Lytic And Lysogenic Cycle Of Viruses Pdf


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19.01.2021 at 08:44
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lytic and lysogenic cycle of viruses pdf

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Viruses are often very specific as to which hosts and which cells within the host they will infect. This feature of a virus makes it specific to one or a few species of life on earth. So many different types of viruses exist that nearly every living organism has its own set of viruses that try to infect its cells.

The lytic cycle results in the destruction of the infected cell and its membrane. Bacteriophages that only use the lytic cycle are called virulent phages in contrast to temperate phages. In the lytic cycle, the viral DNA exists as a separate free floating molecule within the bacterial cell, and replicates separately from the host bacterial DNA, whereas in the lysogenic cycle, the viral DNA is located within the host DNA. This is the key difference between the lytic and lysogenic bacterio phage cycles. The lytic cycle, which is also referred to as the "reproductive cycle" of the bacteriophage, is a six-stage cycle.

Virus: Reproduction

Bacteriophage , also called phage or bacterial virus , any of a group of viruses that infect bacteria. Bacteriophages were discovered independently by Frederick W. Bacteriophages also infect the single-celled prokaryotic organisms known as archaea.

Thousands of varieties of phages exist, each of which may infect only one type or a few types of bacteria or archaea. Phages are classified in a number of virus families; some examples include Inoviridae, Microviridae, Rudiviridae, and Tectiviridae.

Like all viruses, phages are simple organisms that consist of a core of genetic material nucleic acid surrounded by a protein capsid. There are three basic structural forms of phage: an icosahedral sided head with a tail, an icosahedral head without a tail, and a filamentous form. During infection a phage attaches to a bacterium and inserts its genetic material into the cell.

After that a phage usually follows one of two life cycles, lytic virulent or lysogenic temperate. Lytic phages take over the machinery of the cell to make phage components.

They then destroy, or lyse, the cell, releasing new phage particles. Lysogenic phages incorporate their nucleic acid into the chromosome of the host cell and replicate with it as a unit without destroying the cell.

Under certain conditions lysogenic phages can be induced to follow a lytic cycle. Other life cycles, including pseudolysogeny and chronic infection, also exist. In pseudolysogeny a bacteriophage enters a cell but neither co-opts cell-replication machinery nor integrates stably into the host genome.

Pseudolysogeny occurs when a host cell encounters unfavourable growth conditions and appears to play an important role in phage survival by enabling the preservation of the phage genome until host growth conditions have become advantageous again. In chronic infection new phage particles are produced continuously over long periods of time but without apparent cell killing.

Phages have played an important role in laboratory research. The first phages studied were those designated type 1 T1 to type 7 T7. The T-even phages, T2, T4, and T6, were used as model systems for the study of virus multiplication.

In Alfred Day Hershey and Martha Chase used the T2 bacteriophage in a famous experiment in which they demonstrated that only the nucleic acids of phage molecules were required for their replication within bacteria.

The results of the experiment supported the theory that DNA is the genetic material. For his work with bacteriophages, Hershey was awarded the Nobel Prize for Physiology or Medicine in In the s American biochemist George P.

Smith developed a technology known as phage display , which allowed for the generation of engineered proteins. Gene III encodes a protein expressed on the phage virion surface.

Thus, gene III fusion proteins taken up by phages were displayed on the surfaces of virion particles. Researchers could then use antibodies developed to recognize the foreign protein fragment to purify fusion phage cultures , thereby effectively amplifying the foreign gene sequence for further study. British biochemist Gregory P. Winter subsequently refined phage display technology for the development of human antibody proteins. Such proteins could be used to treat diseases in humans with less risk of inducing potentially dangerous immune reactions compared with previous therapeutic antibodies derived from animals.

Adalimumab Humira , used for the treatment of rheumatoid arthritis , was the first fully human antibody made via phage display to be approved by the U. Food and Drug Administration approved in For their discoveries relating to phage display, Smith and Winter were awarded a share of the Nobel Prize in Chemistry.

Phage therapy was not successful, and after the discovery of antibiotics in the s, it was virtually abandoned. With the rise of antibiotic-resistant bacteria, however, the therapeutic potential of phages has received renewed attention. Bacteriophage Article Media Additional Info. Article Contents. Print print Print. Table Of Contents. While every effort has been made to follow citation style rules, there may be some discrepancies.

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Khan Academy - Bacteriophage. The Editors of Encyclopaedia Britannica Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree See Article History. Alternative Titles: bacterial virus, phage. The cycle of infection results in the death of the host cell and the release of many virus particles, called virions. General structure of T4 bacteriophage and a model of its mode of attachment to, and injection of its DNA into, a bacterial cell.

Get a Britannica Premium subscription and gain access to exclusive content. Subscribe Now. Learn More in these related Britannica articles:. From a host cell infected by one bacteriophage, hundreds of bacteriophage progeny are produced.

In American biologists Alfred D. Hershey and Martha Chase prepared two populations of bacteriophage particles. In one population, the outer protein coat…. Transduction is an efficient means of transferring DNA between bacteria because DNA enclosed in the bacteriophage is protected from physical decay and from attack by enzymes in the environment and is injected directly into cells by the bacteriophage. However, widespread gene transfer by means…. History at your fingertips. Sign up here to see what happened On This Day , every day in your inbox!

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21.2B: The Lytic and Lysogenic Cycles of Bacteriophages

The objective of regulation of bacterial virus bacteriophage genes is usually the orderly assembly of new phage particles without destroying the host cell too soon. This provides a paradigm for the complex problem of development in multicellular organisms. It is a temperate phage , meaning that phage infection does not always result in the destruction of the host cell. The DNA of the invading phage has two possible fates Fig. These proteins regulate transcription from a number of promoters in a regulatory region of the phage DNA described below. The functions of the proteins and promoters are summarized in Table l.

We've updated our Privacy Policy to make it clearer how we use your personal data. We use cookies to provide you with a better experience, read our Cookie Policy. Bacteriophage phage are obligate intracellular viruses that specifically infect bacteria. Phage have a very simple structure Figure 1. Their genetic material is contained in a prism shaped head, surrounded by a protein capsid. This is connected to the elongated sheath sometimes called the tail by a neck or collar region. At the bottom of the sheath is the base plate to which the tail fibers normally six that facilitate attachment to the host cell are attached.

Viruses like the ones pictured above are very tiny nonliving particles. Viruses do not carry out respiration. They also do not grow or reproduce on their own. A virus needs a living cell in order to reproduce. The living cell in which the virus reproduces is called a host cell. Let's look at what happens when a virus attacks our cells.

Virus: Reproduction

About Blog Location. When my 9th grade students see the reproductive cycles of viruses, they are often intimidated by the confusing, unfamiliar steps. Bacteriophages that only use the lytic cycle are called virulent … Lytic or Virulent Phages a.

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1 Comments

Adrien G.
27.01.2021 at 23:28 - Reply

Bacteriophage , also called phage or bacterial virus , any of a group of viruses that infect bacteria.

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