05-22-2012, 06:41 PM
Recombination
Exchange of genetic information between two genomes.
"Classic" recombination
This involves breaking of covalent bonds within the nucleic acid, exchange of genetic information, and reforming of covalent bonds.
This kind of break/join recombination is common in DNA viruses or those RNA viruses which have a DNA phase (retroviruses). The host cell has recombination systems for DNA.
Recombination of this type is very rare in RNA viruses (there are probably no host enzymes for RNA recombination). Picornaviruses show a form of very low efficiency recombination. The mechanism is not identical to the standard DNA mechanism, and is probably a "copy choice" kind of mechanism (figure 1) in which the polymerase switches templates while copying the RNA.
Figure 2 Marker rescue
Recombination is also common in the coronaviruses - again the mechanism is different from the situation with DNA and probably is a consequence of the unusual way in which RNA is synthesized in this virus.
So far, there is no evidence for recombination in the negative stranded RNA viruses giving rise to viable viruses (In these viruses, the genomic RNA is packaged in nucleocapsids and is not readily available for base pairing).
Reassortment
If a virus has a segmented genome and if two variants of that virus infect a single cell, progeny virions can result with some segments from one parent, some from the other.
This is an efficient process - but is limited to viruses with segmented genomes - so far the only human viruses characterized with segmented genomes are RNA viruses e.g. orthomyxoviruses, reoviruses, arenaviruses, bunya viruses.
Reassortment may play an important role in nature in generating novel reassortants and has also been useful in laboratory experiments (figure 3). It has also been exploited in assigning functions to different segments of the genome. For example, in a reassorted virus if one segment comes from virus A and the rest from virus B, we can see which properties resemble virus A and which virus B.
Reassortment is a non-classical kind of recombination
Exchange of genetic information between two genomes.
"Classic" recombination
This involves breaking of covalent bonds within the nucleic acid, exchange of genetic information, and reforming of covalent bonds.
This kind of break/join recombination is common in DNA viruses or those RNA viruses which have a DNA phase (retroviruses). The host cell has recombination systems for DNA.
Recombination of this type is very rare in RNA viruses (there are probably no host enzymes for RNA recombination). Picornaviruses show a form of very low efficiency recombination. The mechanism is not identical to the standard DNA mechanism, and is probably a "copy choice" kind of mechanism (figure 1) in which the polymerase switches templates while copying the RNA.
Figure 2 Marker rescue
Recombination is also common in the coronaviruses - again the mechanism is different from the situation with DNA and probably is a consequence of the unusual way in which RNA is synthesized in this virus.
So far, there is no evidence for recombination in the negative stranded RNA viruses giving rise to viable viruses (In these viruses, the genomic RNA is packaged in nucleocapsids and is not readily available for base pairing).
Reassortment
If a virus has a segmented genome and if two variants of that virus infect a single cell, progeny virions can result with some segments from one parent, some from the other.
This is an efficient process - but is limited to viruses with segmented genomes - so far the only human viruses characterized with segmented genomes are RNA viruses e.g. orthomyxoviruses, reoviruses, arenaviruses, bunya viruses.
Reassortment may play an important role in nature in generating novel reassortants and has also been useful in laboratory experiments (figure 3). It has also been exploited in assigning functions to different segments of the genome. For example, in a reassorted virus if one segment comes from virus A and the rest from virus B, we can see which properties resemble virus A and which virus B.
Reassortment is a non-classical kind of recombination