Question

the following diagram shows a double - stranded dna molecule.
which of the following represents the correct mrna molecule that will result from the transcription of the dna molecule?
choose 1 answer:
a 5 u c a g c g u a 3
b 5 u a c g c u g a 3
c 5 a u g c g a c u 3
d 5 a g u c g g c a u 3
image credit ~

Explanation:

Step1: Identify the non - coding strand

The non - coding strand (template strand) of the DNA is read during transcription. From the diagram, the non - coding strand (the one labeled non - coding) has the sequence (from 3' to 5'? Wait, no, for transcription, the DNA template is read in 3' to 5' direction and mRNA is synthesized in 5' to 3' direction. Wait, first, let's get the sequence of the coding strand. Wait, the coding strand (the one labeled coding) has the sequence (from 5' to 3'? Wait, the DNA molecule has the coding strand with 5' at the bottom and 3' at the top? Wait, no, the diagram shows the coding strand with 3' at the top and 5' at the bottom, and non - coding strand with 5' at the top and 3' at the bottom. Wait, the coding strand sequence (from 5' to 3'): let's list the bases. The coding strand (bottom to top? Wait, the coding strand has 5' at the bottom (since the arrow is pointing to it, and the 5' and 3' are labeled: coding strand 3' at top, 5' at bottom; non - coding strand 5' at top, 3' at bottom. So the coding strand sequence (from 5' to 3'): T, A, C, G, C, G, A, T? Wait, no, let's look at the bases. The coding strand (the left column) from bottom (5') to top (3'): T, C, A, G, C, G, T, A? Wait, no, the first base (bottom) of coding strand is T, then C, A, G, C, G, T, A (top). Wait, no, the coding strand's bases (from 5' (bottom) to 3' (top)): T (bottom), C, A, G, C, G, T, A (top). Wait, but during transcription, the mRNA is complementary to the non - coding (template) strand. The non - coding strand (right column) from 5' (top) to 3' (bottom): T, A, G, C, G, C, A, T? Wait, no, the non - coding strand's bases (top to bottom, 5' to 3'): T, A, G, C, G, C, A, T? Wait, no, let's list the non - coding strand (right column) from 5' (top) to 3' (bottom): T (top), A, G, C, G, C, A, T (bottom). Wait, no, the diagram: coding strand (left) has bases (top to bottom): A, T, G, C, G, A, C, T. Non - coding strand (right) has bases (top to bottom): T, A, C, G, C, T, G, A. Wait, I think I messed up. Let's do it properly. The DNA double helix: the coding strand (sense strand) has the same sequence as the mRNA (except T is U in mRNA). Wait, transcription uses the non - coding (antisense, template) strand. So the mRNA sequence is complementary to the non - coding strand, and has the same sequence as the coding strand (with T replaced by U). So first, find the coding strand sequence. The coding strand (left column) from 5' (bottom) to 3' (top): let's list the bases (bottom to top): T, C, A, G, C, G, T, A? Wait, no, the bottom base of coding strand is T (the first base at the bottom of the left column), then C, A, G, C, G, T, A (top). Wait, no, the left column (coding strand) bases: bottom (5'): T, then C, A, G, C, G, T, A (top, 3'). So coding strand sequence (5' to 3'): T C A G C G T A. Then the mRNA should have the same sequence as the coding strand, with T replaced by U. So mRNA sequence (5' to 3'): U C A G C G U A. Now let's check the options. Option A: 5' U C A G C G U A 3' which matches. Wait, let's verify. The non - coding strand (template) is complementary to the coding strand. So non - coding strand (5' to 3'): A G T C G C A T (since coding is T C A G C G T A, complementary is A G T C G C A T). Then mRNA is complementary to non - coding strand: so A (non - coding) pairs with U (mRNA), G with C, T with A, C with G, G with C, C with G, A with U, T with A. So mRNA sequence: U C A G C G U A (5' to 3'), which is option A.

Step2: Check the options

Option A has the sequence 5' U C A G C G U A 3', which matches the mRNA sequence derived from the coding strand (with T→U) or from complementing the non - coding strand. Option B: 5' U A C G C U G A 3' does not match. Option C: 5' A U G C G A C U 3' does not match. Option D: 5' A G U C G G C A U 3' does not match.

Answer:

A. 5' $\boldsymbol{U}$ $\boldsymbol{C}$ $\boldsymbol{A}$ $\boldsymbol{G}$ $\boldsymbol{C}$ $\boldsymbol{G}$ $\boldsymbol{U}$ $\boldsymbol{A}$ 3'