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

Question

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?

Answer 1

A DNA molecule consists of two complementary strands of nucleotides that run antiparallel to one another, that is one strand is 5' to 3' and the other strand is 3' to 5'. The 5' and 3' directionality comes from the orientation of the 5 carbons in the pentose sugar (deoxyribose in DNA or ribose in RNA) found in each nucleotide. During transcription, the enzyme RNA Polymerase reads the template strand of DNA in the 3' to 5' direction and builds the new strand of RNA in the 5' to 3' direction. To answer this question, you need to "think like RNA Polymerase." If the template strand is 5'-AGT-3', you need to read it from the 3' end, so 3'-TGA-5'. Then you build the new mRNA: 5'-ACU-3'.

The coding strand (or sense strand) on the DNA is complementary to the template, so it reads: 5'-ACT-3'.

The non-coding stand (or antisense strand) on the DNA is the template, so it reads: 3'-TGA-5'.

Answer 2

The template, or non-coding (or anti-sense), strand of DNA and the coding (or sense) strand of DNA are complementary and anti-parallel. The template strand is what is "read" during transcription by the RNA polymerase in the 3' to 5' direction to make a complementary RNA that is in the 5' to 3' direction. Since the coding strand of DNA and the RNA are both complementary to the template strand of DNA, the coding (or sense) strand tells you the code (or gives you a sense) of the RNA, with the exception that all T's in the coding strand become U's in the RNA, since RNA uses uracil bases instead of thymine bases (with a few exceptions in tRNA, in which uradine nucleotides are converted to thymidine nucleotides). Hence, if you are given the template (or non-coding) strand sequence of DNA = 5'-AGT-3', you would first re-write it 3' to 5' and then write the coding strand under it.

3'-TGA-5'

5'-ACT-3'

The corresponding codon found in the mRNA would be the same sequence as the coding strand, except replacing all T with U: 5'-ACU-3'

However, if the _coding_ strand sequence is given as 5'-AGT-3', then the RNA sequence is simply 5'-AGU-3'

Answer 3

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.