Question

Describe the temporal sequence of the bacteriophage life cycle.

Short answer

Answer: The bacteriophage life cycle consists of five main stages: (1) Adsorption, where the phage recognizes and binds to specific bacterial surface structures; (2) Penetration and Ejection of Viral Genome, wherein the phage injects its genetic material into the host cell; (3) Replication, during which the phage's genetic material replicates using the host's machinery; (4) Assembly, where the replicated components assemble into mature phage particles; and (5) Release, when the new phage particles are released from the host cell through lysis or budding, ready to infect new host cells.

Step-by-step solution

Introduction to Bacteriophage Life Cycle

A bacteriophage, or simply phage, is a virus that infects and replicates within a bacterial host. The life cycle of a bacteriophage generally comprises of the following stages: (1) Adsorption, (2) Penetration and ejection of the viral genome, (3) Replication, (4) Assembly, and (5) Release. We will discuss each stage in the subsequent steps, emphasizing the order and the events that occur in each stage.

Adsorption

During the first stage of the bacteriophage life cycle, the bacteriophage recognizes and binds to specific receptor sites on the bacterial cell surface. This is called adsorption. The tail fibers of the bacteriophage play a crucial role in recognizing bacterial surface structures, such as flagella, pili, or the bacterial cell wall itself, depending on the bacteriophage and host characteristics.

Penetration and Ejection of Viral Genome

After the adsorption of the bacteriophage to the host bacterial cell, the viral genetic material (either DNA or RNA) needs to enter the host cell. This process, called penetration, may involve the injection of genetic material through the cell wall and membrane or the formation of a small channel. Once inside the bacterial cell, the viral genome takes control of the host machinery to ensure its replication, transcription, and translation.

Replication

During the replication stage, the bacteriophage's genetic material replicates inside the host cell using the host's replication machinery. This stage may include the synthesis of viral proteins necessary for the construction of new viral particles. The host's cellular machinery produces multiple copies of the phage DNA or RNA as well as the viral proteins.

Assembly

In this stage, the replicated viral components (genome and proteins) are assembled within the host cell into mature phage particles. The procapsid, a precursor to the capsid (viral protein coat), often forms first, followed by the packaging of the genome into the assembly. The other components, such as the tail and tail fibers, are assembled separately and then attached to the phage head.

Release

Finally, the newly formed bacteriophage particles are released from the host cell. This can happen through two main mechanisms: (a) Lysis, in which the bacterial cell wall is enzymatically degraded, and the cell bursts, releasing the new phage particles, or (b) budding, where the new phage particles slowly extrude through the cell membrane without directly lysing the host cell. The released phages are now capable of infecting new host bacterial cells, and the life cycle begins anew.