The following points highlight the two main types of reproductive cycle in virus. The types are: 1. Lytic Cycle 2. Lysogenic Cycle.
Type # 1. Lytic Cycle (Fig. 2.5):
It is the reproductive cycle of virulent phages, e.g., T4 bacteriophage. The phage attaches itself to the host cell (e.g., Escherichia coli) through its tail fibres. The fibres bend and bring the tip of tail in contact with the host cell wall. The tip of the tail produces a hole in the bacterial cell wall by means of enzyme lysozyme.
The tail sheath contracts and injects the viral genome into host cell. After entering the host cell, the viral DNA transcribes some early mRNAs to form some enzymes over the host ribosomes. Some of these are nucleases. They degrade host DNA and mRNAs.
Ribosomes and tRNAs remain unaffected. Phage DNA and mRNA are also protected from nucleases due to methylation of their cytosine bases. Parent viral DNA functions as a template and replicates repeatedly with the help of bacterial nucleotides.
Simultaneously, host machinery (ribosomes, tRNAs, amino acids, energy) is used by phage genes to synthesize proteins for vial lysozyme, internal proteins and capsid proteins.
Different components combine to form new viruses or phage particles. The host cell ruptures by means of Lysozyme releasing the phage particles. The period between entry of viral nucleoid into host cell and bursting of host cell to release new viruses is called eclipse period.
Type # 2. Lysogenic Cycle (Fig. 2.6):
Lambda phage (λ phage) has a higher degree of regulation of its genes. The phage is parasitic over Escherichia coli. It does not possess tail fibres for attachment to bacterial cell.
The tail directly comes in contact with bacterial cell, drills a hole in the wall and injects the phage DNA into the cell. In lysogenic cycle, the phage DNA does not take over the control of cellular machinery of the host. Instead, it produces a repressor (e.g., cl) and undergoes reduction to temperate or non-virulent state.
With the help of enzyme integrate the viral genome becomes integrated with the chromosomal DNA of the bacterium at a specific site (e.g., galactose locus in λ phage).
In this form the viral genome is called pro-phage. Pro-phage replicates along with bacterial chromosome and, therefore, gets distributed to the daughter bacteria. Pro-phage does not form virus particles because the genes connected with taking over of host machinery remain repressed due to formation of a repressor.
At times the synthesis of repressor is stopped. Repressor can also be destroyed by chemicals, high energy radiations and other adverse conditions. This converts the temperate or non-virulent virus into virulent or lytic virus. Therefore, the bacterial cell carrying pro-phage is called lysogenic cell and the phenomenon of existence of virus genome in pro-phage state along-with host DNA is termed as lysogeny.