Question

Describe the basis for chromosome mapping in the Hfr \(\times \mathrm{F}^{-}\) crosses.

Short answer

Question: Describe the importance of conjugation, recombination, and interrupted mating in chromosome mapping using Hfr × F⁻ crosses. Answer: Conjugation allows for the transfer of genetic material between Hfr and F⁻ cells, while recombination leads to the incorporation of the transferred genes into the F⁻ cell's chromosome, creating recombinant cells with new gene combinations. Interrupted mating is a technique that stops the conjugation process at different time intervals, enabling scientists to analyze the presence of genes in recombinant cells at various time points. This provides information about the relative order and distance of genes along the bacterial chromosome, allowing for the creation of a chromosome map that helps in understanding the organization and function of bacterial genes.

Step-by-step solution

Introduction to Hfr and F⁻ cells

Hfr is a type of bacterial cell that contains an integrated F (fertility) plasmid in its chromosome. The F plasmid is responsible for the conjugation process, which allows for the transfer of genetic material between bacteria. F⁻ cells are bacterial cells without an F plasmid, meaning they cannot undergo the conjugation process and cannot transfer genetic material to other cells.

Conjugation process in Hfr × F⁻ crosses

In the Hfr × F⁻ crosses, the Hfr cell transfers a portion of its chromosome, including the integrated F plasmid, to the F⁻ cell through a sex pilus (a tube-like structure that connects the two cells). The Hfr cell begins by transferring the genes closest to the F plasmid integration site and progresses linearly along the chromosome in a time-dependent manner.

Recombination of transferred genes

Once the F⁻ cell receives the DNA from the Hfr cell, it can incorporate the new genes into its chromosome through a process called recombination. This results in the exchange of genetic material and can lead to the creation of recombinant F⁻ cells with new combinations of genes from both parent cells.

Interrupted mating technique

In order to map the genes on the bacterial chromosome, scientists use a technique called interrupted mating. This involves stopping the conjugation process at different time intervals and then examining the resulting F⁻ recombinant cells. The longer the conjugation process, the more genes will be transferred to the F⁻ cell. By analyzing which genes are present in the recombinant cells at different time points, scientists can order the genes along the chromosome based on their relative distances from the origin of transfer (the F plasmid integration site).

Chromosome mapping in Hfr × F⁻ crosses

The combination of conjugation, recombination, and interrupted mating in Hfr × F⁻ crosses allows for the determination of the relative order and distance of genes along the bacterial chromosome. By examining the frequency of gene transfer at different time intervals, a map of the bacterial chromosome can be created. This method is useful for understanding the organization and function of bacterial genes.