[net.sci] Restrictions in DNA copies in an organism

alexis@reed.UUCP (Dimitriadis) (11/20/84)

> > If there were enough copies of the genetic material in an organism
> > (and the restriction in the number of copies in higher organisms is no
> > doubt artificial) then all the pieces will probably get a full
> > complement of genetic material, QED. That is, a simple organism does not
> > need specialized structures for replicating.

> Also, re the number of copies of the genetic material being limited
> in higher animals, and artificial, no way.  Experiments done in Drospohila 
> melanogaster (the fruit fly) since the time of T.H. Morgan (about 1920)
> rather clearly demonstrate that having *extra* copies of some genes are very
> detrimental to the organism, i.e. it dies or cannot reproduce, which
> genetically is the same thing.  I think it would be safe to say 
> that there are important constraints on the number of copies of genes.
> especially those important in developmental regulation.


  Just thought I'd defend my claim that the restriction in the number of 
copies of DNA is artificial...

  First of all, I consider Drosophila to be a "higher" organism. Lower
organisms would include bacteria, algae, etc.  Bacteria generally have
more than one copy of their plasmids. Mitochondria, even human, also have
(if I remember correctly) a variable number of copies of their genetic
material.  Some bacteria shuffle their genetic material by forming a
bridge between two individuals, through which a piece of chromosomal DNA
of RANDOM LENGTH is passed.
Higher plants will tolerate mutliple sets of chromosomes.  
They WILL be sterile if spurious chromosomes are added that
upset chromosome pairing during meiosis.  To me, that indicates that at
least some restrictions in the presence of DNA are caused by the
mechanism of sexual reproduction, which is not present in all organisms.
Hence "artificial restriction".

  If Neurospora crassa, the orange bread mold, is artificially given
thousands of extra copies of some of its own genes, it will over the
generations revert to the normal number. Still the increased number is
by no means lethal.  In general, in lower organisms there is a larger
variety in the amount of DNA present, with restrictions getting
progressively tighter in higher species.
  
  In higher organisms, there is no doubt that the number of copies is
critical.  In humans in particular, a missing copy is almost always
lethal, or may cause congenital defects, etc.   But I am pretty sure
that such restrictions result from the fine tuning of other mechanisms
that have come to depend on a constant amount of DNA.  I do not know of
anything to indicate that such mechanisms might be present in a Very 
Primitive organism.  I am open to suggestions.

		    Alexis
		(alexis @ reed)

alexis@reed.UUCP (Dimitriadis) (11/20/84)

> > If there were enough copies of the genetic material in an organism
> > (and the restriction in the number of copies in higher organisms is no
> > doubt artificial) then all the pieces will probably get a full
> > complement of genetic material, QED. That is, a simple organism does not
> > need specialized structures for replicating.

> Also, re the number of copies of the genetic material being limited
> in higher animals, and artificial, no way.  Experiments done in Drospohila 
> melanogaster (the fruit fly) since the time of T.H. Morgan (about 1920)
> rather clearly demonstrate that having *extra* copies of some genes are very
> detrimental to the organism, i.e. it dies or cannot reproduce, which
> genetically is the same thing.  I think it would be safe to say 
> that there are important constraints on the number of copies of genes.
> especially those important in developmental regulation.


  Just thought I'd defend my claim that the restriction in the number of 
copies of DNA is artificial...

  First of all, I consider Drosophila to be a "higher" organism. Lower
organisms would include bacteria, algae, etc.  Bacteria generally have
more than one copy of their plasmids. Mitochondria, even human, also have
(if I remember correctly) a variable number of copies of their genetic
material.  Some bacteria shuffle their genetic material by forming a
bridge between two individuals, through which a piece of chromosomal DNA
of RANDOM LENGTH is passed.
Higher plants will tolerate mutliple sets of chromosomes.  
They WILL be sterile if spurious chromosomes are added that
upset chromosome pairing during meiosis.  To me, that indicates that at
least some restrictions in the presence of DNA are caused by the
mechanism of sexual reproduction, which is not present in all organisms.
Hence "artificial restriction".

  If Neurospora crassa, the orange bread mold, is artificially given
thousands of extra copies of some of its own genes, it will over the
generations revert to the normal number. Still the increased number is
by no means lethal.  In general, in lower organisms there is a larger
variety in the amount of DNA present, with restrictions getting
progressively tighter in higher species.
  
  In higher organisms, there is no doubt that the number of copies is
critical.  In humans in particular, a missing copy is almost always
lethal, or may cause congenital defects, etc.   But I am pretty sure
that such restrictions result from the fine tuning of other mechanisms
that have come to depend on a constant amount of DNA.  I do not know of
anything to indicate that such mechanisms might be present in a Very 
Primitive organism.  I am open to suggestions.

		    Alexis Dimitriadis
		Senior Student in Biology
		      Reed College
		    (alexis @ reed)