The bacterial viruses or bacteriophages are quite selfish entities that totally depend on the bacterial cells for their replication. The nature of the bacteriophage is such that it uses the genetic machinery of the bacteria, takes the shelter of the bacterial cells, grows and reproduces in these cells and lyses the bacterial cells to release its hundreds of progenies. Thus a bacteriophage utilizes the bacterial cells and makes them non-functional later on. From the research point of view, the bacteriophages are easy to study since they require bacteria to grow. The discovery of triplet gene code was possible using the bacteriophage. Francis Crick and his colleagues used these entities for exploring wild-type and mutant phenotypes. Gene mapping is possible with bacteriophages. Recombinant DNA technology uses bacteriophages for performing genetic crosses, construction of chromosome maps, and for intergenic and intragenic mapping. Most of the phages infect E. coli for their survival.
Max Delbruck was the first scientist to initiate genetic studies in Bacteriophages. The structure of a bacteriophage is studied using an electron microscope. The design of a bacteriophage is such that it can sit on the bacterial cell and inject its genetic material into the cell. They consist of two basic structural components such as the head and the filamentous tail. The protein coat forming the head is called a capsid. The phage genome is present in its capsid. The bacteriophage has a filamentous tail. It has additional spider-leg like structures. These typical structures facilitate the landing of the bacteriophages on the bacterial cells thereby helping in the attachment.
The capsid is icosahedral with a three-dimensional structure surrounding the nucleic acid. Sometimes the capsid may have a filamentous or helical structure. Bacteriophages may have DNA or RNA as their genetic material. Lambda bacteriophages infect E. coli and have a double-stranded DNA. ϕX174 phages have a single-stranded circular DNA. MS2 phages have a single-stranded linear RNA. Thus there are different types of phages with different genome structures. In some phages the genomes are segmented, meaning that different RNA molecules carry different genes. ϕX174 phages have extra information in the form of overlapping genes that share nucleotide sequences and code for different gene products.
Studying replication cycles in phages helps us in understanding the life cycle of these entities. There are two types of life cycles observed in bacteriophages such as lytic and lysogenic cycles.
Lytic cycle:
A T4 bacteriophage is used to study the lytic cycle. This bacteriophage has a double-stranded linear DNA. The first step of the lytic cycle involves the landing of the bacteriophage on the bacterial cell surface. The spider-leg like structures on the phage tail facilitate in phage landing and attachment. The phage particle interacts with the receptor protein present in the bacterial cell. The receptor of the T4 phage is known as Omp C or an outer membrane protein. It is a type of porin that forms membrane channels for uptake of nutrients. The next step is to inject the DNA into the bacterial cells. The structure of the phage facilitates an easy injection of the genetic material into the bacteria. Once the phage DNA enters the cell, the bacteria stops synthesizing own materials.
Image 1: Bacteriophage lytic pathway
The phage genome replication starts as soon as the bacterial replication stops. The replication of the phage DNA begins within a minute. It requires only five minutes depolymerizing the phage DNA. The nucleotides help in replication of the phage genome. New phage capsids start developing after twelve minutes. Various phage particles get assembled within the bacterial cell. The genome or the phage DNA gets inserted into the capsid heads. Cell lysis enables release of the phages. It takes twenty minutes to release the progeny. Each lytic cycle produces 200 to 300 bacteriophages. The progeny phages are further capable of infecting the other bacteria. This experiment was carried out by Ellis and Delbruck in 1939. One step growth curve represents the results. The latent period lasts from 0 to 22 minutes revealed that there was no change in the number of cells during the first twenty-two minutes of the infection. The phages are mixed with the bacteria and plated on a culture medium. An entire lawn of bacteria is allowed to grow on the medium. Each phage infects the bacterium on the plate and the phages released from one infection attack the adjacent bacteria. The cycle continues in this way. The result of lysis depicts a clear patch in the lawn of bacteria. The clear patch is known as a plaque. Most of the phages undergo a lytic cycle. Some phages undergo a lysogenic cycle.
Lysogenic cycle:
Image 2: Lysogenic pathway
The lysogenic cycle of a lambda phage is studied. A specialty of a lysogenic cycle is that the phage genome gets integrated into the host DNA. The striking feature of the lysogenic cycle helped researchers to use lambda phage as cloning vectors. A quiescent form of a bacteriophage known as prophage occurs due to the entry of phage DNA into the cells. The phage DNA and the bacterial DNA undergo site-specific recombination. The integration occurs between identical 15 base pair sequences so that the lambda DNA integrates into the same position within the E. coli DNA. The phage may switch over to the lytic cycle in response to physical or chemical stimuli. A second recombination event begins. This event excises the phage genome from the host DNA. Thus the phage genome replication starts. A Lysogeny is a phenomenon in which the phage genome gets integrated into the bacterial genome. These phages are known as temperate phages. The one carrying a temperate phage in a prophage state is known as a lysogenic bacterium. Lysogenization is an experimental production of a lysogenic bacterium by exposing them to temperate phage. The bacteria change their phenotype, sometimes in their morphology, pathogenic capability or synthetic properties accompanying a lysogeny. This phenomenon is known as lysogenic conversion. Hence, through lysogenic conversion, the temperate phage can spread virulence factors such as exotoxins and exoenzymes.
References:
[1] Microbial Genetics, Keya Chaudhary
[2] Molecular Genetics of Bacteria, Jeremy W. Dale, Simon F. Park
[3] Genetics, G. Ivor Hickey
[1] Microbial Genetics, Keya Chaudhary
[2] Molecular Genetics of Bacteria, Jeremy W. Dale, Simon F. Park
[3] Genetics, G. Ivor Hickey
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