Meselson and Stahl grew bacteria for many generations in medium containing heavy
ID: 171925 • Letter: M
Question
Meselson and Stahl grew bacteria for many generations in medium containing heavy 15N, then shifted the bacteria into medium containing light 14N for one, two, or three rounds of DNA replication. The DNA was extracted after each round and the density of the DNA molecules was determined by density gradient centrifugation. After the first round of replication, Meselson and Stahl saw only one DNA band of density intermediate to DNA containing only 15N or 14N. After this observation, which hypothesis for DNA replication could be eliminated?
a: conservative
b: semiconservative
c: one of the above could be ruled out
d: dispersive
Explanation / Answer
The Meselson-Stahl experiment proved that replication of DNA (deoxyribonucleic acid) occurs in semiconservative manner; it states that cell's DNA is composed of one old parental strand and one newly synthesized strand.
Three were 3 alternative models of DNA replication, which include conservative, semiconservative, and dispersive.
The conservative model says that the two DNA strands would separate, and they act as templates to synthesize two new strands, and then rewind.
The dispersive model says that each strand daughter DNA would contain a mixture of new and old parental DNA.
In the 1950's Meselson and Stahl conducted a classic experiment to decide, which one was right.
They used Escherichia coli (E. coli) as their experimental organism, which they cultured for several generations in a medium containing 15N (heavy isotope of nitrogen). They then transferred the bacteria into a new medium containing 14N (lighter isotope of nitrogen). Two DNA samples were subsequently isolated from the bacteria; one after the first replication at 20 minutes and one after the second replication at 40 minutes. They separately centrifuged the sample for identifying the difference in the densities of nitrogen.
If conservative-two clear bands would form; less dense DNA at the top, heavier DNA at the bottom; but first replication sample produced only one band, it proved that conservative replication is wrong.
Hence, the eliminated hypothesis from the given list is (a) Conservative.