14. DNA Synthesis

The Meselson-Stahl experiment demonstrated that DNA replication produces new molecules of DNA each containing…

The DNA of a phage was injected into the bacterial host, but the protein coat remained outside. The viral DNA directed the host to replicate the new phage viruses. Which scientist(s) are associated with this finding?

E. coli cells grown on ¹⁵N medium are transferred to ¹⁴N medium and allowed to grow for two more generations (two rounds of DNA replication). DNA extracted from these cells is centrifuged. What density distribution of DNA would you expect in this experiment?

a. One high-density and one low-density band

b. One intermediate-density band

c. One high-density and one intermediate-density band

d. One low-density and one intermediate-density band

In the late 1950s, Herbert Taylor grew bean root-tip cells in a solution of radioactive thymidine (a precursor to one of the deoxyribonucleotides in DNA) and allowed them to undergo one round of DNA replication. He then transferred the cells to a solution without radioactive thymidine, allowed them to replicate again, and examined their chromosomes for the presence of radioactivity. His results are shown in the following figure, where red indicates a radioactive chromatid.

(b) What would the results of Taylor's experiment be if eukaryotes used a conservative mode of DNA replication?

In the late 1950s, Herbert Taylor grew bean root-tip cells in a solution of radioactive thymidine (a precursor to one of the deoxyribonucleotides in DNA) and allowed them to undergo one round of DNA replication. He then transferred the cells to a solution without radioactive thymidine, allowed them to replicate again, and examined their chromosomes for the presence of radioactivity. His results are shown in the following figure, where red indicates a radioactive chromatid.

(a) Draw labeled diagrams of double-stranded DNA molecules that explain the pattern of radioactivity observed in the sister chromatids after the first and second rounds of replication.