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DNA to mRNA for three diferent strands of DNA, given "5-AGT-3"

My Study Group can't figure out this problem:

"A triplet of bases in the Template Strand of DNA = 5'-AGT-3', The corresponding codon found in the mRNA would be..."

but then they ask the same question for the Non-Coding and Coding strands of DNA for the same "DNA = 5'-AGT-3'". Can you please explain what is different, why they are different, and what the three answers are?

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1 Answer

In DNA A corresponds to T and C corresponds to G, the only difference in RNA is that T is replaced with U, so the mRNA sequence that corresponds to the codon AGT would be UCA.

That is the coding sequence, noncoding DNA sequences can in some instances produce tRNA and rRNA but they never code for mRNA so a noncoding sequence of DNA would not have a corresponding mRNA sequence.

It may be possible that I am misreading the question however, as I found it oddly worded.

The reason there can be only two answers and the reason non-coding DNA doesn't produce mRNA is because coding DNA refers to DNA that codes for proteins, since mRNA is the go between for DNA and the ribosomes for protein synthesis non-coding DNA will not produce mRNA by definition. Since there is only the two types of DNA possible in this problem there is actually only two questions not three.

Comments

Hi Kathryn, 

Here is how mRNA is created: coding DNA gets transcribed to template DNA, then template DNA gets transcribed to mRNA.  

It is important to remember that 5' ends get "transcribed" to 3' ends.  So the template DNA 5'AGT3' is transcribed to the mRNA 3'UCA5'.  But since it is customary to write codons from 5'-end to 3'-end, the mRNA codon is actually ACU.

coding DNA:      5' ACT 3'

template DNA:   3' TGA 5'

mRNA:              5' ACU 3'

Hope this helps!

Whoops, thinking fail, this is what I get for doing these in the wee hours, Emma is quite right.

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