Question

Which of the following mRNA strands contains the longest open reading frame?

A.5′-CCACGAUGCGUUGAGCGC-3′[16%]

B.5′-CAAUGGUAAAGUCUUAGU-3′[51%]

C.5′-GACGAUGUAACUUGCACU-3′[8%]

D.5′-AGAAUGGUAUCCUGAGCC-3′[2

The correct answer is B because there is the largest distance between the start codon and stop codon. I get that. What I don't get is that these strands are written from 5' to 3'. I know that this is the convention, but I thought that DNA was read from 3' to 5' by the enzymes that synthesize the new strand? What am I missing? Thank you

Answer 1

Answer to this questions is definately "B"  as rightly explained in question itself that "there is the largest distance between the start codon and stop codon."

I assume there is bit of confusion between the process of replication and translation-

In replication or in sythesis of new DNA strands, as rightly mentioned in question itself that "the DNA template is read in 3′ to 5′ direction by the enzymes whereas a new strand is synthesized in the 5′ to 3′ direction".

But in the process of translation, in mRNA reading frame codon are always translated in the 5′→3′ direction into amino acids by a ribosome to produce a polypeptide chain

And Open reading frame is only and only related to translation where it decides which nucleotide to start translation, and when to stop.