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 concept of DNA replication: DNA replication is a semi-conservative process where each of the two strands of the DNA helix serves as a template for the synthesis of a new complementary strand. This results in two DNA molecules, each with one old strand and one new strand.
2. Consider the first round of replication: When the bean root-tip cells are grown in radioactive thymidine, the newly synthesized DNA strands incorporate the radioactive thymidine. Therefore, after the first round of replication, each double-stranded DNA molecule will consist of one original non-radioactive strand and one newly synthesized radioactive strand.
3. Visualize the first round of replication: Draw a diagram showing two double-stranded DNA molecules. Each molecule should have one strand labeled as 'non-radioactive' and the other strand labeled as 'radioactive'. This represents the semi-conservative replication where one strand is old and the other is new.
4. Consider the second round of replication: The cells are now in a solution without radioactive thymidine. During this round, each strand (both radioactive and non-radioactive) serves as a template for the synthesis of a new complementary strand. The new strands synthesized in this round will be non-radioactive because they are not exposed to radioactive thymidine.
5. Visualize the second round of replication: Draw a diagram showing four double-stranded DNA molecules. Two molecules will have one radioactive strand and one non-radioactive strand, while the other two molecules will have both strands non-radioactive. This pattern reflects the semi-conservative nature of DNA replication and the absence of radioactive thymidine in the second round.

Key Concepts

DNA Replication

DNA replication is the process by which a cell duplicates its DNA, resulting in two identical copies. It involves unwinding the double helix and synthesizing new strands using the original strands as templates. This process is semi-conservative, meaning each new DNA molecule consists of one original strand and one newly synthesized strand.

Radioactive Labeling

Radioactive labeling involves incorporating radioactive isotopes into molecules to track their presence and movement within biological systems. In this experiment, radioactive thymidine was used to label newly synthesized DNA strands, allowing researchers to visualize and analyze the replication process by detecting radioactivity in the chromatids.

Sister Chromatids

Sister chromatids are identical copies of a chromosome, connected by a centromere, formed during DNA replication. After replication, each chromosome consists of two sister chromatids, which separate during cell division. Observing the pattern of radioactivity in sister chromatids helps understand the distribution of labeled DNA after replication cycles.