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

The key concepts:

1. DNA is double-stranded:
2. One strand is called the Template strand (non-coding strand) — this is the strand used as a template by RNA polymerase to make mRNA.
3. The other is the Coding strand (non-template strand) — it has the same sequence as the mRNA (except T instead of U).
4. Transcription always reads the Template strand in the 3' → 5' direction, making an mRNA strand in the 5' → 3' direction.

What do the strands mean here?

1. Template strand: The DNA strand actually read by RNA polymerase to make mRNA.
2. Coding strand: The opposite DNA strand that has the same sequence as the mRNA (except T→U in mRNA).

Given:

Triplet on Template strand = 5' - A G T - 3'

Step 1: Write the complementary strand (Coding strand)

1. Remember base pairing:
2. A pairs with T,
3. G pairs with C,
4. T pairs with A.
5. Template strand (5' → 3'): A G T
6. Complementary strand (Coding strand) is the reverse complement, 3' → 5':

Let's find the coding strand sequence (5' → 3'):

1. Template strand 5' → 3': A G T
2. Template strand 3' → 5': T C A (just reversing)
3. Coding strand is complementary to template strand (3' → 5' on template pairs with 5' → 3' coding):

Template (3'→5'): T C A

Coding (5'→3'): A G T

So the coding strand is 5' - A G T - 3' — same as the template strand given because the template strand is 5' to 3' but RNA polymerase reads template 3' to 5'. This is unusual, let's double-check carefully...

Important: Template strand is normally given in 3' → 5' direction for transcription.

Since transcription reads the template strand 3' → 5', the triplet for RNA polymerase to read is in the 3' → 5' direction.

Clarifying the problem:

You gave the template strand triplet as 5' - A G T - 3'.

But for transcription, RNA polymerase reads the template strand from 3' to 5', so the codon corresponds to the complementary bases of the template strand read in reverse order.

So first reverse the template triplet to 3' → 5':

1. Template strand 5' → 3': A G T
2. Template strand 3' → 5': T G A

Now, to get the mRNA codon (5' → 3'), take the complement of this 3' → 5' template strand:

1. Template strand (3' → 5'): T G A
2. mRNA (5' → 3'): A C U

Summary:

Given strandSequence (5' → 3')ExplanationCorresponding mRNA codon (5' → 3')
Template strand (given)	5'-A G T-3'	Reverse to 3'-T G A-5' for transcription. mRNA is complement.	5'-A C U-3'
Coding strand	5'-A G T-3' (complement of template)	Same as mRNA but with T instead of U	5'-A G U-3'
Non-coding strand	(same as coding strand)	Same as coding strand sequence	5'-A G U-3'

So the three answers:

1. Template strand triplet: 5'-AGT-3'
2. → mRNA codon = 5'-ACU-3'
3. Coding strand triplet (5'-AGT-3')
4. → mRNA codon = 5'-AGU-3' (same as coding strand except U instead of T)
5. Non-coding strand (usually same as coding strand)
6. → mRNA codon = 5'-AGU-3'

Why are they different?

1. The template strand is complementary to the mRNA and is read in the 3' → 5' direction.
2. The coding (non-template) strand has the same sequence as the mRNA (except T→U).
3. So, when asked for the mRNA codon corresponding to a DNA triplet, you need to know which strand is given, and:
4. If template strand is given, you find mRNA by complementing and reversing (since transcription reads 3'→5').
5. If coding strand or non-coding strand is given, just change T to U for mRNA.

Answer 2

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 3

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 4

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.