throop@cs.utexas.edu (David Throop) (01/22/91)
While we're on the subject of chlorplasts and other organelles, let me
ask, "Why do we still have them?"
The DNA in the organelles is separate from the DNA of the rest of
the genome and each individual inherits all of its organelle DNA from
just one parent. As such, the the organelle DNA gets none of the
advantages of sex. These advantages include:
Diploidy - having two copies of genes, with minor variations - if
some environmental factor makes one copy disfuntional the other copy
may still function and let the organism survive.
Genome shuffling - the chance to take beneficial mutations from two
parents and combine them into a single individual.
And there are other advantages. [My terminology may be off, I'm
not a biologist.]
So if sex is so great why don't organelles have it? Specifically,
why has there not been evolutionary pressure to move the functions of
the organelle out of the organelles' DNA onto the chromosomes? Even
if the organelles originated from symbiotes, why haven't the two
genetic legacies been joined over evolutionary time?
David Throop
overt@antony (Christian Overton) (01/22/91)
In article <1179@ai.cs.utexas.edu>, throop@cs (David Throop) writes: > So if sex is so great why don't organelles have it? Specifically, >why has there not been evolutionary pressure to move the functions of >the organelle out of the organelles' DNA onto the chromosomes? Even >if the organelles originated from symbiotes, why haven't the two >genetic legacies been joined over evolutionary time? > Actually, most mitochondrial and chloroplast proteins are in fact coded for by nuclear genes not organelle genes. A problem with this arrangement is that proteins synthesized in the cytosol must then be targeted for the organelle and then transported across the organelle's membranes. A signal peptide on the protein directs it to the organelle where it is then transported across at 'contact sites (zones)' where the inner and outer membranes join. Characterization of the signal peptide and the mechanisms for transport across the mitochondrial membrane are areas of active research. Chris -- +-------------------------------------------------------------------------------+ | G. Christian Overton || Telephone: (215) 648-2420 | | Center for Advanced Information Technology || Internet: overt@prc.unisys.com | | Unisys || FAX: (215) 648-2288 |
sarima@tdatirv.UUCP (Stanley Friesen) (01/25/91)
In article <1179@ai.cs.utexas.edu> throop@cs.utexas.edu (David Throop) writes: > The DNA in the organelles is separate from the DNA of the rest of >the genome and each individual inherits all of its organelle DNA from >just one parent. As such, the the organelle DNA gets none of the >advantages of sex. ... > So if sex is so great why don't organelles have it? Specifically, >why has there not been evolutionary pressure to move the functions of >the organelle out of the organelles' DNA onto the chromosomes? Even >if the organelles originated from symbiotes, why haven't the two >genetic legacies been joined over evolutionary time? The answer is that there has been such evolutionary pressure. On the assumption that the symbiotic hypothesis is correct, a great deal of the original genome of the proto-organelle has been transfered to the nucleus. What is left is almost (but not quite) vestigial. The genes remaining in the organelles mostly code for some (but not all) of the basic building block of the organelle, but most of the regulatory and 'functional' genes are in the nucleus. (For instance the genes coding for the energy transfer chain that makes ATP are apparently nuclear). -- --------------- uunet!tdatirv!sarima (Stanley Friesen)