Ted Byers wrote:
It has been more than ten years since I last discussed this with a
specialist in the genetics of animal development. At that time at least, it
was believed that the suite of introns used to form a gene varies through
development, so, for example, the actual composition of your hemoglobin
right now is different from what it was when you were a kid, and will be
different again when you're an ol' fossil like me. So, according to what
she told me, what counts as an intron or exon (what some refer to as junk)
will depend on age as well as the gene in question. Alas, she didn't have
answers for many of my questions, like "How exactly does the selection of
introns vs exons happen?" or "How is the gene constructed from the introns
once the introns have been made?"
There is a set of proteins (and some neat RNA molecules) in place
that takes care of splicing in your cells. That is what happens, the
full length message RNA is created from the DNA, exons (the coding bits)
and introns (the non-coding bits) both. It is rapidly processed by the
splicing machinery, which deletes the introns from the message, which is
then used as the template to create a protein. I haven't heard about
the age thing, but it is true that there can be many different messages
made from one template. This is called alternate splicing. Different
forms of a protein can be made by adding or removing exons from the
list, although I think they stay in order. So you can get protein ABCD,
BCD, ACD, etc. (where the letters represent an exon). Some of those
splice forms may be functional, some maybe not. The specifics of how
splicing occurs are pretty well worked out (it is complicated, of
course). The specifics of how the cell controls which splice form to
make are a little less clear, I think, although that isn't my area of
expertise.
or "Is there any intermingling of introns
from different genes (e.g. is it possible to have an intron from one gene in
between the introns of some other gene); if so, how does the cell know which
introns are part of a given gene?
I don't think this is a frequent occurance. There are 'chimeric'
proteins, I know of a few involved in cancer, but I think those are
usually created by recombination of the genomic DNA, not the message. I
have heard of something called 'trans-splicing', which would seem to be
what you suggest, although I know absolutely nothing about it.
or "How is the correct sequence of
introns in the gene stored and later recovered for use?"
The correct sequence is stored in the genomic DNA. This isn't at
all affected by splicing. Message is created from the DNA template,
that RNA (message RNA, or mRNA) is then spliced. So you don't have to
worry about destroying the information. There is something called
recombination, which is particularly prevalent in creation of
antibodies, where the actual DNA of the antibody producing cell is
changed for the immunoglobulin genes. This is what allows you to make
such a large variety of antibodies (the number of possible combinations
is staggering). Or at least that is what I learned several years
ago... It is a special case though, and those rearrangements don't
carry over to your offspring.
or "Does all this
happen in the nucleus?" or
Pretty much.
"How does the mechanism in a given nucleus know
what the age of the organism is in order to know which introns to use at any
given time?"
This isn't my specialty either, but if this does happen, it is
probably a function of telomere length. The telomeres are repetitive
sequence at the end of each chromosome. Due to the way DNA replication
works, you lose a little bit of telomere with every cell division. So,
the sorter the telomeres, the older the organism. Neat... I think this
is one of the more prevalent hypotheses about how the aging function
works. There is probably also a contribution from DNA damage, your DNA
accumulates various mutations and damage over time, and there are
proteins which sense this. Next you are going to ask me how everything
gets reset to 'normal' in the next generation... I don't really want to
go there.
You learn all sorts of things from an orchid newsgroup... *grin*
Rob
--
Rob's Rules:
http://www.msu.edu/~halgren
1) There is always room for one more orchid
2) There is always room for two more orchids
2a. See rule 1
3) When one has insufficient credit to purchase
more orchids, obtain more credit