Rucete ✏ Campbell Biology In a Nutshell
Unit 3 GENETICS — Concept 19.2 Viruses Replicate Only in Host Cells
Viruses are obligate intracellular parasites, meaning they can only reproduce by hijacking the machinery of a host cell. Understanding viral replication involves examining specific cycles, such as the lytic and lysogenic cycles in bacteriophages, as well as distinct processes in animal viruses.
General Features of Viral Replicative Cycles
- Viruses lack metabolic enzymes and ribosomes, relying completely on host cells.
- Viral entry involves binding specific receptors on host cells.
- Entry mechanisms include DNA injection (phages), endocytosis, or membrane fusion (animal viruses).
- Once inside, the viral genome takes over the host machinery to produce new viruses.
- New viruses assemble and exit, often destroying the host cell in the process.
Replicative Cycles of Bacteriophages
Lytic Cycle (Virulent Phages)
- Phage replicates rapidly and causes the host cell to lyse (burst).
- Leads to immediate release of new viruses.
- Example: Phage T4 infecting E. coli.
Lysogenic Cycle (Temperate Phages)
- Phage DNA integrates into the host genome as a prophage.
- Host cell replicates the prophage during normal division.
- Environmental cues (e.g., radiation, stress) can trigger entry into the lytic cycle.
- Example: Phage λ (lambda).
Bacterial Defenses Against Phages
- Surface receptor mutations: prevent phage attachment.
- Restriction enzymes: cut viral DNA at specific sequences.
- CRISPR-Cas system:
- Captures viral DNA snippets from past infections.
- Uses RNA guides and Cas proteins to destroy viral DNA during future attacks.
Replicative Cycles of Animal Viruses
- Animal viruses vary by genome type (RNA/DNA, single/double-stranded) and envelope status.
Viral Envelopes
- Membrane derived from the host cell; aids in entry and exit.
- Typically, enveloped viruses leave host cells without killing them immediately.
RNA Viruses and Retroviruses
- RNA viruses (e.g., influenza): use viral RNA polymerase to replicate their genome.
- Retroviruses (e.g., HIV): use reverse transcriptase to make DNA, which integrates into host genome as a provirus.
Evolutionary Perspective
- Viruses may have evolved from mobile genetic elements like plasmids or transposons.
- Constant arms race between host immune defenses and viral adaptations.
In a Nutshell
Viruses can replicate only within host cells. Phages have distinct lytic (cell-killing) and lysogenic (integrative) cycles. Animal viruses employ diverse mechanisms, often using envelopes and specialized enzymes. Understanding these cycles is crucial for addressing viral diseases and manipulating viral functions.