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    I'm just trying to get my head around stop/start codons

    am I correct in thinking that the start codon on DNA begins the translation of one specific amino acid chain that produces one specific proteins

    so you have a dna strand on one chromosome - ATG starts the mRNA translation

    --------- ATG GTT AGA TAG TAA TGA -----------

    and that mRNA chain then moves to the ribosome simply as ATG GTT AGA TAG TAA TGA and code for a particular protein - and hence multiple mRNA strands are needed on one long strand of DNA with multiple stop/start codons

    or

    does the mRNA copy the entire strand and the ribosome organises the stopping and starting of protein chanins

    many thanks, I apologise if that is hard to follow
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    (Original post by Organ)
    I'm just trying to get my head around stop/start codons

    am I correct in thinking that the start codon on DNA begins the translation of one specific amino acid chain that produces one specific proteins
    so you have a dna strand on one chromosome - ATG starts the mRNA translation

    --------- ATG GTT AGA TAG TAA TGA -----------

    and that mRNA chain then moves to the ribosome simply as ATG GTT AGA TAG TAA TGA and code for a particular protein - and hence multiple mRNA strands are needed on one long strand of DNA with multiple stop/start codons

    or

    does the mRNA copy the entire strand and the ribosome organises the stopping and starting of protein chanins

    many thanks, I apologise if that is hard to follow
    Sorry I can't follow most of it, however the bit in bold is correct, apart from I think you mean RNA
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    It only translates a bit at a time if I remember correctly
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    (Original post by Organ)
    I'm just trying to get my head around stop/start codons
    I struggled to understand what you were on about, but i'll attempt to answer it anyway.

    The stop and start codons are markers for protein synthesis. There is no tRNA anticodon that attached to the STOP codon and thus when the stop codon appears the polypeptide chain formed is finished and cannot be added to.

    (Original post by Organ)
    am I correct in thinking that the start codon on DNA begins the translation of one specific amino acid chain that produces one specific proteins
    Yes, the start codon will produce a specific sequences of amino acids and thus a specific polypeptide chain (unless there is a mutation during protein synthesis). However once the polypeptide chain is formed its structure with regards to primary, secondary, tertiary, etc.. can still be modified by various cell organelles such as the golgi body.

    (Original post by Organ)
    so you have a dna strand on one chromosome - ATG starts the mRNA translation

    --------- ATG GTT AGA TAG TAA TGA -----------

    and that mRNA chain then moves to the ribosome simply as ATG GTT AGA TAG TAA TGA and code for a particular protein - and hence multiple mRNA strands are needed on one long strand of DNA with multiple stop/start codons
    First off, mRNA has uracil (U) in it not thymine (T) and so the fictitious strand you provided would not actually exist.

    Anyhow, when the DNA is transcribed (on the antisense strand of the double helix) a stop codon is part of the DNA sequence. When the RNA polymerase reaches the stop codon in the DNA, the RNA "peels away" from the antisense strand and then becomes a strand of mRNA. The stop codon is the last triplet to be coded for from the DNA onto the mRNA.

    This strand of mRNA then moves into the cytoplasm through nuclear pores and gets translated by a ribosome. The start codon starting the process by attaching to a specific tRNA anticodon and thus letting an amino acid carried by the tRNA to start binding to neighbouring amino acids which lets a polypeptide chain form. When the ribosome reaches the stop codon on the mRNA there is not a tRNA anticodon that can attach to the stop codon and thus the polypeptide chain is released from the ribosome binding site and a specific polypeptide chain has been formed.

    Remember that if there is a start codon after the first start codon, it does not affect the chain, it is just there to ensure a specific amino acid (the one it codes for) is put at that position in the polypeptide chain.

    (Original post by Organ)
    or

    does the mRNA copy the entire strand and the ribosome organises the stopping and starting of protein chanins

    many thanks, I apologise if that is hard to follow
    No, the above statement is false.
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    Transcription (in the nucleus) begins a little upstream of the start codon on DNA to produce an mRNA of the coding sequence with extra bases at the end.

    Translation in the ribosome starts from the start AUG sequence on mRNA (which happens to code for methionine). It finds this by scanning in from the end of the mRNA.

    Translation continues matching each triplet codon with a corresponding amino acid via a tRNA which has a complementary anticodon.

    Translation is terminated by the STOP codon (can be one of three: UAA UAG UGA), which does not code for a tRNA molecule. It 'codes' for a peptide called a release factor which binds and blocks the ribosome to terminate the formation of the peptide sequence and cause the ribosome to dissociate. The finished primary peptide is then released.

    In eukaryotes, one strand of mRNA codes for only one primary peptide sequence. Many separate lengths of mRNA are transcribed to produce different peptides and hence different proteins. One of the key methods used to control the rate of synthesis of a protein is by controlling the rate of transcription of the specific mRNA which codes for it.

    HTH
 
 
 
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