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The following points highlight the two main types of Life Cycle of Phages. The Types are: 1. Lytic Cycle of T-Even Phages 2. The Life Cycle of Lambda Phages.
Type # 1. Lytic Cycle of T-Even Phages:
The lytic cycle (also termed as vegetative life cycle or Infection cycle or sometimes Multiplication cycle) results in the lysis (rupture) of the host cell. As a result a number of newly synthesized viral progeny is produced. Thus bacteriophages undergoing a lytic life cycle (only) are also known as virulent bacteriophages.
The classic example of such virulent phage is T4. There are other such phages that are called T-even phages (e.g., T2, T4 and T6). The life cycle of T4 undergoing a lytic cycle is shown in Fig. 14.1.
The lytic life cycle (starting from the infection to the host cell) consists of five steps:
(1) Attachment
(2) Penetration
(3) Synthesis
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(4) Assembly, and
(5) Release of the new viral progeny
In case of T4 the infection cycle lasts about 25 minutes at 37°C.
Type # 2.The Life Cycle of Lambda Phages:
The λ phages are commonly used in DNA cloning. They can have either lytic or lysogenic cycle, depending on the environment. In the lytic cycle, λ phages replicate rapidly and eventually cause lysis of the host cell. In the lysogenic cycle, the viral DNA circularizes and integrates into the host DNA. Then, λ phages may replicate with the host cell.
Under certain conditions (e.g., ultraviolet irradiation of cells), the λ phages may transform from the lysogenic cycle to the lytic cycle. This transformation is mainly controlled by two proteins: cl (also known as λ repressor) and Cro. increase in cl proteins promotes the lysogenic cycle whereas increase in Cro proteins promotes the lytic cycle.
The phage initially attaches itself to specific host cell molecules (receptors) on the bacterial cell wall. In the next stage of penetration, it introduces its genetic material into the cell. T4 phage practically drills a minute hole through the bacterial cell wall and membrane and then pushes the core of its tail [Fig. 14.1 (b)] into the cytoplasm through contraction of a sheath around the core.
The viral DNA is then injected into the cell by conformational changes in the phage head. Once the phage genome enters the cytoplasm, it subverts the host’s nucleic acid and protein synthesis apparatus and starts the synthesis of viral proteins and DNA. Along with the synthesis of the viral proteins, the newly formed progeny self-assemble them into viral component such as head (containing the phage DNA), tail and fibers.
The assembly step (or process) results in the formation of numerous intact phage particles within the cell. In the next step, the viral proteins cause the lysis of the host cell, and all the viral progeny are released into the environment. Many of the released progeny infect other adjacent bacterial cells, entering another round of replication.
