An interrupted mating study is carried out on Hfr strains 1, 2, and 3 identified in Problem 17. After conjugation is established, a small sample of the mixture is collected every minute for 20 minutes to determine the distance between genes on the chromosome. Results for each of the three Hfr strains are shown below. The total duration of conjugation (in minutes) is given for each transferred gene.

Hfr strain 1           oriT     met     ala     lac     gal
Duration (min)       0          2        8       13      17
Hfr strain 2           oriT     met     leu     thr      azi
Duration (min)       0          2        7       10      17
Hfr strain 3           oriT     gal      pro     trp      azi
Duration (min)       0          3        8       14      19

Explain why azi is the last gene of strain 2 to transfer in the 20 minutes of conjugation time. How many minutes of conjugation time would be needed to allow the next gene on the map to transfer from Hfr strain 2?

Key Concepts

Hfr Strains and Conjugation

Hfr (high frequency of recombination) strains are bacterial strains that have integrated a plasmid into their chromosomal DNA, allowing them to transfer chromosomal genes to a recipient bacterium during conjugation. This process involves the formation of a pilus that connects two bacteria, enabling the transfer of genetic material. The order and timing of gene transfer can provide insights into the relative positions of genes on the chromosome.

Gene Transfer Timing

The timing of gene transfer during conjugation is crucial for mapping bacterial chromosomes. Genes that are closer to the origin of transfer (oriT) are transferred earlier, while those further away are transferred later. By analyzing the duration of gene transfer, researchers can infer the relative distances between genes, allowing for the construction of a genetic map based on the sequence of gene transfer.

Azi Gene Positioning

In the context of Hfr strain 2, the azi gene is the last to be transferred, indicating its position is farthest from the oriT. The time taken for each gene to transfer reflects their physical distance on the chromosome. To determine how many additional minutes are needed for the next gene to transfer after azi, one must consider the established transfer order and the total conjugation time required for the entire gene sequence.