Multiplication in Bacteriophages
bacteriophages exhibit two types of replication cycle- virulent or lytic cycle and temperate or lysogenic cycle.
I) Virulent or lytic cycle
The phages undergaining lytic cycle are called lytic phages or virulent phages. In lytic cycle, a lytic phages infects and kills the host cell to release progeny virions.
The whole process involves fallowing steps
i) Adsorption or infection:
The process of attachment of a virion on the host cell surface is called adsorption. The tips of the tail fiber bind or adsorb to specific receptors on the surface of e.coli. The viral receptors may be f-pili, lipo protein etc.
ii) Penetration or Injection:
The tail fibers of virion bend to bring the spikes and basal plate in contact with the surface of bacterial wall. The tail sheath contracts so that the hollow tail core penetrates the bacterial wall and injects the viral genome into the cytoplasm. After penetration, the empty capsid that remains outside the bacterium is called the ghost.
iii) Synthesis of phage components:
Immediately after penetration, the phage DNA synthesizes early proteins. Some early proteins break down the bacterial DNA and take control of the bacterial cell machinery. The other early proteins used as enzymes for replication of phage DNA. The newly synthesized phage DNA's produces late proteins, which are the protein sub units of the phage capsid.
iv) Virion assembly:
The capsid proteins assemble to form empty head and a condensed viral DNA is packed inside it. Finally the separately assembled tail joins to head to form a daughter or new virion.
v) Lysis or release
During assembly of progeny virions, the bacterial cell becomes spherical. The phage enzymes weaken the cell wall which ultimately burst or lyse to release about 100-200 progeny viruses.
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| Lytic cycle |
II) Temperate or Lysogenic cycle
The phages that exhibit lysogenic cycle are called temperate phages or non-virulent phages. example lambda phages attacking E.coli. During lysogenic cycle, the phage DNA integrate into the bacterial DNA and is now called prophage. The host bacterium containing prophage is called lysogenic bacterium. The prophage passively replicates along with the host DNA for many generations. When a lysogenic bacterium exposed to UV light or chemical. The prophage withdraw from the host DNA to undergo lytic cycle this conversion of a prophage into a lytic phage is called induction.
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| Lysogenic cycle |
