Bacteriophage Life Cycle:
Bacteriophages are the best-understood viruses in terms of their gene structure and expression. T2-bacteriophage infects Escherichia coli and causes lytic cycle. This phage has a double-stranded circular DNA enclosed in a hexadecimal proteinaceous head, a cylindrical hollow tail, and six tentacles for attachment to the host. The phage gets attached to the bacterial wall by its tentacles. The lysozyme found at the tip of the tail dissolves the host cell. Thereafter the viral DNA synthesis of an enzyme known as the nuclease degrades the host DNA. Only after a few minutes, the host DNA is destroyed by the nuclease.
On the other hand, the viral DNA is resistant to nuclease because its cytosine residues are methylated. Now the viral genes make use of host ribosomes for multiplication. First of all the replication of viral DNA takes place. Thereafter, the proteins of the head-tail and tentacles are synthesized. An independent synthesis of viral components occurs within the cell followed by an assembly of those components into complete virus particles. After a well-defined period the bacterial cell bursts and releases the newly formed virus particles. The liberated virus particles again infect the bacterial cells and the lytic cycle is repeated.
Lysogenic Cycle:
An interesting modification of the lytic cycle occurs when a certain type of bacteriophage, termed temperate, infects certain types of bacteria. In this instance, the genetic material of the temperate viruses attaches itself to a bacterial chromosome in a different manner. Here the viral DNA gets attached to bacterial DNA, becomes inactive, and is known as prophage or provirus. The inactivity of the phage DNA is because of the synthesis of repressor protein by phage DNA that causes repression of all the phage genes. Thereafter as the bacterial cell divides, phage DNA also replicates with it and is inherited by all the bacterial cells of the following generations.
In the other option, DNA or viral gene integrates with the bacterial (E.coil) chromosomal DNA and gives rise to a prophage. Here, the phage replicates along with the host. At times, the prophage may be activated due to altered environmental conditions. This inhibits the repressor protein synthesis, and results in the expression of lytic genes. Now, the prophage becomes a lytic phage and resumes the lytic phage and resumes the lytic cycle. Here the bacterial cell (E.coil) shows no immediate sign of infection and continues to grow and divide as if nothing had happened. Such a bacterium is termed as lysogenic and the cycle involved the lysogenic cycle. Occasionally this bacterium will spontaneously start making viral components and these components will be assembled into mature virus particles.