Dna Replication

Answer (b) Ti plasmid
Ti- plasmid. It is the tumour inducing plasmid and is a circular DNA molecule that is found in the strains of Agrobacterium tumefaciens. It will help in the transfer of the genetic material into the genome of the plants. The plasmid also have oncogenes that cause the abnormal growth of the galls in the infected plants and also the virulence genes. These genes help in the interaction between the bacteria and the plants.

The question pertains to the use of radioactive thymidine to trace the synthesis of DNA within chromosomes. Let's analyze the given options and discern which plant species were used in such experiments:

1. Understanding Radioactive Thymidine: Radioactive thymidine is a radioactive form of thymidine (a nucleoside component of DNA) used in scientific experiments to track DNA synthesis because it gets incorporated into new strands of DNA during replication.
2. Choice Analysis:
   • Vicia faba: Commonly known as the broad bean, Vicia faba was extensively used in cytogenetic studies, notably in experiments involving radioactive thymidine. Its larger chromosomes allow for easier observation under a microscope.
   • Pisum satirum: Also known as the garden pea, it is famous for Mendel's genetic experiments, but not specifically known for studies involving radioactive thymidine.
   • Cocus nucifera: The coconut plant, primarily used in studies related to tropical agriculture, does not have a significant history in chromosome marking experiments using radioactive thymidine.
   • Antirrhinum: Known for its role in genetics as a model organism, particularly in studying flower development and mutations, but not inherently linked to radioactive thymidine experiments.
3. Conclusion: The experiments involving radioactive thymidine for detecting new DNA synthesis in chromosomes were primarily performed on Vicia faba due to its cytogenetically favorable features.

Therefore, the correct answer is Vicia faba.

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.

This scenario describes the setup of the classic Meselson and Stahl experiment, which demonstrated the semi-conservative nature of DNA replication.

1. Initial State (Parent DNA): The *E. coli* are initially grown in a medium containing the heavy isotope of nitrogen, ¹⁵N. This means that both strands of the DNA double helix in the parent bacteria are composed of nucleotides containing ¹⁵N. Let's represent this parent DNA as ¹⁵N/¹⁵N. This DNA has a high density.
2. Transfer and First Generation: The bacteria are then transferred to a medium containing only the normal, lighter isotope of nitrogen, ¹⁴N. They are allowed to replicate once (first generation).
3. Semi-conservative Replication: According to the semi-conservative model:
   • The parent DNA molecule (¹⁵N/¹⁵N) unwinds.
   • Each of the original ¹⁵N strands serves as a template for the synthesis of a new complementary strand.
   • Since the available nucleotides are now ¹⁴N-labeled, the newly synthesized strands will contain ¹⁴N.
   • Therefore, each new DNA molecule formed after one generation will consist of one original "heavy" strand (¹⁵N) and one newly synthesized "light" strand (¹⁴N). These are hybrid DNA molecules, represented as ¹⁵N/¹⁴N.
4. Analyzing the Strands of First Generation DNA: The question asks about the two strands within a single DNA molecule of the first generation bacteria.
   • One strand contains ¹⁵N (the original parent strand).
   • The other strand contains ¹⁴N (the newly synthesized strand).
   • Since ¹⁵N and ¹⁴N have different masses, the two strands within the molecule have different densities.
   • Does this molecule "resemble" the parent DNA? Yes, because one of its strands (the ¹⁵N strand) is directly inherited from the parent molecule. It retains half of the original parental material.

Therefore, the two strands of a DNA molecule in the first generation bacteria have different densities but resemble their parent DNA (as one strand is parental).

The concept of semiconservative DNA replication was first demonstrated by the scientists Matthew Meselson and Franklin Stahl in 1958 using Escherichia coli (E. coli) bacteria. They conducted an elegant experiment known as the Meselson-Stahl experiment to investigate how DNA is replicated.

In their experiment, Meselson and Stahl labeled the DNA of E. coli bacteria with a heavy isotope of nitrogen (15N) by growing the bacteria in a medium containing this isotope. They then transferred the labeled bacteria to a medium containing a lighter isotope of nitrogen (14N) and allowed them to replicate their DNA.