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.

Verified step by step guidance

1. Understand the experiment setup: Herbert Taylor's experiment involved incorporating radioactive thymidine into newly synthesized DNA strands during the first round of replication. In the second round of replication, non-radioactive thymidine was used.
2. Recognize the semi-conservative nature of DNA replication: Each new DNA molecule consists of one old (template) strand and one newly synthesized strand. This is crucial to understanding the distribution of radioactivity in the chromatids.
3. Visualize the first round of replication: After the first round of replication with radioactive thymidine, each double-stranded DNA molecule will have one strand containing radioactive thymidine and one non-radioactive strand. This results in each chromosome having one radioactive chromatid (red) and one non-radioactive chromatid.
4. Visualize the second round of replication: In the second round, using non-radioactive thymidine, each of the strands from the first round serves as a template. This results in two types of double-stranded DNA: one with both strands non-radioactive and one with one radioactive strand (from the first round) and one non-radioactive strand.
5. Draw the diagrams: For the first round, draw chromosomes with one red (radioactive) and one black (non-radioactive) chromatid. For the second round, draw some chromosomes with both chromatids black (non-radioactive) and some with one red and one black chromatid, reflecting the semi-conservative replication and the use of non-radioactive thymidine in the second round.