mtDNA question

[Oh_Gee]

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So in TBR BioII chapter 4 passage 10,

"Both strands of human mitochondrial DNA are transcribed from a single promoter region on each strand.
Two giant RNA molecules are thus created. The two molecules are known as the heavy and light strands. The
heavy strand is processed to produce many RNAs, including ten polyadenine-containing RNAs. Roughly
90% of the light strand contains no discernibly useful information.

65. The DNA that codes for the light strand is also known as the:
A. antisense strand, because the light chain is complementary.
B. antisense strand, because the light chain is nearly identical.
C. sense strand, because the heavy chain is complementary.
D. sense strand, because the heavy chain is nearly identical.

answer: D is correct. We are told from the passage that the heavystrand of RNA is responsible for encoding many proteins, while the light chain of RNA is over 90% nonsense. One should realize that the heavy chain of RNA is nearly identical to the strand of DNA that gave rise to the light chain of RNA. The only difference is that the thymines in the DNA have been replaced by uracils. Since the heavy chain of RNA contains all of these functional codons, the strand that is nearly identical to it is called the sense strand. The piece of DNA that actually gave rise to the heavy strand of DNA is called the antisense strand. The correct choice is D"

This answer does not make sense to me. I thought the sense strand in DNA was the coding strand (usually the one written on top in 5'>3', whereas the anti sense strand was complementary strand) which is why I selected A. Also since 90% of the light strand doesn't code, how is that strand the sense strand?

 

[sxld_cu]

I'm not using TBR so I'm not familiar with the passage but here's my understanding of the problem:

You are correct, in that the sense strand is the segment of (double-stranded) DNA that runs 5' --> 3'. It is complementary to the antisense strand that runs complementary, from 3' --> 5'.
However, the sense strand has the same sequence as the mRNA (except for the uracil/thymine difference) so if the mRNA is the same as the sense strand, it must be derived from the antisense strand. (In other words, mRNA is based off the template of the antisense strand, which is why it is the same as the sense strand.) We know that the heavy strand produces useful RNA and the light strand is useless, so it's fair to assume that the heavy strand is mRNA, so the DNA that codes for the heavy strand is the antisense strand. Therefore, the DNA that codes for the light strand is the sense strand (C or D). As we established before, the heavy chain mRNA is the same as the sense strand, narrowing it down to answer choice D.

 

[Oh_Gee]

oooh i see. thank!