Question

Experiments involving use of radioactive thymidine to detect distribution of newly synthesized DNA in the chromosome was performed on which plant ?

• A. Vicia faba
• B. Pisum Satirum
• C. Cocus nucifera
• D. Antirrhinum

Answer 1

The Correct Option is A

The question refers to the experiments conducted to demonstrate that DNA replication in eukaryotic chromosomes is semi-conservative, similar to how Meselson and Stahl showed it for *E. coli* DNA molecules.

This specific experiment using radioactive thymidine (specifically, tritium-labeled thymidine, [³H]thymidine) and autoradiography to visualize the distribution of the labeled, newly synthesized DNA in chromosomes over successive cell divisions was performed by J. Herbert Taylor, Philip Woods, and Walter Hughes in 1957. 

They used the root tip cells of the broad bean plant, Vicia faba, as their experimental organism. *Vicia faba* was chosen because it has relatively large and easily distinguishable chromosomes, making it suitable for cytological studies and autoradiography.

The experiment involved:

1. Growing root tips in a medium containing radioactive thymidine, which gets incorporated into newly synthesized DNA during the S phase.
2. Transferring the roots to a non-radioactive medium and allowing cells to undergo mitosis.
3. Observing the distribution of radioactivity in the chromosomes at metaphase of the first and subsequent mitotic divisions using autoradiography.

Their results showed that after one replication, both chromatids of each chromosome were labeled. After a second replication in non-radioactive medium, only one of the two sister chromatids in each chromosome was labeled, consistent with the semi-conservative model of replication at the chromosome level.

Therefore, the plant used for this experiment was Vicia faba.

Answer 2

In 1958, Taylor and colleagues conducted experiments using radioactive thymidine to trace the distribution of newly synthesized DNA within chromosomes. They utilized Vicia faba (broad bean) as their model organism. This research provided evidence for the semi-conservative replication of DNA, demonstrating that each new DNA molecule consists of one original strand and one newly synthesized strand.

The correct answer is (A) : Vicia faba.