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Problem from MIT OCW

enter image description here Could somebody please explain the answer for (c). I assumed that since transcription started at the bolded/underlined T/A base pair, the second codon would be UCU? Thanks!

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    $\begingroup$ Welcome to Biology StackExchange! Beware: Transcription and translation are two very different processes. Transcription indeed begins at the bold&underlined T/A, but translation begins at the start codon. $\endgroup$
    – Domen
    Jul 31, 2023 at 18:29
  • $\begingroup$ Hi! Thanks for your response. If it were to begin at the start codon, wouldn't the second codon be TTT and the anticodon would be AAA? Where does the answer, CCU come from? $\endgroup$
    – kikisq
    Jul 31, 2023 at 18:58

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This is a question best understood carefully and in parts. We know that transcription begins at the bolded AT pair, and that once transcription begins we are looking for a translation start codon (i.e., ATG).

From the transcription start, we have the sequence as follows:

5' TGCAGAGATGAGGAAAA...AGT 3'

Here is the start codon for translation:

5' TGCAGAGATGAGGAAAA...AGT 3'

If our start codon is there, then the next codon would be 5' AGG 3'. We know that an anticodon is the complement of the codon, so it would be 3' UCC 5'. However, we normally write sequences in the 5'to 3' direction, so that's how we get 5' CCU 3'.

I think another commenter was right in that you seem to be mixing up transcription and translation. The start codon refers to translation start, not transcription start. Make sure to slow down a bit and read the question, as it even gives you that information in part B ("give the first ten nucleotides of the transcript and box the start codon for translation").

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