H

The cells were moved into medium containing light isotopes (14N and 12C)...

Bacteriophage added i

... and infected with bacteriophage.

-Medium with 14N and 12C E. coli culture

Increasing density

After phage proteins were produced, ribosomes were separated by equilibrium density gradient centrifugation.

Only old ribosomes containing heavy isotopes (15N and 13C), were found.

^ New ribosomes produced after phage infection would contain 14N and 12C, and would be relatively light.

After phage proteins were produced, ribosomes were separated by equilibrium density gradient centrifugation.

Increasing density

Only old ribosomes containing heavy isotopes (15N and 13C), were found.

Conclusion: Ribosomes are not produced during phage reproduction.

14.4 Brenner, Jacob, and Meselson demonstrated that ribosomes do not carry genetic information.

labeled ("hot") uracil was added to the medium (which would become incorporated into newly produced phage RNA). After a few minutes, they transferred the cells to a medium that contained unlabeled ("cold") uracil. This type of experiment is called a pulse-chase experiment: the cells are exposed to a brief pulse of label, which is then "chased" by cold, unlabeled precursor. Pulse-chase experiments make it possible to follow, by tracking the presence of the radioactivity, products of short-term biochemical events, such as RNA synthesis immediately following phage infection.

Gros and his coworkers found that the newly produced phage RNA was short lived, lasting only a few minutes, and was associated with ribosomes but was distinct from them. They concluded that newly synthesized, short-lived RNA carries the genetic information for protein structure to the ribosome. The term messenger RNA was coined for this carrier.

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