Question

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

Short answer

Answer: The basis for chromosome mapping using Hfr × F⁻ crosses lies in the process of conjugation, gene transfer, and recombination between the donor (Hfr) and recipient (F⁻) strains. By analyzing the frequency of gene transfer and recombination in interrupted mating experiments, the linear order and relative distances between genes on the chromosome can be determined.

Step-by-step solution

Understanding Hfr and F⁻ strains

Hfr stands for 'high-frequency recombination', which is a bacterial strain that has the F (fertility) factor integrated into its chromosome. The F factor allows for the transfer of genetic material from the Hfr (donor) strain to the F⁻ (recipient) strain. F⁻ strains are those that lack the F factor and cannot initiate genetic transfer in conjugation, but they can receive genetic material from the Hfr strains.

Conjugation and gene transfer

During conjugation, the Hfr and F⁻ cells come into contact, and a thin tube-like structure, called a pilus, forms between them. The genetic material from the Hfr strain starts to transfer to the F⁻ strain through the pilus. The genes that are closer to the F factor in the Hfr strain are transferred first, followed by those that are farther away.

Recombination and mapping

Once the genetic material from the Hfr strain enters the F⁻ strain, recombination events may occur, replacing the original genes in the F⁻ strain with the newly received genes. After conjugation, the bacterial cultures are exposed to a selective medium that allows for the growth of specific recombinant categories, depending on the genes transferred. By analyzing the frequency of transfer and recombination of different genes, a chromosome map can be created, showing the relative order and distances between the genes on the chromosome.

Interruption of gene transfer

Conjugation between Hfr × F⁻ strains can be interrupted by breaking the pilus, preventing the complete transfer of the donor chromosome to the recipient. By carrying out interrupted mating experiments, where conjugation is terminated at different time intervals, a linear order of gene transfer can be obtained. The order in which genes are transferred corresponds to their position on the Hfr chromosome, and these genetic markers can be used to create a chromosome map.

Calculating recombination frequency

The frequency of gene transfer (recombination frequency) is influenced by the distance between genes on the chromosome, with genes that are closer together having a higher transfer frequency. By comparing the recombination frequency of different gene pairs, the relative distance between these genes on the chromosome can be calculated. In conclusion, the basis for chromosome mapping in Hfr × F⁻ crosses lies in the process of conjugation, gene transfer, and recombination. By analyzing the frequency of gene transfer and recombination in interrupted mating experiments, the linear order and relative distances between genes on the chromosome can be determined.