Target Health Blog

Viruses

March 30, 2020

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Quiz
Source:

Two rotaviruses: the one on the right is coated with antibodies which stop it attaching to cells and infecting them
Photo credit: By GrahamColm at en.wikipedia;
https://commons.wikimedia.org

A 1) ____ is a microscopic parasite which can infect living organisms and cause disease. It can make copies of itself inside another organism's cells. Viruses consist of nucleic acid + a protein coat. Usually the nucleic acid is RNA; sometimes it is DNA. Viruses are able to cause many types of diseases such as polio, ebola and hepatitis. Virology is the study on viruses.

Viruses reproduce by getting their nucleic acid strand into a prokaryote or eukaryote (cell). The 2) _____ or _____ strand then takes over the cell machinery to reproduce copies of itself and the protein coat. The cell then bursts open, spreading the newly created viruses. All viruses reproduce this way, and there are no free-living viruses. Viruses are ubiquitous (everywhere) in the environment, and all organisms can be infected by them.

Viruses are much smaller than bacteria. They were not visible until the invention of the electron microscope. A virus has a simple structure, it has no internal cellular structure, no cell wall or cell membrane, just the protein coat that holds the string of nucleic acid. With eukaryote cells, the virus protein coat is able to enter the target cells via certain cell membrane receptors. With prokaryote bacteria cells, the bacteriophage physically injects the nucleic acid strand into the host cell. When the host cell has finished making more viruses, it undergoes lysis, or breaks apart. The viruses are released and are then able to infect other cells. Viruses can remain intact for a long time, and will infect cells when the time and conditions are right.

Some special viruses are worth noting. Bacteriophages have evolved to enter bacterial cells, which have a different type of cell wall from eukaryote cell membranes. Envelope viruses, when they reproduce, cover themselves with a modified form of the host cell membrane, thus gaining an outer lipid layer that helps entry. Some of our most difficult-to-combat viruses, like influenza and HIV, use this method.

Viral infections in animals trigger an immune response which usually kills the infecting virus. 3) _____ can also produce immune responses. They give an artificially acquired immunity to the specific viral infection. However, some viruses (including those causing AIDS and viral hepatitis) escape from these immune responses and cause chronic infections. Antibiotics have no effect on viruses, but there are some other drugs against viruses.

There are  many genomic structures in viruses. As a group they have more structural genomic diversity than plants, animals, archaea, or bacteria. There are millions of different types of viruses, but only about 5,000 of them have been described in detail.

The life cycle of viruses differs greatly between species but there are six basic stages in the life cycle of viruses:

1.     Attachment is a specific binding between viral capsid proteins and specific receptors on the host cellular surface.

2.     Penetration follows attachment: Virions (single virus particles) enter the host cell through receptor-mediated endocytosis or membrane fusion. This is often called viral entry.

3.     The infection of plant and fungal cells is different from that of animal cells. Plants have a rigid cell wall made of cellulose, and fungi have one of chitin. This means that most viruses can only get inside these cells by force. An example would be: a virus travels on an insect that is a vector of disease. The same insect feeds on plant sap. while feeding, the insect damages the cell walls, thus letting the virus enter.

4.     Bacteria, like plants, have strong cell walls that a virus must get through to infect the cell. However, bacterial cell walls are much thinner than plant cell walls, and some viruses have mechanisms that inject their genome into the bacterial cell across the cell wall, while the viral capsid remains outside.

5.     Uncoating is a process in which the viral capsid is removed: This may be by degradation by viral enzymes or host enzymes or by simple dissociation; the end-result is the releasing of the viral nucleic acid.

6.     Replication of viruses involves multiplication of the genome. This usually requires production of viral messenger RNA (mRNA) from “early“ genes. This may be followed, for complex viruses with larger genomes, by one or more further rounds of mRNA synthesis: “late“ gene expression is of structural or virion proteins.

In some viruses the viral genome is put by genetic recombination into a specific place in the host's chromosome. The viral genome is then known as a “4) _____“ or, in the case of bacteriophages a “prophage“.

Whenever the host divides, the viral genome is also replicated. The viral genome is mostly silent within the host; however, at some point, the provirus or prophage may give rise to active virus, which may lyse the host cells.

5) _____ viruses (e.g. HIV) typically are released from the host cell after the virus acquires its envelope. The envelope is a modified piece of the host's plasma membrane.

The genetic material within virus particles, and the method by which the material is replicated, varies considerably between different types of viruses.

Reverse transcribing viruses with RNA genomes (retroviruses) use a DNA intermediate to replicate. Those with DNA genomes (pararetroviruses) use an RNA intermediate during genome replication. They are susceptible to antiviral drugs that inhibit the reverse transcriptase enzyme. An example of the first type is HIV, which is a retrovirus. Examples of the second type are the Hepadnaviridae, which includes 6) _____ B virus.

RNA interference is an important innate defence against viruses. Many viruses have a replication strategy that involves double-stranded RNA (dsRNA). When such a virus infects a cell, it releases its RNA molecule. A protein complex called dicer sticks to it and chops the RNA into pieces. Then a biochemical pathway, called the RISC complex, starts up. This attacks the viral mRNA, and the cell survives the infection.

The production of 7) _____ is an important host defense mechanism. This is a hormone produced by the body when viruses are present. Its role in immunity is complex; it eventually stops the viruses from reproducing by killing the infected cell and its close neighbors. Vertebrates have a second, more specific, immune system. It is called the adaptive immune system. When it meets a virus, it produces specific antibodies that bind to the virus and render it non-infectious. Two types of antibodies are important. The first, called 8) _____, is highly effective at neutralizing viruses but is produced by the cells of the immune system only for a few weeks. The second, called IgG, is produced indefinitely. The presence of IgM in the blood of the host is used to test for acute infection, whereas IgG indicates an infection sometime in the past. IgG antibody is measured when tests for immunity are carried out.

Another vertebrate defense against viruses is cell-mediated immunity. It involves immune cells known as 9) _____ cells. The body's cells constantly display short fragments of their proteins on the cell's surface, and, if a T cell recognizes a suspicious viral fragment there, the host cell is destroyed by killer T cells and the virus-specific T-cells proliferate. Cells such as macrophages are specialists at this antigen presentation.

Not all virus infections produce a protective immune response. These persistent viruses evade immune control by sequestration (hiding away); blocking antigen presentation; cytokine resistance; evading natural killer cell activity; escape from apoptosis (cell death), and antigenic shift (changing surface proteins). HIV evades the immune system by constantly changing the amino acid sequence of the proteins on the surface of the virion. Other viruses, called neurotropic viruses, move along nerves to places the immune system cannot reach.

A recent project discovered nearly 1500 new RNA viruses by sampling over 200 invertebrate species. “The research team extracted their RNA and, using next-generation sequencing, deciphered the sequence of a staggering 6 trillion letters present in the invertebrate RNA libraries. The research showed that viruses changed bits and pieces of their RNA by a variety of genetic mechanisms.

A group of large viruses infect amoebae. The largest is Pithovirus. Others in order of size are Pandoravirus, then Megavirus, then Mimivirus. They are bigger than some bacteria, and visible under a light microscope.

And yes, there are 10) _____ viruses. Most plant viruses are rod-shaped, with protein discs forming a tube surrounding the viral genome; isometric particles are another common structure. They rarely have an envelope. The great majority have an RNA genome, which is usually small and single stranded (ss), but some viruses have double-stranded (ds) RNA, ssDNA or dsDNA genomes. Although plant viruses are not as well understood as their animal counterparts, one plant virus has become iconic: tobacco mosaic virus (TMV), the first virus to be discovered.

ANSWERS: 1) virus; 2) RNA or DNA; 3) Vaccines; 4) provirus; 5) Enveloped; 6) Hepatitis; 7) interferon; 8) IgM; 9) T; 10) plant

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