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Why are start and stop codons present at translation level but not at transcription level? And how will I obtain a protein at transcription level?

Thanks in advance

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closed as off-topic by Remi.b, David, mgkrebbs, AliceD Nov 23 '18 at 12:35

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  • $\begingroup$ After transcription, you don't get a protein but an mRNA. Is this unclear to you or is something else unclear to you? $\endgroup$ – Remi.b Nov 16 '18 at 16:24
  • $\begingroup$ @Remi.b Ya that's unclear too :(. Why is that the case? And why aren't any start and stop codons present at that time? Thanks $\endgroup$ – Ankit Kumar Nov 16 '18 at 16:55
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    $\begingroup$ It is hard to know how many things are unclear to you from your question. Hence, the best anyone can do would be to write a whole introduction to the so-called "central dogma of molecular biology", which makes your question too broad, IMO. Plus, many such introductions already exist online. For example, have a look at Khan Academy, wikipedia or many other source of information. $\endgroup$ – Remi.b Nov 16 '18 at 17:23
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Codons are only meaningful for translation, since that is when the nucleo-based DNA/RNA sequence information is translated into a protein sequence based on the triplet code. Before that step codons don't really have 'meaning'.

During transcription the start & stop codons are already present, but they are ignored by the RNA polymerase, because it works on a single base at a time.

The equivalent 'start' and 'stop' signs for transcription are promoters and poly-adenylation-sites (at least for eukaryotic mRNA). Both are recognised by additional proteins, that help regulate transcription:

  • transcription factors bind the promoters on the DNA and recruit RNA polymerase to that site so that transcription can start
  • poly-adenylation-site are specific sequences that lead to the addition of the poly-A tail to the nascent mRNA. This process also leads to termaination of transcription, however the exact mechanism isn't quite clear.
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  • $\begingroup$ Thanks a lot. It cleared a lot of my concepts. I just have one more specific question. I am given a particular DNA sequence. What will be the mRNA molecule as a result of transcription of that DNA segment? $\endgroup$ – Ankit Kumar Nov 16 '18 at 18:30
  • $\begingroup$ I hope the question is clear. Please help. I'll be grateful to you! $\endgroup$ – Ankit Kumar Nov 16 '18 at 18:31
  • $\begingroup$ The mRNA that binds to a template strand will complement the template. For the sequence AATC, the mRNA will be UUAG (remember that in RNA, uracil is used instead of thymine) $\endgroup$ – Astrolamb Nov 16 '18 at 20:05

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