Animal Virus And Bacteriophage
Fig 1. Three types of viruses: a bacterial virus, otherwise called a bacteriophage (left center); an animal virus (top right); and a retrovirus (bottom right). Viruses depend on the host cell that they infect to reproduce. When found outside of a host cell, viruses, in their simplest forms, consist only of genomic nucleic acid, either DNA or RNA (depicted as blue), surrounded by a protein coat, or capsid.
A virus is a disease-causing entity on the borderline between life and non-life. Viruses are capable of reproducing only within a living host cell. They effectively reprogram the cells they invade, turning the cellular machinery into a biological factory for manufacturing fresh copies of themselves. For more details, see viral reproduction. The simplest viruses consist of a single helical strand of RNA coated with protein molecules. Other viruses are more complex and may have a diameter of up to 0.2 microns.
All viruses contain nucleic acid, either DNA or RNA (but not both), and a protein coat, which encases the nucleic acid. Some viruses are also enclosed by an envelope of fat and protein molecules. In its infective form, outside the cell, a virus particle is called a virion. Each virion contains at least one unique protein synthesized by specific genes in its nucleic acid. Viroids (meaning viruslike) are disease-causing entities that contain only nucleic acid and have no structural proteins. Other viruslike particles called prions are composed primarily of a protein tightly integrated with a small nucleic acid molecule.
An Introduction To The Viruses
Viruses are generally classified by the organisms they infect, animals, plants, or bacteria. Since viruses cannot penetrate plant cell walls, virtually all plant viruses are transmitted by insects or other organisms that feed on plants. Bacterial viruses are known as bacteriophages.
Viruses are further classified into families and genera based on three structural considerations: 1) the type and size of their nucleic acid, 2) the size and shape of the capsid, and 3) whether they have a lipid envelope surrounding the nucleocapsid (the capsid enclosed nucleic acid).
There are predominantly two kinds of shapes found amongst viruses: rods, or filaments, and spheres. The rod shape is due to the linear array of the nucleic acid and the protein subunits making up the capsid. The sphere shape is actually a 20-sided polygon (icosahedron).
Structure And Function Of Viruses And Bacteria
A bacteriophage (Fig 2), also known simply as a phage, is a virus that attacks and infects bacteria. The infection may or may not lead to the death of the bacterium, depending on the phage and sometimes on conditions. Each bacteriophage is specific to one form of bacterium.
Bacteriophages make up a diverse group of viruses, some of which have complex structures, including double-stranded DNA. Since the first was discovered in 1915, bacteriophages have been important in the study of genetics.
Conceivably, complex combinations of molecules could exist on other worlds which, although not living organisms in any familiar sense, could behave like viruses given a suitable host. This would present a threat to humans and other terrestrial life-forms which would have no immunity to such alien infection. It was the reason that elaborate quarantine precautions were planned for the return of the first astronauts from the Moon. Even more care will be required when people first come into contact with Martian rocks and soil (see back-contamination).
Going Viral With An Ancient Global Player
The possibility that terrestrial viruses could mutate into dangerous, alien forms if subjected to highly ionizing radiation, such as that present above the protective blanket of the Earth's atmosphere, was exploited by Michael Crichton in The Andromeda Strain.Viruses are the most abundant creature to be living on the planet. A virus is a small infectious agent that has either DNA or RNA as their genetic material. They infect animals, plants as well as bacteria. Depending on the organism they infect and the type of genetic material they have, viruses are divided into different types. Two of those types are retrovirus and bacteriophage. Let us discuss these viruses in detail.
A virus that is composed of single-stranded RNA as their genetic material are referred to as retroviruses. The RNA is mostly of a positive polarity and is 7-10 kb in size. They belong to the family Retroviridae. They have an outer protective envelope that is made up of lipids and glycoproteins.
The hallmark of a retrovirus is the ability to synthesise reverse transcriptase enzymes. Because their genetic material is RNA, they need to transcribe back into double-stranded DNA. This is made possible by the process of reverse transcription. The virus incorporates itself into the host’s genome and the host machinery is compelled to replicate and transcribe the viral DNA.
Chapter 6 An Introduction To Viruses
Retroviruses are of two types, based on the number of polyproteins they encode. Simple retroviruses that encode pol, gag and env proteins, and complex retroviruses that code six more accessory proteins in addition to the three polyproteins.
The infection of retrovirus is seen commonly in plants and animals. The glycoproteins on the envelope bind to receptor proteins on the cell surface on the host. The protective coat of the virus degrades and becomes a part of the host cell. The RNA and reverse transcriptase enzyme enters the host cell and starts replication. It is then integrated into the host genome. The host genome then replicates capsid proteins and viral RNA, and new retroviruses are gathered together which then attack the host cell.
A virus that infects bacteria and archaea are referred to as bacteriophages. They have a proteinaceous capsule on the outside. Their genetic material is either DNA or RNA, that is single or double-stranded. They are also known as a phage, informally.
Future Animal Health Applications For Bacteriophages
The bacteriophage was discovered by Frederick W. Twort in Great Britain (1915) and Félix d’Hérelle in France (1917), separately. This particular class of virus is ubiquitous on this planet and can be found everywhere.
The phages are specific to a certain or a group of bacteria that it can infect. It usually binds to protein receptors on the surface of bacteria to infect them. After infection, the phage undergoes two types of life cycles. The first bacteriophage life cycle is lytic cycle, where the host cell is lysed as soon as the viral genetic material is replicated to aid in the release of new viral particles. Another is the lysogenic cycle, where the host cell is not immediately lysed but the genetic material is incorporated into the host genome and the phage may enter the lytic cycle at some later stages.
Examples of phage include T4, T2, T6, M13 and ƛphage. These were the very first phages to be studied. The T2 phage was used in the Hershey and Chase experiment to demonstrate that only the nucleic acids of the virus are injected into the host cell for replication and not protein molecules.
Virus‐like Particles: Next‐generation Nanoparticles For Targeted Therapeutic Delivery
The RNA is released in the host cell where it is reverse transcribed to form a DNA intermediate. The intermediate then gets incorporated into the host’s genome for replication.
The phage either undergoes lytic cycle where the viral cells take over the host cell’s machinery for replication, or lysogenic cycle where the genetic material is incorporated into the host’s genome.
Retroviruses are characterised by single-stranded, positive sense RNA as their genetic material. They synthesise reverse transcriptase enzymes to form a double stranded DNA to incorporate themselves into the host genome.
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