Question

Meselson and Stahl were able to demonstrate semiconservative replication in \(E\). coli by a. using radioactive isotopes of phosphorus to label the original strand and visually determining the relationship of original and new DNA strands. b. using different enzymes to eliminate old strands from DNA. c. using isotopes of nitrogen to label the DNA and determining the relationship of original and new DNA strands by density differences of the new DNA molecules. d. labeling viral DNA before it was incorporated into a bacterial cell and visually determining the location of the DNA after centrifugation.

Short answer

The correct answer is Option c. Meselson and Stahl used isotopes of nitrogen to label the DNA and determined the relationship of original and new DNA strands by the density differences of the new DNA molecules.

Step-by-step solution

Understand the Given Choices

Analyze all the given options. Each option describes a different method of observing DNA replication.

Recall Meselson and Stahl's Experiment

Remember the details of the real experiment conducted by Meselson and Stahl. They used nitrogen isotopes to label the DNA and observed the replication through differences in density of the original stand and new DNA molecules.

Identify the Correct Choice

Match the accurate description of the experiment to the options given. The option stating that Meselson and Stahl used nitrogen isotopes to label the DNA and determine the relations of the original and new strands through density differences is the correct one.

Key concepts

Meselson and Stahl experiment

The Meselson and Stahl experiment is a landmark in molecular biology, demonstrating how DNA replicates. Their main achievement was proving the semiconservative model of DNA replication. This model proposes that each new DNA molecule consists of one old strand and one new strand. Their experiment was ingenious and well-designed. They used bacteria, specifically _E. coli_, and employed a clever method to explore DNA replication.

Rather than just hypothesizing, they wanted solid, observable evidence. They chose to use stable isotopes of nitrogen as markers, allowing them to track the DNA molecule's path during replication. Through their experiment, Meselson and Stahl conclusively showed that each new double helix was made by pairing one old strand with a newly synthesized strand. This discovery was pivotal in advancing our understanding of genetic inheritance and DNA replication.

Nitrogen isotopes

Nitrogen isotopes played a crucial role in the Meselson and Stahl experiment. Nitrogen is an essential element in all DNA molecules. It is found in the nucleotide bases that make up the strands of the DNA double helix. Meselson and Stahl used two isotopes of nitrogen: the heavier, rare isotope known as \( ^{15}N \), and the more common, lighter isotope \( ^{14}N \).

Initially, the _E. coli_ were grown in a medium containing \( ^{15}N \), which resulted in DNA that was heavier due to the denser isotope. When these bacteria were then transferred to a \( ^{14}N \) medium, any new DNA polymerized was lighter than the original. By using these isotopes as markers, they could distinguish between old and new strands within the replicated DNA, allowing them to track the process of replication by identifying these differences through clever analytical techniques.

DNA density gradient

The DNA density gradient technique was a central method in the Meselson and Stahl experiment to distinguish between old and new DNA strands. To perform this, centrifugation in a medium like cesium chloride was essential. Centrifugation causes substances in a solution to separate based on density.

When DNA prepared in their experiment was spun in a high-speed centrifuge, it formed distinct bands according to its density. Initially, the DNA, cultivated entirely in \( ^{15}N \), appeared as a single heavy band. As replication occurred in a \( ^{14}N \) environment, hybrid DNA molecules with intermediate densities were formed. In subsequent generations, DNA molecules of both intermediate and lighter densities appeared. These results provided strong evidence of semiconservative DNA replication; each new generation of bacteria consistently showcased one strand from the parent and one newly synthesized strand.

E. coli replication

_Escherichia coli_, commonly abbreviated as _E. coli_, was the organism of choice for Meselson and Stahl's experiments on DNA replication. This bacterium is a favorite model organism in genetics and molecular biology research for several reasons. It is easy to grow in laboratory conditions and has a relatively simple genome.

The selection of _E. coli_ offered practical advantages. For one, its rapid growth cycles meant that multiple generations could be observed over a short period. In the context of the experiment, _E. coli_'s simplicity allowed clear observation of the effects of altering its culture environment, from \( ^{15}N \) enriched media to a \( ^{14}N \) environment.

Using _E. coli_ as a model, Meselson and Stahl could conclusively observe the shifts in DNA density across generations, establishing evidence that spoke to the underlying mechanisms of DNA replication.