ScreamYourHeartOut
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is the difference between them that plasmids replicate at the same time as the hosts chromosomes and that viruses can replicate independantly of the hosts chromsomes?
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nexttime
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(Original post by ScreamYourHeartOut)
is the difference between them that plasmids replicate at the same time as the hosts chromosomes and that viruses can replicate independantly of the hosts chromsomes?
Plasmids can replicate independently of the chromosomes also.

Plasmids tend to reside in bacteria (although there are exceptions), whereas viruses are called phages if they infect bacteria.

As far as i'm aware, viruses tend to disperse at the host's eventual destruction, which is not necessarily true of plasmids.

A key difference would be in what they encode - viruses code proteins that let them re-form and infect other cells. They typically encode a protein coat which helps them survive and specific mechanisms that can induce entry into cells. Plasmids, as far as i can tell, are accepted (or rejected) by bacteria rather than an active infection. This would be the key difference.

EDIT: There are viruses which are dependent on other viruses to infect cells e.g. Hepatitis D and Hepatitis B. I guess the key difference there will be its extracellular structure.
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l4ith
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It's not necessary that viral reproduction leads to host destruction with bacteriophages in the initial stages - there are actually two reproductive pathways: lytic and lysogenic replication. In lytic replication - as the name suggests - when the viral proteins have been made (in varying ways depending on the type of virus) - then the bacterial cell lyses and the virions burst out. In the lysogenic replication, the viral DNA can be incorporated into the bacterial chromosome. Thus as the bacterium undergoes bacterial division, the DNA along with viral DNA is passed on to more and more bacteria. An event such as UV radiation can trigger protein synthesis from the viral DNA, and hence also leads to destruction of the bacterial cell.

So, whilst in both cases there is in both cases eventual destruction of the host cell, it does not necessarily happen immediately.
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alijimi
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(Original post by ScreamYourHeartOut)
is the difference between them that plasmids replicate at the same time as the hosts chromosomes and that viruses can replicate independantly of the hosts chromsomes?
It is true that plasmid present in bacteria, replicates independently of the main DNA because it codes for proteins that are not essential for bacterial growth. It is rather involved in the resistance to antibiotics.

Viruses can either enter the cell and remain silent, that is known as lysogenic cycle; the viral DNA gets integrated in the host's DNA and replicates with it, but does not express viral proteins.
Or in the lytic cycle, the viral DNA replicates and is translated by the host's ribosomes into viral proteins, to form new viral particles or virions that are released by cell death. In the case of eukaryotic cell, virions are enveloped by a fragment of the plasma membrane from the host cell.
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nexttime
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(Original post by alijimi)
In the case of eukaryotic cell, virions are enveloped by a fragment of the plasma membrane from the host cell.
^ in the case of enveloped viruses.
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alijimi
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(Original post by nexttime)
^ in the case of enveloped viruses.
Enveloped by a plasma membrane fragment from the infected eukaryotic cell. Virions are then released without necessarily cell rupture. That is not the case in prokaryotic cell where rupture of cell wall and plasma membrane is a must for virions release
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l4ith
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Alijimi describes the way I thought it was. As an example, an HIV virion can exit a T helper cell without necessarily destroying it (although eventually the cell dies as many HIV virions escape). However in the case of bacteriophages, they produce an enzyme that breaks the cell wall of the bacterium, causing it to lyse.
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BioSam
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As far as I'm aware the only real distinction between the two is that viruses move between host cells in a protective protein coat (capsid) whereas plasmids exist as naked DNA.

Plasmids are not necessarily 'accepted' by bacteria - they can be and often are infectious and perceived by the host as a threat (hence the presence of restriction sites on plasmids, its a natural mechanism enabling bacteria to degrade harmful plasmids) -and there are other bacterial immune mechaisms that specifically target certain plasmids

Plasmids "selfishly" manipulate their host in much the same way of viruses, for instance some plasmids encode a poisonous protein which they release into a parent bacterium. The plasmid also encodes the antidote, so that unless all of the daughter bacteria take a plasmid with it, they will be killed by the poison (so this selectively favours bacteria who take the plasmid along with them).

Not all bacteriophages reproduce lytically - take M13 for example which has no lytic effect on its host.
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nexttime
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(Original post by BioSam)
...
Thank you for clearing that up

Out of interest - you say there are plasmids that kill non-plasmid infected cells. Are there any you know of that have active cell-entry mechanisms? - they could produce factors which make them more likely to be taken up. Or do they have to be 'accepted' (or rejected) by a bacteria? - a potential difference between phages and plasmids i guess (although generally secondary to the lack of structures when out of the cell, of course).
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BioSam
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(Original post by nexttime)
Thank you for clearing that up

Out of interest - you say there are plasmids that kill non-plasmid infected cells. Are there any you know of that have active cell-entry mechanisms? - they could produce factors which make them more likely to be taken up. Or do they have to be 'accepted' (or rejected) by a bacteria? - a potential difference between phages and plasmids i guess (although generally secondary to the lack of structures when out of the cell, of course).

Conjugative plasmids encode the proteins necessary for their transfer to another bacterium which I guess you could say is active cell entry (the recipient is fairly passive in the process as far as I'm aware)

Rejection can occur when the recipient bacterium already has a plasmid similar to the one trying to 'gain access' - this is called plasmid incompatibility but this is a pretty 'selfish' mechanism on the part of the plasmid too because the point of incompatibility is that having two similar plasmids in one bacterium would make it difficult to ensure that both plasmids were equally segregated and represented in daughter cells (so it's in the plasmid's "interest" to be rejected by incompatible cells.
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CoiT
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Plasmids replicate independently of the host chromosomes but viruses use the hosts metabolic machinery to replicate.
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