Question 12

[w-huang36150]

12. If replication occurred in a bidirectional manner, the evidence would be that shortly after initiation:

1. each gene in the E. coli genome would be represented only once. [x]
2. each gene in the E. coli genome would be represented twice. [x]
3. DNA duplication would begin on both sides of the origin of replication. [x]
4. gene frequencies should be very high for regions symmetrically disposed about the origin.

For this question, I picked "D" because the passage says, "convincing evidence of bidirectional replication was obtained by measuring gene frequencies during replication." Therefore, I thought the evidence should relate to gene frequency. Thank you

[admin]

I suppose that if you had not read the passage (which I'm not suggesting that you should avoid doing!), you would have certainly chose answer choice C since it comes across as a gift: we know that replication occurs in both directions because replication (duplication) occurs in both directions.

Undoubtedly the passage mentions gene frequency. So what is a gene? A segment of DNA that codes for a protein. We know that it is a triplet code where 3 nucleotides code for one amino acid. We also know that it takes 40-1000 amino acids to make a protein. So one gene is, in biochemical terms, very long!

So back to the question: "shortly after initiation," we would expect that DNA replication would begin (answer choice C) but to be able to measure multiple gene frequencies "shortly after initiation" given the size of just one gene, seems much less likely.

Don't worry, there will only be 1-2 questions like this per test!

[tnesbit4508229]

I didn't like this question either. My problem with it is that we were trying to "prove" that bidirectional replication is occurring. And, the answer to this question is like saying "we can prove bidirectional replication is occurring because the replication is occurring bidirectionally", which doesn't make sense to me, personally.

Add to that the fact that the passage makes the following statement: "Eventually, convincing evidence of bidirectional replication was obtained by measuring gene frequencies during replication." And I personally disagree with the answer.

[JerryWTu7580]

Could someone explain what answer choices A and B meant?

[mybirdnest7483]

I too also had a problem with this question. I picked D as well because C seems like such a terrible answer and D makes sense.

Answer A and B just means there is only one or two copy of each gene shortly after replication. This makes no sense because # of identical genes in a genome does not have to be unique to begin with.

[admin]

"Shortly after initiation" negates gene frequency as possible evidence meaning D. cannot be correct as described above.

[dwumi1604]

bump

[hua8986059]

no way in hell this question should be labeled easy. lol.

Got an 11 on this bio section and I'm missing all these easy ones.