Final answer:
Hfr strains of E. coli, which contain F plasmids integrated into the bacterial chromosome, are essential for gene mapping as they reveal the order and linkage of genes through conjugation experiments. Differences in Hfr strains help determine the sequence of genes and their relative distances on the chromosome, thereby creating a map of the bacterial genome.
Step-by-step explanation:
The Hfr strains of E. coli play a crucial role in gene mapping due to their ability to transfer parts of the bacterial chromosome into a recipient F- cell during conjugation. Each Hfr strain has the F plasmid integrated into a different location on the bacterial chromosome. When an Hfr cell conjugates with an F- cell, it can initiate transfer of chromosomal DNA starting at the integration site of the F plasmid and proceeding in a linear fashion. However, because the connection between the two cells is temporary, usually not the entire chromosome is transferred. The order in which genes are transferred and integrated into the recipient F- cell's chromosome corresponds to their locations on the donor's chromosome. By using different Hfr strains with the F plasmid integrated at various sites, researchers can determine the sequence and relative positions of genes on the chromosome. The experimental data have shown that genes are arranged linearly on bacterial DNA, and the timing of gene transfer has been so refined that it also reflects the size of the genes.
For example, using Hfr strain 4 in a conjugation experiment may result in the transfer of genes in the order of V-W-X-Y-Z-A-B if the conjugation is allowed to proceed for longer periods, allowing scientists to determine their linkage and the order of genes on the chromosome. The question regarding the chemical rationale behind using different salt concentrations to extract different nucleosome structures relates to the physical properties of chromatin. Higher salt concentrations are typically required to extract 30 nm solenoid structures due to the stronger interactions between nucleosomes in these more compact forms of chromatin. In contrast, 10 nm nucleosome fibers, which are less tightly packed, can be extracted with lower salt concentrations. The salt disrupts the ionic bonds between histones and DNA as well as the histone-histone interactions, leading to the release of nucleosome fibers of varying compactness.