In the 1950s the heavy isotope of nitrogen (15N) and an ultracentrifuge were used in an experiment to investigate the manner in which DNA replicates. Bacteria were grown in a culture containing 15N nucleotides for several generations to ensure that only heavy 15N would be found in their DNA. These bacteria were then grown in a culture containing normal 14N nucleotides for one generation only. Bacteria from this generation were then removed, their DNA was extracted and centrifuged. DNA composed of 15N nucleotides has a greater density than DNA formed of normal 14N nucleotides. This process allowed DNA molecules with different densities to collect and form in bands in different regions of the centrifuge tube.

Question

In the 1950s the heavy isotope of nitrogen (15N) and an ultracentrifuge were used in an experiment to investigate the manner in which DNA replicates. Bacteria were grown in a culture containing 15N nucleotides for several generations to ensure that only heavy 15N would be found in their DNA. These bacteria were then grown in a culture containing normal 14N nucleotides for one generation only. Bacteria from this generation were then removed, their DNA was extracted and centrifuged. DNA composed of 15N nucleotides has a greater density than DNA formed of normal 14N nucleotides. This process allowed DNA molecules with different densities to collect and form in bands in different regions of the centrifuge tube.

How did this experiment demonstrate that the replication of a DNA molecule is semi-conservative?

http://en.wikipedia.org/wiki/Meselson-Stahl_experiment

Answer 1

The Meselson-Stahl experiment in the 1950s demonstrated that the replication of a DNA molecule is semi-conservative. Here's how the experiment worked and what it revealed:

1. Bacteria were grown in a culture containing heavy nitrogen isotope, 15N. This ensured that the bacteria DNA incorporated the heavy 15N nucleotides and resulted in the DNA having a higher density.

2. The bacteria were then transferred to a culture containing the normal, lighter nitrogen isotope, 14N, for one generation only. This allowed the bacterial DNA to replicate once in the presence of 14N nucleotides.

3. After one generation in 14N, bacteria were removed, and their DNA was extracted and centrifuged in an ultracentrifuge.

4. Due to the differential densities of the DNA molecules with heavy (15N) and light (14N) nucleotides, the DNA molecules separated into distinct bands in different regions of the centrifuge tube.

5. The results of the experiment showed that the DNA bands formed after replication were intermediate in density compared to the DNA bands composed solely of 15N or 14N nucleotides. This suggested that the DNA replication process involved each original DNA strand serving as a template for the synthesis of a new complementary strand.

6. This conclusion supported the semi-conservative replication model proposed by James Watson and Francis Crick, where each new DNA molecule consists of one original (parental) strand and one newly synthesized (daughter) strand.

Therefore, the Meselson-Stahl experiment demonstrated that during DNA replication, each strand of the original DNA molecule serves as a template for the synthesis of a new complementary strand, resulting in two daughter DNA molecules that each contain one parental strand and one newly synthesized strand, hence showing semi-conservative replication.