DNA Replication

This forum made possible through the generous support of SDN members, donors, and sponsors. Thank you.

fataliz

Full Member
7+ Year Member
Joined
Dec 27, 2014
Messages
23
Reaction score
9
Hi, came across this question online.

Question 2. A cell is placed in a medium containing radioactively labeled thymidine. After the cells undergo replication three times, what percentage of the cells will have both strands of DNA labeled?

A. 25 percent

B. 50 percent

C. 75 percent

D. 100 percent

Answer: C is correct. This question asks you to determine the percentage of cells with both strands of DNA labeled after three replications of one cell in radioactive thymidine. After three rounds of replication, there will be eight cells: 2 3=8. After the first round, each cell will have one labeled strand and one not labeled. After the second round, two cells will have one labeled and one unlabeled strand, and two cells will have both strands labeled. After the third round, six cells will have both strands labeled. No matter how many rounds of replication occur, there will always only remain two cells that contain one labeled DNA strand and one unlabeled DNA strand.

Answer A is the percentage of cells that do not have both strands labeled; B and D are both the result of miscalculation.

i totally have no idea. how is it even possible to have any cells with both strands labelled? After the first replication, both cells have one labelled and one unlabelled strand (as the answer mention as well). After the second, wouldn't you have 4 cells with 2 cells having one unlabelled strand, and 2 cells having both labelled strands?

Appreciate the help!!

Members don't see this ad.
 
This is a silly question in my opinion, but basically they want you to know that DNA replication is semi-conservative. So in the original cell, let's say we have 1 chromosome (1 double-helix). As this cell is preparing for division (mitosis), this DNA undergoes replication and the two 'parental' strands separate (no radioactive DNA here). Each parental strand acts as a template which directs the synthesis of a new strand complementary to it (this new strand will contain the radioactive DNA). Fast forward a bit: the cell undergoes mitosis and divides. Now each daughter cell differs in that half of their DNA content is radioactive (the newly synthesized complementary strand). When each of these 2 daughter cell divides yet again: one of the two cells they produce will contain the original parental DNA (non-radioactive) along with the newly synthesized complementary DNA (radioactive). The second cell gets the radioactive DNA strand (synthesized in the previous round of mitosis) along with the complementary DNA (also radioactive).

The basic idea here is, because of the semi-conservative nature of DNA replication, the two original strands of the parental DNA double-helix will always be found in 2 of the cells, each with 1 parental DNA strand from the original cell that is associated with radioactive complementary DNA. Realizing this, you should realize that 6 of the 8 cells would contain entirely radioactive DNA. 6/8 reduces down to 3/4 or 75%.
 
Last edited:
  • Like
Reactions: 1 user
i totally have no idea. how is it even possible to have any cells with both strands labelled? After the first replication, both cells have one labelled and one unlabelled strand (as the answer mention as well). After the second, wouldn't you have 4 cells with 2 cells having one unlabelled strand, and 2 cells having both labelled strands?!
That's exactly right: of the 4 cells produced by the second round of division, two will contain (50% parental, 50% radioactive), the other two will be 100% radioactive. When these 2 cells (100% radioactive) undergo another round of division, they also produce cells that are 100% radioactive for a total of 6 cells that are 100% radioactive. Hope that isn't confusing. I think if you drew it out, it might be easier to follow.
 
holy crap. i know where my confusion came from. i assumed the original cell was already stained with the radioactive DNA, now it makes sense. thanks!
 
  • Like
Reactions: 1 user
Top