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enter image description here

I am slightly confused by the diagram above.

The first codon of the unaltered DNA is AAG. During transcription, isn't this coverted to UUC (mRNA). So doesn't UUC code for phe and not Lys?

Likewise, how does TAG code for a stop codon? Isn't it transcibed to AUC, which codes for ile?

Also, is that the coding strand or template strand? If there is a mutation in the coding strand, is there any impact on the protein?

enter image description here

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  • $\begingroup$ translation is making protein out of mRNA. Transcription is making mRNA (UUC) out of DNA (AAG). TAG transcribes to UAG. Check wikipedia page for en.wikipedia.org/wiki/Transcription_(biology) $\endgroup$ Commented Dec 17, 2018 at 1:35
  • $\begingroup$ Isn't there complementary base parigng for the template strand? So won't AAG become UUC as A pairs with U and C pairs with G? $\endgroup$ Commented Dec 17, 2018 at 2:08
  • $\begingroup$ codons are defined by mRNA sequence, defined by gene sequence. Correspondence is 1:1 except Ts become Us. What materials you are using? Do you have access to some Cell Bio textbook? $\endgroup$ Commented Dec 17, 2018 at 3:35
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    $\begingroup$ @ChristopherU'Ren it's not lysosomes, but "lysine." Also, no, AAG would be transcribed as UUC which is translated to Phenylalanine. See a codon chart (source: here). $\endgroup$ Commented Dec 17, 2018 at 4:56
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    $\begingroup$ However, if the book was showing you the coding strand of DNA (i.e., the strand not actually being used in transcription), then they're correct -- the coding strand triplet that is complimentary to the base triplet on the template strand used in transcription is going to be AAG when the resulting amino acid is lysine (and the codon in AAG). To summarize how the coding DNA strand, template DNA strand, and mRNA strand compare, see the link in my previous comment. $\endgroup$ Commented Dec 17, 2018 at 5:01

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In the first image, AAG is the true codon for Lysine. So when the ribosome hits "AAG" in the mRNA it recruits a Lysine-tRNA.

What can be confusing is the use of the term "coding strand" when talking about the DNA. The coding strand is illustrated in the second image you posted, and it is the coding strand of DNA that the first image is depicting. What "coding strand" means here is, this is the strand that looks like the mRNA will look. Depicting the DNA coding strand like this--i.e. as a series of codons--can be slightly misleading since we know codons really only mean anything when in the form of mRNA. Nevertheless, it is a common way of thinking about DNA.

Back to the first image. Because it is showing the "coding strand" of DNA, the "template strand" for 5'-AAG-3' then must be 5'-CTT-3'. Hence, when this strand is transcribed by RNA polymerase, the 5'-CTT-3'(DNA) becomes 5'-AAG-3'(mRNA).

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  • $\begingroup$ Why does CTT become AAG? Isn't it GAA? $\endgroup$ Commented Dec 17, 2018 at 5:45
  • $\begingroup$ You need to always be thinking about this in terms of 5'-to-3'. I indicted this above and I think that may be why you are getting confused. $\endgroup$
    – DavidR
    Commented Dec 18, 2018 at 2:33
  • $\begingroup$ Please provide support for your answer. Preferably, cite or link to reputable sources that back-up the claims you make. Thanks $\endgroup$ Commented Dec 20, 2018 at 14:37

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