It's worth noting that some of these chemicals are secreted by plants as part of a defense against
bacteria. Thus we probably have a kind of natural chemical arms race going on here. Other
metabolic plasmids allow bacteria to degrade herbicides like 2,4-D, as well as certain detergents!
People are investigating the use of such plasmids to help biodegrade pollution.
Tumor-Causing Plasmids
"Crown gall" is a cancer of plants caused by a bacterium known as Agrobacterium tumefaciens.
But actually, the disease is caused by a plasmid having this bacterium as its host! When the plasmid
passes from the bacterium to the cells of infected fruit trees, some of the genes contained in the
plasmid cause tumors. Do these tumors help spread the bacteria to other trees?
Transposons:
Transposons, or "transposable elements", are sequences of DNA that move within their host's
genome from one position to another. They were first discovered in the 1940s by Barbara
McClintock, who later won the Nobel prize for this work. They exist in all known organisms, often
in large quantities. Their main "function" appears to be simply their own self-replication, rather
than any benefit to the host, or even any direct effect whatsoever on the host phenotype. For this
reason, people sometimes refer to transposons as "selfish DNA".
In addition to transposons, there is plenty of other DNA in our chromosomes that doesn't seem to
code for proteins. This is sometimes called "junk DNA". It comes in various distinct forms, such
as "introns", "satellite DNA", and "pseudogenes". In fact, junk DNA makes up about 97% of the
human genome!
Satellites:
A satellite is a "sub-viral agent composed of nucleic acid molecules that depends for its
reproduction on co-infection of a host cell with a helper virus". In other words, just as a planet can
have a moon orbiting it, a virus can have a satellite orbiting it!
There are various kinds of satellites:
Satellite viruses
Satellite nucleic acids
Satellite DNAs
Double-stranded satellite RNAs
Single-stranded satellite RNAs
Satellite virus
A good example is the tobacco mosaic satellite virus, which goes along with the well-known
tobacco mosaic virus (TMV).
5
bacteria. Thus we probably have a kind of natural chemical arms race going on here. Other
metabolic plasmids allow bacteria to degrade herbicides like 2,4-D, as well as certain detergents!
People are investigating the use of such plasmids to help biodegrade pollution.
Tumor-Causing Plasmids
"Crown gall" is a cancer of plants caused by a bacterium known as Agrobacterium tumefaciens.
But actually, the disease is caused by a plasmid having this bacterium as its host! When the plasmid
passes from the bacterium to the cells of infected fruit trees, some of the genes contained in the
plasmid cause tumors. Do these tumors help spread the bacteria to other trees?
Transposons:
Transposons, or "transposable elements", are sequences of DNA that move within their host's
genome from one position to another. They were first discovered in the 1940s by Barbara
McClintock, who later won the Nobel prize for this work. They exist in all known organisms, often
in large quantities. Their main "function" appears to be simply their own self-replication, rather
than any benefit to the host, or even any direct effect whatsoever on the host phenotype. For this
reason, people sometimes refer to transposons as "selfish DNA".
In addition to transposons, there is plenty of other DNA in our chromosomes that doesn't seem to
code for proteins. This is sometimes called "junk DNA". It comes in various distinct forms, such
as "introns", "satellite DNA", and "pseudogenes". In fact, junk DNA makes up about 97% of the
human genome!
Satellites:
A satellite is a "sub-viral agent composed of nucleic acid molecules that depends for its
reproduction on co-infection of a host cell with a helper virus". In other words, just as a planet can
have a moon orbiting it, a virus can have a satellite orbiting it!
There are various kinds of satellites:
Satellite viruses
Satellite nucleic acids
Satellite DNAs
Double-stranded satellite RNAs
Single-stranded satellite RNAs
Satellite virus
A good example is the tobacco mosaic satellite virus, which goes along with the well-known
tobacco mosaic virus (TMV).
5
