While bacteriophages inject their nucleic acid directly into the host cell, leaving the viral capsid outside, animal viruses enter host cells entirely through either receptor-mediated endocytosis or membrane fusion.

Comparison of Bacteriophage and Animal Virus Entry

Penetration is the second stage of the lytic cycle, where the bacteriophage contracts its tail sheath like a hypodermic needle to inject its viral genome through the host cell wall and membrane, while the phage head and other components remain outside the cell.

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During the lytic cycle, a virulent bacteriophage takes over the host bacterial cell, replicates its viral genome, synthesizes proteins to assemble new phages, and ultimately destroys the host cell by cell lysis to release progeny viruses.

Lytic Cycle

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Biosynthesis is the stage of bacteriophage infection where the virus degrades the host chromosome using virus-encoded endonucleases. The virus then hijacks the host cell's machinery to replicate, transcribe, and translate key viral components (such as capsomeres, sheaths, base plates, tail fibers, and viral enzymes) for assembling new viruses, with polymerase genes expressed early and capsid and tail proteins expressed later in the cycle.

Biosynthesis of Bacteriophage

The replication steps for bacteriophages generally include attachment to the host, entry or penetration of the viral genome, replication and synthesis of viral components (biosynthesis), assembly of new phages (maturation), and release of progeny virions (lysis).

Steps of Bacteriophage Replication

During attachment, the first stage of the lytic cycle, a bacteriophage interacts with specific surface receptors on the host bacterium (such as lipopolysaccharides and OmpC proteins). Phages typically exhibit a narrow host range, restricting infection to particular bacterial species or specific strains.

Attachment of Bacteriophage

Penetration of Bacteriophage (Lytic Mechanism)

Maturation is the stage of the bacteriophage lytic cycle in which newly synthesized viral components are assembled to form complete, new phage particles within the host cell.

Maturation of Bacteriophage

Lysis is the final stage of the bacteriophage replication cycle, where the bacterial cell wall is disrupted by phage proteins such as holin or lysozyme. This disruption causes the host cell to burst, releasing the newly assembled progeny viruses into the environment to infect new cells.

Lysis of Bacteriophage

Generalized transduction is a process of horizontal gene transfer in which a bacteriophage incorrectly packages a random fragment of the host bacterium's degraded chromosome into its capsid instead of viral DNA during the lytic cycle. When this phage infects a new bacterial cell, it injects the previous host's DNA. This asexual gene transfer can lead to DNA recombination, providing the recipient bacterium with new genes, such as those for antibiotic resistance or sugar metabolism.

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