BACKGROUND Watson and Crick’s discovery of the structure of DNA in 1953 suggested a mechanism by which DNA is replicated. Experimental evidence came with research by American molecular biologists Matthew Meselson and Franklin Stahl in 1958.
HYPOTHESIS DNA replicates in a semiconservative manner, meaning that each new DNA molecule consists of one parental strand and one newly synthesized strand.
ALTERNATIVE HYPOTHESIS DNA replicates in a conservative manner, meaning that one DNA molecule consists of two parental strands, and the other consists of two newly synthesized strands.
METHOD Meselson and Stahl distinguished parental strands (“old”) from newly synthesized strands (“new”) using two isotopes of nitrogen atoms. Old strands were labeled with a heavy form of nitrogen with an extra neutron (15N), and new strands were labeled with the normal, lighter form of nitrogen (14N).
EXPERIMENT The researchers first grew bacterial cells on medium containing only the heavy 15N form of nitrogen. As the cells grew, 15N was incorporated into the DNA bases, resulting, after several generations, in DNA containing only 15N. They then transferred the cells into medium containing only light 14N nitrogen. After one round of replication in this medium, cell replication was halted. The researchers could not observe the DNA directly, but instead they measured the density of the DNA by spinning it in a high-speed centrifuge in tubes containing a solution of cesium chloride.
Figure 12.2
Part I: Understanding the Meselson and Stahl Experiment
How Do We Know
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Instructions: Read through Figure 12.2 carefully and then answer the questions below:
1.
What does “semi-conservative” DNA replication mean? One newly synthesized DNA molecule created after one round of replication consists of …
A.
B.
C.
D.
E.
F.
G.
H.
2.
Meselson and Stahl’s alternative hypothesis was that DNA replicates conservatively. What does “conservative” DNA replication mean? One newly synthesized DNA molecule created after one round of replication consists of…
A.
B.
C.
D.
E.
F.
G.
H.
3.
Meselson and Stahl’s experimental setup required growth of E. coli bacteria on media containing either “heavy” 15N or “light” 14N. Which of the following most accurately describes the characteristics of a bacterial chromosome? Full points will only be given for choosing all that apply.
Bacteria have single-stranded DNA molecules.
Bacteria have double-stranded DNA molecules.
Bacterial DNA is composed of Adenine, Thymine, Cytosine and Guanine nitrogenous bases.
Bacterial DNA is composed of Adenine, Uracil, Cytosine and Guanine nitrogenous bases.
Bacteria have a single circular chromosome.
Bacteria have a single linear chromosome.
Bacteria have multiple linear chromosomes.
Bacterial chromosomes have a single origin of replication.
Bacterial chromosomes have multiple origins of replication.
Correct.
Incorrect.
4.
Meselson and Stahl’s experimental setup required growth of E. coli bacteria on media containing either “heavy” 15N or “light” 14N. When DNA replicates, where is the “heavy” 15N or “light” 14N incorporated into a molecule of DNA?
A.
B.
C.
D.
E.
5.
What are the similarities and difference between “heavy” 15N and “light” 14N ?
15N has one more than 14N.
15N and 14N have the same number of and .
The molecular weight of 15N is as 14N.
Correct.
Incorrect.
Question 1 of 7
After two rounds of replication, DNA was extracted from the bacterial cells and centrifuged the DNA in a solution of cesium chloride. Cesium chloride (CsCl) solution will produce a density gradient when centrifuged for several hours, with a high density at the bottom of the centrifuge tube, and a lower density at the top of the centrifuge.
When another substance, such as DNA, is added to the tube and centrifuged, that substance equilibrates at a location where its density matches that of the CsCl.
6.
After one round of replication in the presence of 14N & centrifugation, Meselson and Stahl only observed one band of DNA at an intermediate density. Why?
A.
B.
C.
D.
Correct.
Incorrect.
7.
After one round of replication in the presence of 14N & centrifugation, what would you expect given the conservative model of DNA replication (Meselson and Stahl’s alternative hypothesis)?
A.
B.
C.
D.
Correct.
Incorrect.
8.
After one round of replication in the presence of 14N & centrifugation, could Meselson and Stahl rule out their alternative hypothesis (conservative DNA replication)? Why or why not?
A.
B.
C.
Correct.
Incorrect.
9.
After two rounds of replication in the presence of 14N & centrifugation, what would you expect given the conservative model of DNA replication (Meselson and Stahl’s alternative hypothesis)?
A.
B.
C.
D.
Correct.
Incorrect.
10.
After two rounds of replication in the presence of 14N & centrifugation, Meselson and Stahl observed two bands – intermediate density and light density DNA. They began the experiment with heavy DNA. What happened to the heavy DNA?
A.
B.
C.
Correct.
Incorrect.
11.
What results would you expect to see following three rounds of replication in the presence of 14N & centrifugation? Select all that apply to receive full credit.
One light band, one heavy band
One intermediate density band
One light band, one intermediate density band
One intermediate density band, one heavy band
25% of the newly synthesized DNA molecules contain one strand of heavy DNA
50% of the newly synthesized DNA molecules contain one strand of heavy DNA
75% of the newly synthesized DNA molecules contain one strand of heavy DNA
100% of the newly synthesized DNA molecules contain one strand of heavy DNA
Correct.
Incorrect.
12.
If Meselson and Stahl first grew the E. coli cells in 14N media, moved them to 15N media, and then allowed for cells to go through two rounds of replication, what would their results have been? Select all that apply to receive full credit.
One light band, one heavy band
One intermediate density band
One light band, one intermediate density band
One intermediate density band, one heavy band
25% of the newly synthesized DNA molecules contain one strand of heavy DNA
50% of the newly synthesized DNA molecules contain one strand of heavy DNA
75% of the newly synthesized DNA molecules contain one strand of heavy DNA
100% of the newly synthesized DNA molecules contain one strand of heavy DNA
25% of the newly synthesized DNA molecules contain one strand of light DNA
50% of the newly synthesized DNA molecules contain one strand of light DNA
75% of the newly synthesized DNA molecules contain one strand of light DNA
100% of the newly synthesized DNA molecules contain one strand of light DNA