This article explains the lytic and lysogenic cycle of bacteriophages. I tried to write it as an interesting essay, and I am very pleased with the results.
English: bacteriophage lysogenic and lytic cycle (Photo credit: Wikipedia)
Viruses are inactive until they infect a suitable host cell. Some infect plant cells; others prefer animal cells. Even human beings are not exempt.
As the name implies, bacterial viruses infect bacteria. These viruses are also called bacteriophages or simply phages.
Not all phages act the same way. They all destroy their host when their life cycle is complete. However, some do it more rapidly than others. The reproductive cycle of the fast-acting phages is called the lytic cycle. A phage that takes its time before killing its host is said to have a lysogenic reproductive cycle.
“Botany: An Ecological Approach” and other sources explain in detail the lytic cycle of phage T-4, a phage that infects the bacterium Escherichia coli. This bacterium is found in human intestines. One variety of this species causes severe diarrhea. Sometimes it even kills the people it infects.
Phage T4 consists of a rather large double-stranded DNA molecule covered by a protein coat called a capsid. The head of the capsid has an icosahedral form and its tail is cylindrical. At the end of its tail are a hexagonal base plate and some tail fibers.
The phage T4 begins its lytic cycle by attaching itself to the surface of an E. coli bacterium. There is a specific site on the surface of the bacterium where the attachment can be made. The attachment is effected by means of the tail fibers.
After the phage makes a small hole in the bacterium, the viral DNA enters the cell. The protein capsid remains outside. It is no longer useful to the virus, so it is cast aside like an old coat.
Once inside the cell, the viral DNA deposes the powers that be and assumes command. The enzymes of the bacterium, its ribosomes, and other machinery are forced to assist in the production of new bacteriophages.
The phage needs nucleotides to make copies of itself, so it orchestrates the destruction of the chromosome of the bacterium. The bacterium is forced to produce a substance that breaks down its own DNA into its component nucleotides. These are then used to make many copies of the viral DNA. Moreover, instead of making the proteins that the bacterium needs for itself, the bacterium is forced to make the type of proteins needed to make capsids for new phages.