Conservative replication of DNA

[ashtonjam]

45. After Watson and Crick proposed the structure of DNA, it was postulated that DNA could replicate in a conservative fashion. After two rounds of conservative DNA replication, the original duplex parental strands would give rise to:
A. two duplexes of DNA, each containing one daughter strand and one parental strand.
B. four duplexes of DNA, each containing one daughter strand and one parental strand.
C. two duplexes of DNA, one duplex containing two daughter strands and one duplex containing two parental strands.
D. four duplexes of DNA, two duplexes each containing two daughter strands and two duplexes each containing two parental strands.

Answer in white: D

This is from TBR Chapter 9, #45.

Why would conservative replication make two parent duplexes and two daughter duplexes? I thought conservative replication always kept the two parent strands together. The answer seems to indicate that the number of parental strands doubles. Is this how it works? (P = parent strand, D = daughter strand)

Initial:      PP
1 round:     PP DD
2 rounds: PP DD DD DD

So there would be four DNA duplexes total after two rounds of replication, eliminating A and C. But wouldn't there be 3 daughter duplexes and only 1 parent duplex as opposed to the answer?

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[sps27]

45. After Watson and Crick proposed the structure of DNA, it was postulated that DNA could replicate in a conservative fashion. After two rounds of conservative DNA replication, the original duplex parental strands would give rise to:
A. two duplexes of DNA, each containing one daughter strand and one parental strand.
B. four duplexes of DNA, each containing one daughter strand and one parental strand.
C. two duplexes of DNA, one duplex containing two daughter strands and one duplex containing two parental strands.
D. four duplexes of DNA, two duplexes each containing two daughter strands and two duplexes each containing two parental strands.

Answer in white: D

This is from TBR Chapter 9, #45.

Why would conservative replication make two parent duplexes and two daughter duplexes? I thought conservative replication always kept the two parent strands together. The answer seems to indicate that the number of parental strands doubles. Is this how it works? (P = parent strand, D = daughter strand)

Initial:      PP
1 round:     PP DD
2 rounds: PP DD DD DD

So there would be four DNA duplexes total after two rounds of replication, eliminating A and C. But wouldn't there be 3 daughter duplexes and only 1 parent duplex as opposed to the answer?

Yes, I think so too. So am quite surprised by the ans choices here....

 

[phys2med]

From what I learned in class and just checking now again yes you are correct. It would seem the book is wrong

 

[ashtonjam]

Sweet, so it's not me sucking at biology 😀

 

[Mommymed]

45. After Watson and Crick proposed the structure of DNA, it was postulated that DNA could replicate in a conservative fashion. After two rounds of conservative DNA replication, the original duplex parental strands would give rise to:
A. two duplexes of DNA, each containing one daughter strand and one parental strand.
B. four duplexes of DNA, each containing one daughter strand and one parental strand.
C. two duplexes of DNA, one duplex containing two daughter strands and one duplex containing two parental strands.
D. four duplexes of DNA, two duplexes each containing two daughter strands and two duplexes each containing two parental strands.

Answer in white: D

This is from TBR Chapter 9, #45.

Why would conservative replication make two parent duplexes and two daughter duplexes? I thought conservative replication always kept the two parent strands together. The answer seems to indicate that the number of parental strands doubles. Is this how it works? (P = parent strand, D = daughter strand)

Initial:      PP
1 round:     PP DD
2 rounds: PP DD DD DD

So there would be four DNA duplexes total after two rounds of replication, eliminating A and C. But wouldn't there be 3 daughter duplexes and only 1 parent duplex as opposed to the answer?

I'm not going to lie, the wording in the question can throw you off a bit. But during the second round of replication, while the initial DD strand is the daughter strand of the original DNA, when it replicates its going to remain intact acting like the parent strand.

Replication and division goes on and on. So that PP strand was at some point in time the DD strand of another cells replication. In other words, division isn't as clean as one PP and the rest of replications forever are DD. With every new round of replication, the PP now becomes the DD. I hope this makes sense... If not, ask away 🙂

(and we know that replication is NOT like this. Conservative replication was disproved)

 

[ashtonjam]

So it goes more like this?

Initial:      PP
1 round:     PP DD  (<- But this daughter duplex is PP for the next round?)
2 rounds: PP DD [U]PP[/U] DD

 

[Mommymed]

So it goes more like this?

Initial:      PP
1 round:     PP DD  (<- But this daughter duplex is PP for the next round?)
2 rounds: PP DD [U]PP[/U] DD

Yes, but do you understand why? Genetics is highly conceptual, and extremely difficult to explain over the Internet!

Concervative replication, as I'm sure you know, means that after a cell replicates, one cell gets the ENTIRE original DNA molecule, where as the other cell gets an entirely new DNA molecule. No 50/50 which we know to be true.

When you say "parent" strand it doesn't mean the absolute original because there was only one absolute original in everyone (when you were an embryo). So think of parent as just the DNA that gives rise to another DNA molecule. So in round 2, that daughter DD strand from round one will replicate and give rise to another new molecule. It acts as a parent/template PP molecule. Even though it's not the "original" parent molecule from round 1, it acts as the parent for the new molecule.

It might be easier to think of conservative as "old/old" and "new/new"

Round 1: "old/old". "new/new"
Round 2: "old/old" "new/new" AND (that original new/new becomes the template/old for the new and gives rise to🙂 "old/old" "new/new"

I really hope I'm not confusing you, because it's really not confusing just hard to put into words.

 

[ashtonjam]

When you say "parent" strand it doesn't mean the absolute original because there was only one absolute original in everyone (when you were an embryo). So think of parent as just the DNA that gives rise to another DNA molecule.

Oh I see, that's what the confusing part was. Thanks!

 

[Mommymed]

No problem. Genetics is my favorite, so I hope it helped.

And as always, when in doubt pick the best answer. You were right in eliminating A and C but B makes no sense either, because that describes semi conservative. So even if you didn't understand why, D was the best choice.

 

[cagger68w]

B is obviously wrong since it is saying the resulting "duplexes" are comprised of one parent and one daughter strand, which would be indicative of semi-conservative replication. leaving you with D and u can make sense of the wording since it probably is talking about the newer and the older generations of "parental" strands. oh crap. someone already said that...