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Invasion of a host cell by a DNA virus

About microscopic forms of life, including Bacteria, Archea, protozoans, algae and fungi. Topics relating to viruses, viroids and prions also belong here.

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Invasion of a host cell by a DNA virus

Postby littlexsparrow » Wed Sep 23, 2009 7:14 pm

I need help I've never sat higher biology before and have an essay to write and if I don't get 7 or more out of 10 then I'm being moved down but I need the grade. I think I've answered it as well as I can and I've cross referenced everything but I need to know if I can pass with this essay or if I reffered to anything wrong, so please help. It's greatly appreciated.

The question was: Give an account of the events that occur during the invasion of a host cell by a DNA virus.
My answer:
There are four general morphologies identified in viruses. Icosahedral Virions - an icosahedral protein shell (capsid) surrounds nucleic acid and proteins (the core). The capsid and core form a nucleocapsid. Helical Virions – long rods of nucleic acid are surrounded by a cylindrical capsid which has a helical structure. Enveloped Virions – the nucleocapsid, either icosahedral or helical, is surrounded by a loose membranous envelope. The envelope is roughly spherical but makes varying shapes because it is not rigid. Complex Virions – these do not possess clearly defined capsids, but have several coats around the nucleic acid core. Nucleocapsids that are not enveloped are called naked virions (i.e. Iscosahedral or Helical virions).

Viruses only multiply in particular host cells which can be broken down into 3 general classes, animal viruses, bacterial viruses (bacteriophages) and plant viruses. Within each of these groupings the virus is still only able to infect set cells and this is determined by attachment specificity and the available of cellular factors needed for virus replication. The attachment specificity is dictated by the properties of the virions coats and the surface of the cells receptors. However, when it is a “naked” virus these receptions difficulties disappear as specificity receptors are not there. The naked cells bind to the host’ membrane and digest it until they can enter the cytoplasm fully intact or they enter by endocytosis where they enter the host cell through a endocytic vesicle.

When a virion enters the body it searches for a host cell that has complimentary receptors to its “anti-receptors”, when found it joins to a specific receptor site on the host cell it does this by adsorption where glycoproteins on the viral envelope adsorb to complimentary receptors on the host cells membrane, it does this by tricking the cell into believing it’s a harmless protein. When the virion is attached it then can enter by membrane fusion, by endocytosis or by genetic injection.

By the way of membrane fusion viral receptors attach to the surface receptors of the cell and secondary receptors may be present to initiate the puncture of the cell membrane or fusion with the host cell this is followed by the unfolding of the viral envelope. This simply means the virus's envelope blends with the cell membrane, releasing its contents into the cell. This can only be done with viruses that contain an envelope such as HIV or influenza.

By the way of endocytosis in which the virus tricks the cell into believing that the virus is nothing more than nutritious, harmless goods and since the cell takes in these goods naturally from the surrounding enviroment, it tricks it by attaching proteins to the surface of the cell the cell then engulfs them in an edoctytic vacoule, when the virus is in the cell it must then break out of the vacuol into the cells cytoplasm. This method is used by the poliovirus and the Hepatitus C virus.

By way of Genetic injection is done by simply attaching to the surface of the cell via receptors on the cell, and injecting only its gene into the cell, leaving the rest of the virus on the surface. This is only done by viruses which only need the gene to infect the cell, usually done by RNA viruses, and is also restricted to viruses that actually exhibit this behavior. The best studied example includes the phages.

The virus then must make its way into the nucleus of the cell it does this in one of three ways. In some forms of virus the viral DNA is already in the cytoplasm of the cell and so makes its way through the pores in the nucleus’s membrane. In others a nucleocapsid of the virus interacts with the nuclear membrane of the host cell allowing the viral DNA to enter the nucleus through the pores in the nuclear membrane and leaving the capsid behind in the cells cytoplasm. The third way is done by certain small DNA viruses where the entire nucleocapsid enters the nucleus through the pores of the nuclear membrane and then the capsid is dissolved releasing the viral DNA into the nucleus.

It then goes onto a lysogenic cycle or a lytic cycle, the majority of viruses continue straight on to the lytic cycle. Many virions contain special enzymes or virion enzymes not available in the host cell. These are used to aid in the manipulation of the host cell once invaded.

During the lysogenic cycle the viral nucleid acid joins to a specific site of the host cells chromosome, this is known as the provirus. This provirus lays dormant and as the host cell reproduces it replicates its own DNA as well as the provirus meaning that both daughter molecules are infected with the dormant provirus. At any time the provirus may leave the host chromosome and then enter the lytic cycle.

The lytic cycle is when the DNA of the virus cell starts taking over the host cell. The virus' nucleic acid uses the host cell’s machinery to make large amounts of viral components. In the case of DNA viruses, the DNA transcribes itself into messenger RNA (mRNA) molecules, to produce viral mRNA molecules, host cell-DNA-dependent RNA polymerase enzymes copy the (-) DNA strand into (+) viral mRNA. The (+) viral mRNA can then be translated into viral proteins by host cell ribosomes. Examples include most bacteriophages, Papovaviruses, Adenoviruses, and Herpesviruses.One of the first polypeptides to be translated is one that destroys the hosts' DNA. Once this is done the virus effectively turns the host cell into a virsu factory. The broken down hosts DNA nucleotides are used in reproducing the virions DNA. In the later phases of the lytic cycle the DNA starts creating protein “heads” and “tails” for the viruses. Once all needed virus components are created they are assembled into complete viruses once this is complete many virus complete the cycle by lysis, or destrution of the cell. This is when the phage directs the production of an enzyme that breaks down the cell wall allowing fluid to enter and then the cell becoms filled with viruses, typically between 100 and 200, and fluid which cause the cell to “burst”. This allows the viruses to infect other cells and start the cycle all over again.

However, it is possible to have the lytic cycle without lysis this occurs where there is a membrane and not a cell wall. This is often referred to as the produtive cycle but is still classified as the lytic cycle because it has large similarities. This is when the virus “buds” off of the host cell by removing part of the membrane with them meaning the cell is not completely destroyed, this is common with the influenza virus and HIV.
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Postby canalon » Thu Sep 24, 2009 1:48 am

First paragraph is not related to the question.

Para 2: Partly on subject, but lot of filler. Identify the relevant bits (specificity) and mix them with the next para.

Para 3: Virion do not have desires or any thinking abilities, nor does cells, they do not believe anything. In a solution virion diffuse until they bind to specific receptors and this will be a valid objection to a lot of the rest of your text, improve that. And cell just ingest/adsorb proteins (harmless or not) that are bound to the right receptors/channels.

Para 4, 5 & 6: Not 100% clear, but OK (transition with para above could be made smoother).

Para 7: OK, but you can link with para above (naked DNA follow DNA injection etc..)

Para 8: Info OK, but poorly organized. You intorduce what is going to follow, but adds a bit that is not relevant to what you are speaking. This could have been introduced above, proteins and DNA enter at the same time.

para 9: OK, but not all lytic viruses do destroy the DNA. Be careful with your informations, you make general statement that are far from being general:
-Many viruses will not kill the host cell and have lower constant output rather than synthetic burst and release of virions accompanied by the destruction of the cell (see your para 10 which is correct)
-Not all viruses have heads and tails
The fact that those are just special cases should be more evident.

In general, I would say that you are roughly OK, but you need to work on your wording and to improve the hierarchy of your informations, from general to the anecdote.
Patrick

Science has proof without any certainty. Creationists have certainty without
any proof. (Ashley Montague)
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