Scientists recognize semiconservative replication as the mechanism by

Official Answers from VeritasPrep:

Q 1

The point of the 3rd paragraph is that the experiment applies not only to "organisms that lack complex cell structures. . ." but also to "eukaryotes, organisms with complex cells such as animals and plants."
A - Since E. coli are prokaryotes, this answer choice does not make sense.
B - The passage contradicts this point by claiming that semiconservative replication applies to simple organisms (prokaryotes) and complex ones (eukaryotes).
C - We are not given any suggestions as to Watson and Crick's thoughts on this subject.
E - This is presented as a piece of evidence, not a reason for making the point that semiconservative replication applies to eukaryotes.

Q 2

In the 2nd paragraph, the passage states that "In the conservative replication model, a completely new double helix would be formed, leaving the original intact." Therefore, the original heavy strands would still be present, as would an equal number of light strands.
A - This is a description of dispersive replication
B - The conservative model does not mix, it keeps the originals untouched. D - This is not stated or implied in the passage. E - Nowhere is this implied; instead each method is said to double the DNA through replication.

Q 3

The 2nd generation produced light and intermediate density DNA, and the passage states that "the lighter density DNA . . . would incorporate into any newly synthesized DNA." Therefore the intermediate DNA of the 2nd generation will get lighter (but still be intermediate) and the light DNA will remain light. The explanation above should eliminate any of the incorrect choices.

Q 4

The author suggests that autoradiographic visualization

A. gave Watson and Crick insights into the structure of DNA

B. made the Meselson-Stahl experiment possible

C. shows that before a cell can divide, the bases of the double helix must break

D. does not, by itself, provide a way to differentiate between original strands of DNA and future generations

E. put an end to the possibility of conservative replication in bacteria

OA in next post.

Official Answers from VeritasPrep:

Q 4

Since semiconservative replication involves the breaking of the double helix (from the 1st paragraph), and since it is known that semiconservative replication is accurate, then autoradiographic visualization, which confirms the Meselson-Stahl experiment, would confirm this.

A - The passage does not mention that these men used autoradiographic visualization.
B - The passage does not state that autoradiographic visualization was employed or relied on in the Meselson-Stahl experiment.
D - This cannot be implied from the passage; autoradiographic visualization might be able to differentiate between the two.
E - The 2nd paragraph states that "Prior to the Meselson-Stahl experiment of 1958, other methods of DNA replication were suggested." Therefore this experiment put an end to the possibility of conservative replication

VeritasKarishma
Do we have a better explanation for question 3? I am not convinced with official explanation and with other explanations provided above.
Why do we assume third generation of bacteria should be of 25% intermediate density?

There are two thing you need to understand:

What is semiconservative replication?
Semiconservative replication - In this process, the bases of the double helix are broken, and the molecule "unzips" into two strands. Each strand then joins with a new strand, so that each copy is an equal mix of old and new DNA.
In semiconservative replication, the two interwoven strands are separated and each joins with a new strand. So one strand will be old and one will be new.

In the Meselson-Stahl experiment, the bacteria grown in heavy nitrogen were then transferred to medium containing the lighter density DNA, which would incorporate into any newly synthesized DNA. So bacteria grown in heavy nitrogen were transferred to light nitrogen so that new DNA has light DNA.
After one generation, the DNA was of a density exactly intermediate to the heavy and light DNA - makes sense since the heavy strands will recombine with the light strands (new ones) to give intermediate DNA.
After two generations, half of the DNA was intermediate and half was light - makes sense since each double helix will have one heavy and one light strand. The heavy will combine with light to give HL (intermediate) and light will combine with light (since new medium is all light) to give LL (light).
After three generations, HL will combine with light to give HL and LL while LL will combine with light to give LL and LL. So we will have 25% intermediate and 75% light (hence some intermediate, some light)

Hence option (A) is correct.

Passage Map- Overall the passage discusses the experiments that led to the semiconservative method as the prevailing and currently accepted theory of DNA replication
P1: Explain basis of Semiconductive method but point out that it wasn't always accepted
P2: Discuss the experiment that led to the invalidation of other DNA replication theories
P3: state that W+C found hypothesised semiconservative method first, but it wasn't validated until M-S experiment

Question 1
In semiconservative replication the basis of the double helix is broken up.

Most info is contained in P2.

E. coli are bacteria, and are thus prokaryotes, organisms that lack complex cell structures, but semiconservative replication also occurs in eukaryotes, organisms with complex cells such as animals and plants. Since each DNA molecule in a eukaryote is incorporated in one chromosome, human chromosome replication is also semiconservative'.

A is incorrect. E.Coli are not Eukaryotes. E.Coli are mentioned to discuss the implications.
B is actually false. We are told at the end of P2 that bacteria lack complex cell structures, yet semiconservative replication occurs in bacteria. So how can it depend on complex structures?
C is incorrect - it wasn't W+C's experiment it was M-S'.
D is correct - look at the last sentence of P2. The discovery was that human chromosome replication was also semiconservative; thus, the experiments had implications on non-E-coli (bacteria) biological matter i.e humans.
E is incorrect. We are literally told E in the last sentence of P2. It is told as a basis for a deduction though (i.e. a fact), not to point out anything.

Question 2

According to the passage, which of the following is the reason that, had the conservative model been correct, the same number of heavy and light strands of DNA would have resulted?

The answer to this comes from P1, not P2 as it may seem.

"In the conservative replication model, a completely new double helix would be formed, leaving the original intact. "

If you used P2 for the answer, as I did, then you'll struggled with this question.

A - the original DNA is left intact. Incorrect
B - the original DNA is left intact, it is not mixed. Don't get confused by the equal mix of densities discussed in P2. Incorrect
C - Correct as per P1.
D - false. This also happens in semiconservative method per sentence 1.
E - false. Original DNA is left intact. Incorrect.

Question 3
The answer to this is in P2. I'll draw a diagram for ease.

"After one generation, the DNA was of a density exactly intermediate to the heavy and light DNA."

DNA Density Scale:
Heavy-----{Gen1 DNA (Intermediate)}-----------------------Light

After two generations, half of the DNA was intermediate and half was light.
Heavy---1/2---{Gen2 DNA}---1/2---Light

Prediction for Gen 3:
Heavy--------{Gen3 DNA}---Light


Based on the trend across Gen 1 to Gen 2, in that each subsequent generation has contained fewer heavy density DNA molecules, we could logically infer that any subsequent generations would continue this trend and contain even fewer heavy density molecules.

Thus

A is correct.
B - close, but we would likely contain intermediate density than heavy density DNA
C - no. The trend is a decease in DNA density
D - no. It would be hard to substantiate complete eradication of one density after only seeing some progress towards lower density DNA.
E - same logic applies to D; thus, incorrect.

Paragraph 1: Semiconservative replication - each strand is an equal mix of old and new DNA.

Paragraph 2: In the past, other methods were suggested. Two opposing methods: conservative replication model and Dispersive replication model.

Paragraph 3: Meselson-Stahl experiment allowed scientists a way to distinguish between old and new DNA -- eliminating possibility of conservative replication. Later on, dispersive replication was also eliminated.

Paragraph 4: Watson and Crick confirmed semiconservative -- actually only a hypothesis. Later on confirmed by Meselson and Stahl.

_________________________________

The author mentions that eukaryotes replicate through semiconservative replication in order to make the point that

A) the choice of E. coli bacteria was a good one for the Meselson-Stahl experiment
B) semiconservative replication depends on an organism having a complex cell structure
C) Watson and Crick realized that their work would have implications for humans
D) the Meselson-Stahl had implications for forms of life beyond simple bacteria
E) each DNA molecule in a eukaryote is incorporated in one chromosome

"This eliminated the possibility of dispersive replication, which would have produced DNA that was all of a density between the light and exactly intermediate DNA. E. coli are bacteria, and are thus prokaryotes, organisms that lack complex cell structures, but semiconservative replication also occurs in eukaryotes, organisms with complex cells such as animals and plants"

The sentences above suggest that the experiment impacted more than simple bacteria.

_________________________________


According to the passage, which of the following is the reason that, had the conservative model been correct, the same number of heavy and light strands of DNA would have resulted?

A) Conservative replication breaks up the original DNA into small segments.
B) An equal mix of old and new DNA is formed using the conservative model.
C) Conservative replication would keep the original strands untouched.
D) The double helix of DNA is separated only in the conservative model of replication.
E) The conservative replication model splits the original DNA into four discrete strands.

Paragraph 2 states "In the conservative replication model, a completely new double helix would be formed, leaving the original intact." The original stands are left untouched.

_________________________________

It can be inferred from the passage that in the Meselson-Stahl experiment, a third generation of bacteria would produce bacteria with

A) some light density and some intermediate density DNA
B) some light density and some heavy density DNA
C) some heavy density and some intermediate density DNA
D) only light density DNA
E) only intermediate density DNA

We have 3 densities: heavy, intermediate, light.

1 generation: intermediate to the heavy and light DNA.

2 generations: half intermediate and half light (no more heavy)

A third generation would produce bacteria with some light density and some intermediate density DNA. Answer is A.

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