. Generalized transduction occurs when lytic phages accidentally package random bacterial DNA fragments during the lytic cycle. Specialized transduction occurs when temperate phages (like lambda) integrate into the chromosome and excise incorrectly, taking adjacent bacterial genes. Generalized transduction transfers random DNA fragments, while specialized transduction transfers specific genes adjacent to the integration site. Both processes contribute to horizontal gene transfer and bacterial evolution.

Generalized transduction transfers any bacterial DNA fragment (chromosomal or plasmid) through random packaging during viral assembly. Specialized transduction occurs when prophage excision accidentally carries adjacent bacterial genes, transferring specific genes to recipient bacteria. Both mechanisms enable horizontal gene transfer between bacteria.

Bacteria evolved defense mechanisms against viral infection including receptor reduction, polysaccharide coat secretion, and restriction-modification systems that cleave foreign DNA. The CRISPR-Cas system provides adaptive immunity: during adaptation, foreign DNA is cleaved into protospacers and incorporated into the CRISPR array; during interference, these spacers guide Cas proteins to recognize and cleave matching viral DNA. This system has been adapted for gene editing technology.

Bacteria use restriction-modification systems as defense against viral infections. When bacteriophage (bacterial virus) DNA enters a bacterial cell, restriction endonucleases recognize and cut the foreign viral DNA. The bacterial genome is protected through methylation at recognition sites, marking it as 'self.' This system distinguishes between bacterial DNA (protected) and viral DNA (destroyed), providing innate immunity to bacterial cells.

Restriction-modification systems are the most common bacterial immune systems, consisting of restriction endonucleases that cleave unprotected foreign DNA and modification enzymes that protect host DNA through methylation. CRISPR-Cas systems provide adaptive immunity through a two-stage process: adaptation (integration of phage DNA fragments into the CRISPR locus) and interference (guide RNAs direct Cas proteins to cleave complementary phage DNA). These systems carry risks of autoimmunity, limiting their accumulation. Toxin-antitoxin systems, originally studied for plasmid stability, also contribute to antiviral defense by being activated during viral infection when host transcription is inhibited.

The restriction-modification system is a bacterial defense mechanism against viral infection. Bacteria produce restriction enzymes that recognize and cut specific DNA sequences. The bacteria's own DNA is protected by methylation at these recognition sites, preventing self-destruction. This system allows bacteria to distinguish between self and foreign DNA, providing protection against bacteriophage infection.

Bacteria face constant viral infection pressure from bacteriophages, which are the most abundant entities on Earth (10 times more than bacteria). Bacteria have evolved two main immune arms: CRISPR-Cas provides adaptive immunity by memorizing past infections through stored viral DNA fragments that block future infections; restriction-modification systems provide innate immunity using restriction enzymes that cleave foreign DNA while modification enzymes protect bacterial DNA. These systems represent fundamental strategies that bacteria use to survive in environments teeming with viruses.

Antiviral drugs inhibit viral replication at any step of the viral life cycle. The complete sequence of viral replication includes: (1) Fusion of the virus with the host cell membrane and penetration inside the cell; (2) Uncoating and early protein synthesis; (3) Nucleic acid synthesis; (4) Late protein synthesis and processing; (5) Packaging and assembly of viral particles; (6) Release of new viral particles to infect other cells. Different classes of antiviral drugs target each specific step in this process.

Antiviral drugs inhibit viral reproduction by targeting specific stages of the viral life cycle, including attachment/inhibition (e.g., CCR5 antagonists for HIV), uncoating (e.g., influenza inhibitors), reverse transcriptase inhibition (nucleoside/nucleotide analogs and non-nucleoside inhibitors for HIV), integrase inhibition (for retroviruses), DNA polymerase inhibition (for DNA viruses), protease inhibition (for HIV), neuraminidase inhibition (for influenza), and antisense therapy (targeting viral mRNA).

Antiviral drugs target specific stages of the viral replication cycle: (1) Entry inhibitors (amantadine) block viral uncoating by binding to the M2 protein of influenza A virus; (2) Nucleoside/nucleotide analogs (acyclovir, lamivudine, tenofovir, sofosbuvir) mimic natural nucleotides and are incorporated into viral DNA or RNA during replication, causing chain termination; (3) Protease inhibitors (ritonavir, darunavir) prevent the maturation of viral proteins by blocking the viral protease enzyme; (4) Neuraminidase inhibitors (oseltamivir, zanamivir) block the neuraminidase enzyme on the surface of influenza viruses, preventing viral release from infected cells; (5) Immunomodulators (interferon-alpha) enhance host immune response against viruses. Nucleoside analogs require phosphorylation by viral kinases to become active, while nucleotide analogs are already phosphorylated and do not require activation.

Viruses are unique entities between living and non-living organisms, lacking cell walls, cell membranes, and nuclei. They consist of a protein coat, nucleic acid, and replication enzymes. The viral replication cycle involves six sequential steps: (1) Fusion/Entry into host cell, (2) Uncoding of outer layers, (3) Early protein synthesis, (4) Nucleic acid synthesis, (5) Late protein synthesis, and (6) Assembly and release of new virus particles. Antiviral drugs target specific stages: fusion inhibitors (enfuvirtide, T-20) prevent virus-host cell membrane fusion; amantadine and rimantadine block influenza virus uncoating; CCR5 receptor blockers (maraviroc) interfere with viral attachment; acyclovir, famciclovir, ganciclovir, and valacyclovir are nucleoside analogs that competitively inhibit viral DNA polymerase by incorporating into viral DNA and causing chain termination; neuraminidase inhibitors (oseltamivir, zanamivir) block influenza virus release by inhibiting the neuraminidase enzyme essential for cleaving sialic acid residues from host cell surfaces.

Viruses are acellular entities that cannot reproduce independently and must hijack host cell machinery to replicate through a six-stage process: attachment (binding to specific host cell receptors), penetration (entry into the cell), uncoating (release of genetic material from the protein coat), replication (genome multiplication), assembly (formation of new virus particles), and release (exit from the host cell). Antiviral drugs target specific stages of this cycle: fusion inhibitors (e.g., enfuvirtide) block viral entry; reverse transcriptase inhibitors (e.g., AZT) prevent RNA-to-DNA conversion in retroviruses; integrase inhibitors (e.g., raltegravir) block viral DNA integration into host genome; protease inhibitors (e.g., ritonavir) prevent proper protein processing for virus assembly; and neuraminidase inhibitors (e.g., oseltamivir) block virus release from host cells.