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Protein Synthesis

  1. Jun 25, 2005 #1
    In transcription, the mRNA, with the help of RNA polymerase, creates a code complementary to the sense strand on the DNA (i think). So does that mean that the mRNA's codon is opposite of the actual codon that codes for a protein? This would mean that the anticodon on the tRNA and the codon on the initial DNA would be the same. So, for example, for the amino acid methiomine, would the codon "sequence" be AUG (which is on the mRNA), or UAC (which is on the DNA and tRNA)?
     
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  3. Jun 25, 2005 #2

    iansmith

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    Some correction. During transcription, RNA polymerase creates mRNA that is complementatry to anti-sense (the non coding strand) DNA strand. So the mRNA is the same as the sense DNA strand.

    Yes you are right in term of your logic.
     
  4. Jun 25, 2005 #3
    Nay

    NO. DNA does not contain the base Uracil. It contains Thymine instead. So it would be:

    Original DNA: TAC
    mRNA Strand: AUG
    tRNA Strand: UAC

    DNA would not be the exact same as tRNA. It would be the same except for the Thymine/Uracil difference.

    DNA is not the same as mRNA. mRNA is complimentary to DNA, that is correct. But it will have a Uracil anywhere there was a Thymine in the DNA that the mRNA was coded from.
     
    Last edited by a moderator: Jun 25, 2005
  5. Jun 25, 2005 #4
    i think that when he said "the same" iansmith already assumed the U/T difference.
     
  6. Jun 25, 2005 #5
    The tRNA gets a little more interesting because its tertiary (3 dimensional structure) allows a tRNA charged with an amino acid (tRNA-aa) to bind to more than one codon on the mRNA because of a thing called "wobble" pairing. This position corresponds to the 3rd codon position on the mRNA (5' to 3') and the 1st position of the tRNA (5' to 3'). This means that a single charged tRNA can bind to more than one codon.
    3' UAC 5'
    5' AUG 3'
     
  7. Jun 26, 2005 #6
    Was clarifying, since he said "UAC (which is on the DNA" made me wonder if he knew.
     
  8. Jun 26, 2005 #7
    yes, and this is why the genetic code is said to be degenerate.

    it is also interesting that single mutations in a gene can be overshadowed by mutations in a corresponding tRNA to set everything back again!
     
  9. Jun 26, 2005 #8
    Or you don't even need a mutation in a corresponding tRNA. I can't think of an example off the top of my head, but if you have a silent point mutation in mRNA, some codons code for the same amino acid. There's about 60 codons and 20 amino acids I believe. So you have doubles, even triples.

    EDIT: Just did a google search, and here's an example:

    UUU (Phe/F)Phenylalanine
    UUC (Phe/F)Phenylalanine

    Both code for Phe, so if you had a UUU have a silent mutation to UUC, it wouldn't matter.

    Source: http://en.wikipedia.org/wiki/Genetic_code
     
  10. Jun 26, 2005 #9
    you misunderstand me. there are single mutations that would matter.

    let's say that you have a mutation in a gene that winds up substituting Ser instead of Leu(TTG->TCG). it has been observed that frequently this will be accompanied by a mutation in the sequence of DNA that makes the tRNA for Leu to match the new mutated anticodon for Ser, thus undoing the mutation.
     
  11. Jun 26, 2005 #10
    Oh I see what you are saying. But then it's still technically a mutation, and it screws up the amino acid sequence, because you have a different amino acid than originally intended, hence, a different protein.
     
  12. Jun 26, 2005 #11
    no, there isn't since the mutation in the tRNA will exactly undo the mutation in the gene.

    however, you are right in that it is still a mutation since the genetic sequence has been changed - but not the amino acid sequence.
     
  13. Jun 26, 2005 #12
    You said previously that Leu would be replaced with Ser. That would be a change in the amino acid sequence and the genetic sequence. That's called a mutation.
     
  14. Jun 26, 2005 #13
    yes, you are right, but in the case that i gave the tRNA would also undergo a mutation so that everything would be set right (so even though the mRNA codon specifies a Ser, the anticodon region of a Leu tRNA would be mutated to be complementary to a Ser codon). so in the end, the correct amino acid is added to the peptide even though the genetic sequence has been changed.

    i forget what this particular process is called, but it does occur in life.
     
  15. Jun 26, 2005 #14
    But the amino acid still ends up being different fron what it originally was supposed to be, hence MUTATION. A mutation is a change in genes and/or chromosomes, etc. It would be considered a point mutation, even though the tRNA mutates to match the mRNA. At least, according to the Biology course I learned this year.
     
  16. Jun 26, 2005 #15
    maybe we are miscommunicating...the amino acid that gets put on the peptide does NOT change. the original sequence specified a Leu, the genetic mutation in the DNA changed it to make a Ser, but before the peptide is actually made a tRNA is mutated to make a Leu. Therefore, the peptide is exactly what it was supposed to be.
     
  17. Jun 26, 2005 #16
  18. Jun 27, 2005 #17
    O_O We never learned that when we were learning protein synthesis.
     
  19. Jun 27, 2005 #18
    Sorry, but back 2 my initial thing. So, to be clear on it:

    The sense strand on the DNA is what actually codes for amino acids, the mRNA uses the antisense strand as a template so that it can get a similar coding scheme as the sense strand, and the anticodon on the tRNA has a sequence that is opposite of both the sense strand on the DNA and on the mRNA, right?
     
  20. Jun 27, 2005 #19

    iansmith

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    Yes, and as TheLoneWolf pointed out, just remember the difference between DNA and RNA.
     
  21. Jun 28, 2005 #20
    My teacher taught us to call it "tag-c" and that way we'd remember T binds to A and G binds to C.
     
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