Life Cycle of Bacteriophage | Lytic and lysogenic cycle| Acellular life

Replicative Cycles of Phages
Phages are the best understood of all viruses, although some of them are also among the most complex. Research on phages led to the discovery that some double-stranded DNA viruses can replicate by two alternative mechanisms: the lytic cycle and the lysogenic cycle.
The Lytic Cycle

A phage replicative cycle that culminates in death of the host cell is known as a lytic cycle. The term refers to

the last stage of infection, during which the bacterium lyses (breaks open) and releases the phages that were produced within the cell. Each of these phages can then infect a healthy cell, and a few successive lytic cycles can destroy an entire bacterial population in just a few hours. A phage that replicates only by a lytic cycle is a virulent phage.

The Lysogenic Cycle

In contrast to the lytic cycle, which kills the host cell, the lysogenic cycle allows replication of the phage genome without destroying the host. Phages capable of using both modes of replicating within a bacterium are called temperate phages. A temperate phage called lambda, written with the Greek letter l, has been widely used in biological research. Phage l resembles T4, but its tail has only one short tail fiber. Infection of an E. coli cell by phage l begins when the phage binds to the surface of the cell and injects its linear DNA genome (Figure 19.6). Within the host, the l DNA molecule forms a circle. What happens next depends on

the replicative mode: lytic cycle or lysogenic cycle. During a lytic cycle, the viral genes immediately turn the host cell into a l-producing factory, and the cell soon lyses and releases its virus progeny. During a lysogenic cycle, however, the l DNA molecule is incorporated into a specific site on the E. coli chromosome by viral proteins that break both circular DNA molecules and join them to each other. When integrated

into the bacterial chromosome in this way, the viral DNA is known as a prophage. One prophage gene codes for a protein that prevents transcription of most of the other prophage genes. Thus, the phage genome is mostly silent within the bacterium. Every time the E. coli cell prepares to divide, it replicates the phage DNA along with its own chromosome such that each daughter cell inherits a prophage. A single infected cell can quickly give rise to a large population of bacteria carrying the virus in prophage form. This mechanism enables viruses to propagate without killing the host cells on which they depend. The term lysogenic signifiies that prophages are capable of generating active phages that lyse their host cells. This occurs when the l genome (or that of another temperate phage) is induced to exit the bacterial chromosome and initiate a lytic cycle. An environmental signal, such as a certain chemical or high-energy radiation, usually triggers the switchover from

the lysogenic to the lytic mode.

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