The function can be used and called from the resource function repository: https://resources.wolframcloud.com/FunctionRepository/resources/DNAtoAminoAcid

Feel free to check it out!

You are mistaken about the meaning of "coding strand." In the bioinformatics community coding strand refers to the DNA sequence that matches the mRNA (except for having Ts instead of Us). This means the coding strand is what is produced by the process of transcription, and the coding strand (in RNA form) is the template the ribosome would use to make a polypeptide.

See here: coding strand from wikipedia

In essence the coding strand contains the actual codons, so when a ribosome "sees" TAC in the coding strand which is the template the ribosome is attached to, it will insert a tyrosine. The cell does not take the TAC from the coding strand and transcribe it to AUG as you suggest because the coding strand had already been made by transcription.

Think about it and see if you can realize your code is producing incorrect output.

Let's take your second-to-last sentence: In that case, a "TAC" on the template strand would code for an "AUG"/methionine on the mRNA/ribosome.

You have to remember there is chemical directionality to the DNA.

So, TAC on the template strand is really 5' - TAC - 3'

Which means, on the coding strand you would have: 3' - AUG - 5'

Since ribosomes always scan mRNA 5' to 3', the ribosome would actually "see" 5' - GUA - 3', so it would insert valine in the polypeptide.

Don't get frustrated. This is not intuitive stuff that naturally flows out of most textbooks. You are learning as you push through this.

That's correct, Samikshaa!

You've got it!

Do you see a path forward to update your code to produce correct output?

Are you adopting a non-standard genetic code? In the "standard" genetic code "TAC" codes for tyrosine, whereas ATG codes for methionine.

This is very interesting work, Samikshaa. Since you already use BioSequence[], are you already aware of the functions BioSequenceComplement[] and BioSequenceTranslate[]? For example,

BioSequenceTranslate[BioSequenceComplement[BioSequence["DNA", "AGTCGTAGTACGGAT"]]]
   BioSequence["Peptide", "SASCL", {}]

BioSequenceTranslate[BioSequenceComplement[BioSequence["DNA", "TACTTTTCGTCCGGTATAATT"]]]
   BioSequence["Peptide", "MKSRPY.", {}]

where a stop is represented by a period in BioSequence[].

Well, once you have the BioSequence[], it isn't overly difficult to get the same result as your function:

Lookup[EntityValue[Entity["BioSequenceType", "Peptide"], 
                   EntityProperty["BioSequenceType", "AlphabetRules"]], 
       Characters[BioSequenceTranslate[BioSequenceComplement[
                  BioSequence["DNA", "AGTCGTAGTACGGAT"]]] @ "SequenceString"],
        Nothing]

I invite you to study the documentation for BioSequence[] and functions related to it in more detail.

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It recently was uploaded to the WFR, actually! Here's the link.