Dna Replication

The experimental proof for semiconservative replication of DNA was first demonstrated by Matthew Meselson and Franklin Stahl in 1958 using bacteria. The bacterium Escherichia coli (E. coli) was used in their experiment to conclusively show how DNA replicates in a semiconservative manner. Let's break down the reasoning and why "Bacterium" is the correct answer:

• Understanding Semiconservative Replication:
  • In semiconservative replication, each of the two strands of the original DNA molecule serves as a template for the formation of a complementary new strand. Thus, each new DNA molecule consists of one old (parental) strand and one newly synthesized strand.
• The Experiment:
  • Meselson-Stahl Experiment: They grew E. coli in a medium containing a heavy isotope of nitrogen, ^{15}N , until the DNA of the bacteria was fully labeled with the heavy nitrogen.
  • Then, they transferred the bacteria to a medium containing the normal isotope of nitrogen, ^{14}N , and allowed the bacteria to replicate.
  • After each round of replication, the DNA was extracted and analyzed using equilibrium density gradient centrifugation.
  • After one round of replication, the DNA formed a single band intermediate between the expected native DNA density and the density of fully ^{15}N -labeled DNA, supporting the semiconservative model.
• Excluding Other Options:
  • Fungus, Plant, Virus: None of these were used in the original experiments that demonstrated semiconservative replication. Although DNA replication occurs in fungi, plants, and viruses, the initial experimental proof came from work with bacteria.

Thus, the correct answer is Bacterium.

To determine which vector is commonly used for introducing a DNA fragment in human lymphocytes, let's analyze the options provided:

1. Retrovirus:
   • Retroviruses are widely used in gene therapy for transferring genetic material into the cells. They have the unique ability to integrate their own genetic material into the host cell's genome. This makes them particularly useful for introducing DNA fragments into dividing human cells, such as lymphocytes, as they ensure stable expression.
   • Retroviruses are often engineered to be non-pathogenic and are a popular choice for ex vivo gene therapy, where cells are modified outside the body and then reintroduced.
2. λ Phage:
   • Lambda (λ) phage is a virus that infects bacteria (specifically, E. coli). It is commonly used as a cloning vector in bacteria rather than in human cells.
   • Due to its specificity for bacterial systems, it is not ideal for use in human lymphocyte genetic engineering.
3. Ti Plasmid:
   • Ti plasmid is derived from the bacterium Agrobacterium tumefaciens, which is used in plant genetic engineering.
   • This vector is particularly useful in modifying plant cells but not typically used for human lymphocytes.
4. pBR322:
   • pBR322 is a plasmid vector commonly used in bacterial transformations and cloning applications.
   • It is not suitable for use in human cells or for gene therapy involving lymphocytes.

Based on the above analysis, Retrovirus is the most appropriate and commonly used vector for introducing DNA fragments into human lymphocytes, primarily due to its ability to integrate genetic material into the host cell's genome, making them ideal for genetic engineering in this context.