What are the stages of the bacteriophage?

 5.2 Life Cycle of Bacteriophage

There are two types of life cycle of bacteriophages.

•  Lytic cycle (Master-slave relationship) 
• Lysogenic cycle (Host-guest relationship)

 Lytic Cycle (Master-slave relationship)

The lytic cycle of bacteriophage consists of the following steps.

Attachment:

In the first step, the bacteriophage attaches to the cell wall of the host that is bacterium at a specific site known as the receptor site. The attachment occurs by tail fibers.

Penetration:

The tail of virion releases an enzyme known as lysozyme which dissolves a small portion of bacterial cell wall. Now the tail sheath contracts and injects DNA into the host cell. The protein coat of the virus remains outside of the cell.

Multiplication of virion:

After entering the host cell the virion DNA takes over the control of the biosynthetic machinery of the host cell and forces the host cell to synthesize necessary viral components, i.e., DNA and protein. In this way, the virus starts multiplying. After 25 minutes of initial infection about 200 new bacteriophages are found in bacterial cells.

Lysis:

In this final step, the new daughter phages exert pressure on the bacterial cell wall and the enzyme lysozyme also attacks the cell wall from the inner side. Eventually, the cell wall bursts, and the progeny of viruses is released. This process is called lysis of bacterium. Newly formed phages are now ready to attack new hosts to start the lytic cycle again.

Lysogenic Cycle: (Host-Guest Relationship)

All bacteriophages do not cause lysis of bacterium. In the lysogenic cycle, the phage does not destroy the host cell. In fact, both phage and host cells live together in a peaceful co-existence. Therefore, such a relationship is called temperate. 

In the lysogenic cycle first two steps i.e., attachment and penetration are similar to the lytic cycle. After penetration into the host instead of taking over the control of the biosynthetic machinery of the host, the DNA of the phage is incorporated into the bacterial chromosome. Now the DNA of phage is called prophage and this process is known as lysogeny. 

In this condition bacterium does not lyse but lives and reproduces" normally. The viral DNA remains attached to the bacterium and replicates as bacterial chromosome replicates. This may continue for many generations until the process of induction takes place. In this process, viral DNA detaches from bacterial chromosomes and starts the lytic cycle again.

Usage of Bacteriophages in Genetic Engineering

Bacteriophages are used in genetic engineering as vectors along with plasmids. In genetics, a vector is a DNA molecule used as a vehicle to artificially carry foreign genetic material into another cell where it can be replicated and expressed.

The bacteriophage vector is called lambda and they are more efficient than bacterial plasmid. The phage vector can carry larger fragments of DNA usually 15-50 base pairs.

The enzymes of viruses like holins and lysins are used to degrade the bacterial cell wall so bacteriophages have been proposed as alternatives to antibiotics for many antibiotic-resistant bacterial strains.

5.2.3 Influenza or Flu Virus

It is an RNA-enveloped virus, that belongs to the family Orthomyxoviruses. It includes seven genera but out of seven three genera usually cause influenza in humans and some other vertebrates. These three genera are influenza virus A, influenza virus B, and influenza virus C. Each genus includes only one species, i.e., influenza A, B, and C virus.

Influenza A and C cause infection in different vertebrates including humans but influenza B almost exclusively infects humans.

Vaccines and drugs are available for the treatment of influenza virus infection but flu viruses develop resistance against these vaccines and drugs. Therefore, vaccines and drugs have to be reformulated regularly.

Human Immunodeficiency Virus (HIV):

Human immunodeficiency virus (HIV) is an RNA-enveloped virus. It is spherical in shape. It is a retrovirus, i.e., it can convert its RNA into DNA in the host cell. It causes acquired immunodeficiency syndrome (AIDS) in humans. It belongs to the family Retroviridae and the genus Lentivirus.