DNA replication...

[Temperature101]

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A cell is placed in a medium containing radioactively labeled thymidine. If the cell undergoes two rounds of replication while in this medium, the radioactivity will appear:

A . in both strands of all the DNA molecules.

B . in both strands of half the DNA molecules.

C . in one of the strands of every DNA molecule.

D . in both strands of half the DNA molecules; the other half of the DNA molecules will have one labeled strand.

Why is the answer D instead of C?

 

[sakabato93]

A cell is placed in a medium containing radioactively labeled thymidine. If the cell undergoes two rounds of replication while in this medium, the radioactivity will appear:

A . in both strands of all the DNA molecules.

B . in both strands of half the DNA molecules.

C . in one of the strands of every DNA molecule.

D . in both strands of half the DNA molecules; the other half of the DNA molecules will have one labeled strand.

Why is the answer D instead of C?

If the cell had undergone 1 round of replication, C would be correct. But 2 rounds results in D.

 

[Mommymed]

DNA replicates semi conservatively, meaning after replication each of the 2 resulting DNA molecule will have 1 "old" strand and one completely new strand.

This question is best visualized by making yourself a sketch. Draw 2 straight lines, representing the original DNA. Below that branch in 2 directions, and redraw the straight lines but be sure to separate them. Next to each straight line draw a zigzag line, representing new radioactive strands. Now you see a model corresponding with C, BUT we have to replicate again.

Below the picture above, draw 4 lines separated from each other. Be sure to respect the nature of the parent strand, ie you should have 2 straight lines and 2 zigzag lines all SEPERATED from each other. The DNA is still in the radioactive medium, so all 4 new strands will be zigzag. Draw this out and see how it looks, and which answer corresponds 🙂


Btw: I see also why you could have picked C. Yes, every molecule will have ATLEAST 1 strand of radioactive DNA, but C is not the best possible answer.