A quick comment on this interesting thread. With respect to the retrieval
of the control regions and loss of information. Another phenomena
connected at times with reverse transcription is RNA editing which I posted
about (although not very articulately) several months ago. Information
(nucleotide code) may actually be changed after it is transcribed and
before it is translated. Certain bases may be changed (usually C-T) in
order to provide a messenger RNA that will produce a functional protein.
Additionally there are cases in fungi (Physarum) in which the 17SrRNA
(which here is not tranlated of course) actually acquires 40 cytidine
insertions AFTER transcription.
>
>Next, the introns are lost when RNA goes to mRNA. HOw this could be
>reconstructed is also a mystery to me.
>
>Lastly, the information about the specific codons used is lost in the
>degeneracy of the genetic code when mRNA is translated to protein.
>
Retrieval of 5'and 3' control regions would not be retrieved by the reverse
transcription itself but requires the reinsertion of the cDNA into a
position in the genome already equipped with the appropriate up- and
downstream sequences.
By inference it has be assumed that this has happened in the past with
respect to organellar genomes in eukaryotes. The endosymbiontic theory
hold that both the chloroplast and mitochondrial genome represent bacterial
lines caught (were probably parasites on originally) in other cells and
since has lost a large component of their genome but in addition have moved
many of their genes to the nucleas. The latter act requires some real
contortions since the genes in the cp and mt are essitially utilized/copied
in a procaryotic manner whereas when moved they had to attain the
eukaryotic control seqeunces in order to be expressed in the nucleas.
Examples - two subunits are requried to form a functional protein (rbcl)
that is used in all photosynthetic plants/algae. One subunit is coded for
in the cpDNA genome while the other is in the nucleas. The nuclear subunit
is transcribed and then translated in the cytoplasm and the subunit
trasported in the chloroplast to be joined with the other subunit. Another
example is that both the chloroplast and mitochondrial genomes have only a
subset of the tRNAs requrired and must transport many of the tRNAs from the
nucleus.
There are several examples of genes that are found in the cpDNA of all
algae studies that are found in nucleus of plants. There are several
examples of cpDNA genes that have been moved to the mitochondria.
Another quick note of RNA editing. If an edited RNA is reverse transcribed
and reinserted in the DNA would not just represent a gene duplication but
in fact would represent a different DNA sequence. The addition of a "more
efficient" copy (meaning it wouldn't need to be edited a potentially costly
effort) might relax the contraints on the original sequence allowing it to
be transcribed without being edited potentially resulting in a very
different protein (albeit this would be a longshot).
Hope this doesn't muddy the waters too much.
Joel
Postdoctoral Fellow
Department Plant Biology
Southern Illinois University
http://www.intrnet.net/~virkotto/index.html
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