Chapter 1. How Do We Know 12.2

1.1 Figure 12.2

How is DNA replicated?

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

Instructions: Read through Figure 12.2 carefully and then answer the questions below:

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.

Question 1.6

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

Correct.

Incorrect.

Question 1.7

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

Correct.

Incorrect.

Question 1.8

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

Correct.

Incorrect.

Question 1.9

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

Correct.

Incorrect.

Question 1.10

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

Correct.

Incorrect.

Question 1.11

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.

kcVtUDKfaIWTKrqEI7di7A==	One light band, one heavy band
kcVtUDKfaIWTKrqEI7di7A==	One intermediate density band
R6Xlb9MAHXF7O4A0MNT1YQ==	One light band, one intermediate density band
kcVtUDKfaIWTKrqEI7di7A==	One intermediate density band, one heavy band
R6Xlb9MAHXF7O4A0MNT1YQ==	25% of the newly synthesized DNA molecules contain one strand of heavy DNA
kcVtUDKfaIWTKrqEI7di7A==	50% of the newly synthesized DNA molecules contain one strand of heavy DNA
kcVtUDKfaIWTKrqEI7di7A==	75% of the newly synthesized DNA molecules contain one strand of heavy DNA
kcVtUDKfaIWTKrqEI7di7A==	100% of the newly synthesized DNA molecules contain one strand of heavy DNA

Correct.

Incorrect.

Question 1.12

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.

kcVtUDKfaIWTKrqEI7di7A==	One light band, one heavy band
kcVtUDKfaIWTKrqEI7di7A==	One intermediate density band
kcVtUDKfaIWTKrqEI7di7A==	One light band, one intermediate density band
R6Xlb9MAHXF7O4A0MNT1YQ==	One intermediate density band, one heavy band
kcVtUDKfaIWTKrqEI7di7A==	25% of the newly synthesized DNA molecules contain one strand of heavy DNA
R6Xlb9MAHXF7O4A0MNT1YQ==	50% of the newly synthesized DNA molecules contain one strand of heavy DNA
kcVtUDKfaIWTKrqEI7di7A==	75% of the newly synthesized DNA molecules contain one strand of heavy DNA
kcVtUDKfaIWTKrqEI7di7A==	100% of the newly synthesized DNA molecules contain one strand of heavy DNA
kcVtUDKfaIWTKrqEI7di7A==	25% of the newly synthesized DNA molecules contain one strand of light DNA
R6Xlb9MAHXF7O4A0MNT1YQ==	50% of the newly synthesized DNA molecules contain one strand of light DNA
kcVtUDKfaIWTKrqEI7di7A==	75% of the newly synthesized DNA molecules contain one strand of light DNA
kcVtUDKfaIWTKrqEI7di7A==	100% of the newly synthesized DNA molecules contain one strand of light DNA

Correct.

Incorrect.