Bacteriophages are viruses that infect bacteria. They accomplish their infection and propagation with two cycles that work in concert: the lytic cycle and the lysogenic cycle. These life cycles are the driving force for the spread of bacteriophage infection.
LYTIC CYCLE
The lytic cycle is the most common form of replication for viruses, and results in the destruction of the host cell.
The beginning of the lytic cycle, known as penetration, involves the virus entering the host cell. They either attach to a receptor on the cell surface or simply invade by brute through the plasma membrane. Once inside, the virus releases it’s DNA to begin the bio-synthesis of other viral components. The virus utilizes the host’s cellular machinery to create necessary proteins and DNA for replication.
After the necessary components are produced, they are combined to form complete viruses, which then cause the host bacterium to lyse. The broken cell releases these newly synthesized viruses to begin infecting other bacteria.
Some viruses do not completely destroy the host cell, instead budding from the membrane to be released. Viruses that infect eukaryotic cells often replicate this way and it is not common among bacteriophages.
LYSOGENIC CYCLE
As opposed to the lytic cycle, in which viruses utilize cell machinery to immediately reproduce, the lysogenic cycle is a dormant cycle in which the virus inserts its genetic elements into the host’s genome. The genetic element is transferred to daughter cells via the normal replication cycle of the host, until some external stimuli (chemical, UV radiation, etc) allows the reactivation of the viral elements. At this point, the lytic cycle begins again.
Lysogeny is an effective tool for molecular biologists. It can be utilized to insert genes of interest, via lambda phage, into a target bacterium for expression.
GENETIC MATERIAL REPLICATION
Bacteriophages use a combination of the lytic and lysogenic cycles. Under certain conditions, a primarily lytic bacteriophage may be induced to lysogeny in order to preserve the genetic elements for later activation.
A key difference among different kinds of bacteriaphage is the status of the genetic material. Most bacteriophages have double stranded DNA (dsDNA), while others have single stranded DNA or even RNA as the source of genetic information. These different nucleic acids are organized either in a linear fashion, similar to eukaryotes, or in a circular plasmid-like structure. The copying of genetic material is critical for replication and bacteriophage life cycles.
