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RNA: a few questions

  1. Oct 19, 2003 #1
    Greetings.
    I have a few questions about RNA.

    1-In Nirenberg's experiment (building a protein from mRNA made only of U-U-U-U-U ...), how was the protein actually built if it didn't start with AUG (the starting Codon). If the codon AUG is not neccesary for starting the building of a protein, can you please explain how the building starts otherwise ?.

    2-During mRNA processing, how are the introns identified ?

    3-Why are the introns there from the first place ? If they are not translated to any protein, what is their usage ?
    If they have no usage, is it possible that (with time, and evolution) that the introns will go from the original DNA molecule ?

    4-As far as i understand, the DNA is transcripted to mRNA, which is then translated to a protein, how does the cell know when it should transceipt a certain part of DNA to mRNA and went it shouldn't (iow, when the protein that the mRNA builds is not needed).

    5-Are mRNA molecules re-used after building the protein molecule (iow, if the mRNA stays in the cytoplasm, will it be re-translated .. and re-translted ... and re-translated .. ?).

    Thanks in advance (that's all .. for now ).
     
  2. jcsd
  3. Oct 19, 2003 #2

    iansmith

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    Can you point out where you got nirenberg's experirement. I don't know what you are takling about?

    introns have a specific sequences at the 5' end, which is the donor, and at the 3' end, which is the acceptor. It goes like this
    Exon-5'CG/AGGUA/GAGU---Intron-----T/CNAHN3'-Exon--
    ________Donor site______________Acceptor site

    Nobody knows why introns are there but it is observed that there is alternative splicing sites. There could be a different 5' end, 3' end or a different internal structure.

    For example, Calcitonin has 2 polyadenylation signals which are tissue specific. In the thyroid the 1st signal will be used whereas in the brain the 2nd signal is used. The exon at 3' end is different.

    There is region upstream of the gene that is called the regulator region. These region control the expression of the DNA. Regulation of the DNA is complex. There can be DNA-protein, DNA-DNA and DNA-RNA interaction that control the expression. For example, some protein are produce only in specific tissues and these protein have a role in promoting tissue specific gene and these gene are can only be express when the protein is present.

    You are rigth, mRNA is recylce untill it is degraded and many ribosomes (translation untis) can bind to the same mRNA strand.
     
  4. Oct 19, 2003 #3
    Ok, i will come back to questions 2-5. But for now, let me talk about 1.
    It is weird that you don't know this experiment.
    His experiment is also known as the "Poly-U" experiment.
    Mainly, what he did is that he made an RNA molecule made only of Uracil units.
    He then got 20 tubes, and put differnt kinds of Amino acids into the 20 tubes (and also he put Ribosomes, and Enzymes, and other stuff). He put one molecule of this Poly-U RNA into each tube.
    He waited, then came back, and found that only one tube had protein made in it (the one with the amino acid phenylalanine).
    Anyway, the point is that the protein was made although the RNA did not start with AUG (the starting codon).
     
  5. Oct 19, 2003 #4

    Monique

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    Well, although AUG is a predominant initiator codon, there are other possibilities such as GUG or CUG. You say the molecule was only composed of Us? This was a eukaryotic or prokaryotic system? Maybe there were impurities? I haven't heard of the experiment before either :S

    You might be interested in going through the following tutorial, it is quite nice!
    http://www.lsic.ucla.edu/ls3/tutorials/
     
  6. Oct 19, 2003 #5
    Ok, i am browsing it.
    In the meanwhile, you might browse this link too :) http://profiles.nlm.nih.gov/JJ/
     
  7. Oct 19, 2003 #6

    iansmith

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    I have look at the paper publish by Nirenberg (http://profiles.nlm.nih.gov/JJ/B/B/D/K/_/jjbbdk.pdf). From what I read, it appears that they did not used all the ribosome sub-unit, they only used the 30S fraction or they added a lot 30S rRNA. I think that the start codon was not required because of the condition used during the experiement.
     
  8. Oct 19, 2003 #7
    Ok, for Q1, i got what i need (not only AUG is a starting codon).
    Now, the other questions.
    Q2
    good till now
    You lost me ..

    Q5
    So suppose that the cell does not need a certain protein anymore (or has enough of it after some time), the ribosomes will still build from that protein?
     
  9. Oct 19, 2003 #8

    iansmith

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    For question 2 i gave you the sequence that is recognize as being the start and end of an intron. So the 5' end of the intron is start and the 3' end is the end. when the intron is cut out the 5' end will be donate to the 3' end which will accept it in order to connect the two exons

    Yeah the cell keeps on build the protein untill there is no more mRNA but mRNA is very unsatble so it degraded rapidly.
     
  10. Oct 19, 2003 #9

    Monique

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  11. Oct 19, 2003 #10

    Monique

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    Basically, small nuclear ribonucleoprotein particles, snRNPs recognize signals (that Ian gave) at the intron/exon boundaries. These snRNPs cause conformational change of the RNA and cleavage.

    Four different sites have been found:
    5'splice site
    3'splice site
    branchpoint (in the middle)
    polypyrimidine track (after the branchpoint)
     
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