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?

Verified step by step guidance

1. Understand the concept of DNA replication modes: In conservative replication, the original DNA molecule remains intact, and a completely new copy is made. In contrast, semi-conservative replication results in each new DNA molecule consisting of one original strand and one new strand.
2. Consider the experimental setup: Taylor's experiment involved growing cells in radioactive thymidine, which would incorporate into newly synthesized DNA strands during replication.
3. Analyze the first round of replication: If DNA replication were conservative, after the first round, one daughter DNA molecule would be entirely radioactive (newly synthesized) and the other would be non-radioactive (original).
4. Examine the second round of replication: In conservative replication, the original non-radioactive DNA would remain unchanged, and the radioactive DNA would produce a completely new non-radioactive copy. Thus, you would expect one pair of chromatids to be radioactive and the other pair to be non-radioactive.
5. Predict the experimental outcome: If eukaryotes used conservative replication, after two rounds of replication, you would observe one chromatid pair entirely radioactive and the other pair entirely non-radioactive, with no mixed chromatids.

Key Concepts

DNA Replication

DNA replication is the process by which a cell duplicates its DNA, ensuring that each daughter cell receives an exact copy of the genetic material. It involves unwinding the double helix and using each strand as a template for synthesizing a new complementary strand. This process is crucial for cell division and is typically semi-conservative, meaning each new DNA molecule consists of one old and one new strand.

Semi-Conservative vs. Conservative Replication

In semi-conservative replication, each of the two resulting DNA molecules contains one original strand and one newly synthesized strand. In contrast, conservative replication would result in one molecule with two original strands and another with two new strands. Taylor's experiment demonstrated semi-conservative replication, as the presence of radioactivity in only one chromatid after the second replication cycle indicates that only one strand of each DNA molecule was newly synthesized.

Radioactive Thymidine Labeling

Radioactive thymidine is used to label newly synthesized DNA strands during replication. By incorporating this labeled nucleotide into DNA, researchers can track which strands are newly synthesized. In Taylor's experiment, the presence of radioactivity in chromatids after replication cycles helped determine the mode of DNA replication, as only newly synthesized strands would incorporate the radioactive label.