I can only think of one reason, which is because different codons can specify the same amino acids. However I am having trouble thinking of another reason.
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1$\begingroup$ Think carefully about how a gene's sequence is turned into protein - the gene is transcribed, mRNA is created, it runs through the ribosome where tRNAs match the codons and bring in the appropriate amino acids, then the polypeptide chain is spit out. Is there anywhere in that series of steps where the sequence of bases written in DNA could somehow change? Are there any spots where bases might be "clipped off" for some reason? $\endgroup$– MattDMoMar 8, 2015 at 7:01
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$\begingroup$ @MattDMo You should convert that into an answer. $\endgroup$– March HoMar 8, 2015 at 7:03
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$\begingroup$ @MarchHo well, I didn't exactly give an answer, though, just pointed the OP in the right direction. Vaguely. $\endgroup$– MattDMoMar 8, 2015 at 7:05
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1$\begingroup$ This sounds a bit too much like a homework question $\endgroup$– cmbarbuMar 8, 2015 at 8:29
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2 Answers
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I can think of at least 3 reasons in addition to the one you gave:
1: As mentioned in the comments, RNA splicing takes place on most messenger RNA encoding proteins in eukaryotes. Sections of the mRNA may be spliced out, therefore multiple mRNAs with different codon sequence can encode the same gene.
2: Translation is a stateful process, since it depends on the frame of the codon. Therefore, a gene with the sequence GGATGATGATGTAA will encode the same protein as a gene with the sequence ATGATGATGTAA, due to the start codon shifting the frame of translation downwards.
3: Genes contain untranslated regions in regions before the start and after the stop codon. These nucleotides cannot be predicted from protein sequence, but are generally important in regulating protein expression.
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Introns are not included in the polypeptide sequence.
