EK Biology. Question on DNA replication

[eeyevon]

5-bromouracil resembles thymine enough to become incorporated into DNA during replication. Once incorporated, however, it rearranges to resemble cytosine. If 5-bromouracil were present during the replication of the sense strand shown below, which of the following might be the sense strand formed in the following replication?

5'-GGCGTACG-3'​

A. 5'-GGCGCACG-3'
B. 5'-GGCGATCG-3'
C. 3'-CCGCAGGC-3'
D. 5'-GGCGTGCG-3'

The correct answer is D.
My approach was to replace the "T" in the original strand with a "C." After the first replication the "C" becomes "G", and in the following replication it would be "C."
My final answer was choice A: 5'-GGCGCACG-3'

Can someone please explain how to get answer choice D? I always have problems with these types of questions. (I was confident i finally got it this time)

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[aldol16]

This replication created the ANTISENSE strand to the SENSE strand in the question stem. The question asks for the sense strand to this new antisense strand: 5'-GGCGTGCG-3'

I just really dislike this question because once the second replication occurs, by the information in the question, the T should rearrange again to give 5'-GGCGCGCG-3', which is not one of the answer choices.

 

[eeyevon]

Sense Strand: 5'-GGCGTACG-3'
Replicate normally: 3'-CCGCATGC-5'
5-bromouracil is present during replication, so replace every T with 5-bromouracil: 3'-CCGCA(5-bromouracil)GC-5'
5-bromouracil rearranges to to resemble cytosine, replace 5-bromouracil with C: 3'-CCGCACGC-5'
This replication created the ANTISENSE strand to the SENSE strand in the question stem. The question asks for the sense strand to this new antisense strand: 5'-GGCGTGCG-3'

Note for your education: DNA is synthesized from 5'->3' because the 3'-OH performs nucleophilic attack on the 5'-phosphate on the next nucleotide in the sequence, therefore DNA polymerase would move from 3' to 5' on the sense strand while creating the antisense strand.

Thank you! The explanation provided by EK just ended up confusing me even more. Cheers mate 🙂

 

[aldol16]

It's a very poor question. There's no reason to assume the second replication wouldn't take place with 5-bromouracil in solution.

Exactly! It's likely that the second replication would take place with that in solution. Because there's almost no way to stop the replication arbitrarily after one time!