Lytic cycle

The lytic cycle describes for viruses, the development phase, in which the host cell is lysed after new virions were formed. This produces a cytopathic effect. A limiting case form viruses that get their host cell in the lytic cycle, as they attach themselves directly to the cell membrane and pinch off a vesicle, the host cell exit ( budding ). In contrast, the lysogenic cycle receives the host cell and is often associated with decreased synthesis of viral genes and is sometimes referred to as the resting phase or latent phase. Viruses with a fierce but transient course of infection usually do not have a lysogenic cycle ( engl. hit and run viruses ).

General course of the cycle

• Host cell - Contact with virus ( adsorption)
• " Injection " of the phage genome or penetration of the cell or endosomal membrane
• Extract of the viral genome, and in some viruses transport into the cell nucleus
• Replication of the viral genome (DNA or RNA ) or retroviruses, reverse transcription is performed first and subsequently the transcription.
• Integration of the viral genome into the host genome, or retroviruses takes place only in the reverse transcription of DNA, the integration. RNA viruses (except retroviruses ) and some DNA viruses do not integrate.
• Gene expression, possibly a modulation of host restriction and ( in animals ) and the immune system,
• Packaging of the genome for producing the Tochtervirionen, in some viruses, the replication of which is same with the package ( for example, hepatitis B virus)
• Lysis of the host cell or budding of virions
• Infection of other cells

In bacteriophage integration is sometimes also associated with the lysogenic cycle.

Course of the cycle in bacteriophages

The lytic replication cycle of a bacteriophage is connected to the death of the host cell. For lysis, the newly produced viruses are released and can be re new cells affected, we then called virulent phages. The lysed bacteria releases between 100 and 200 phages ( litter size, Eng. Burst size). The lytic cycle of E. coli with the T4 phage at 37 ° C lasts 20 to 30 minutes. Phage species not lysed is known as Temperate. Following describes the procedure for an even phages of the T- series is described, other phages can in some detail to show different processes. The lytic cycle of infection goes exactly according to the scheme described in section 3 ): Where a phage to a suitable host cell and comes in contact with her, he adsorbed with tail fibers and spikes specific cell wall components of the host cell. These receptors include, among others to antigen structures, lipoproteins, lipopolysaccharides or on the host cell surface, for which the corresponding phage has an affinity. This affinity explains the strict host specificity of the phage subject - only the phage proteins of the end plate, tail fibers and spikes fit to the receptor on the host cell surface, can "dock" to the cell. In the absence of the receptor naturally (because it is the wrong bacterium) or it has been lost by mutation, the cell is insensitive, ie resistant to infection by the phage in question. These may give rise resistant clones.

If the phage adsorbed to the cell wall (the process is irreversible, and thus an antigen -antibody reaction, similar), a contraction of the tail sheath with simultaneous penetration of the tail pipe runs through the cell wall. This process prepares the injection of phage DNA and is supported by simultaneous delivery of lytic phage enzymes that make the cell wall throughout. Wherein injection of the phage genome, the protein shell of empty capsid and injection apparatus to the surface of the host cell remains. This residual virus particles without DNA is called a ghost.

Immediately after the injection of the phage DNA in the bacterial cell can be any phage or no phage particles to demonstrate this period is called eclipse or latency. Escherichia coli, this latency time is about 10 minutes. During this time, nothing happens only apparent in the cell. In fact, during this phase, the entire metabolism of the host cell is changed to the production of phage components. Thus, the cell division and synthesis of bacterial DNA is instantaneously adjusted by the injection of the viral DNA by the viral DNA is transcribed into a mRNA by means of cellular enzymes and proteins are synthesized which disrupt the bacteria 's DNA synthesis, the existing bacterial DNA break down into fragments thereof and synthesize new phage DNA. In the further course of the infection are also so-called "late" genes transcribed, which cause the synthesis of structural proteins (such as head, tail pipe, tail fibers, etc.). These components are initially individually in the host cell, and then assembled to form new phage. The occurrence of the first assembled phage ends the eclipse. From now on phage are assembled, until the internal pressure of the host cell above a certain level and an, the cell wall ruptures by a lysozyme - like enzyme which is synthesized in parallel with the assembly of the phage particle. Thus, the newly formed phages are released. This process is called lysis. Phages that induce lysis in a host cell, they are called virulent.