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
In minutes, what is the total length of the chromosome in the donor species?

Verified step by step guidance

1. Step 1: Understand the concept of interrupted mating studies. These experiments measure the time it takes for specific genes to be transferred during conjugation, which helps determine the relative positions of genes on a bacterial chromosome.
2. Step 2: Note that the total length of the chromosome corresponds to the time it takes for the last gene to be transferred in the Hfr strain with the longest duration. This is because the chromosome is circular, and the oriT (origin of transfer) can start at different points in different strains.
3. Step 3: Examine the data provided for each Hfr strain. Identify the gene transferred last and its corresponding duration for each strain: Hfr strain 1 (17 minutes for gal), Hfr strain 2 (17 minutes for azi), and Hfr strain 3 (19 minutes for azi).
4. Step 4: Determine the maximum duration among the three strains. This maximum duration represents the total length of the chromosome in minutes, as it accounts for the longest observed transfer time.
5. Step 5: Conclude that the total length of the chromosome in the donor species is equal to the maximum duration observed in the interrupted mating study, which is the longest time required for the transfer of the last gene.

Key Concepts

Hfr Strains and Conjugation

Hfr (high frequency of recombination) strains are bacterial strains that have integrated a plasmid into their chromosome, allowing them to transfer chromosomal genes to a recipient bacterium during conjugation. This process involves the formation of a pilus and the transfer of genetic material, which can be mapped based on the time it takes for different genes to be transferred.

Gene Mapping by Time of Transfer

In interrupted mating studies, the time taken for specific genes to be transferred from the donor to the recipient is used to determine their relative positions on the chromosome. The earlier a gene is transferred, the closer it is to the origin of transfer (oriT). By analyzing the duration of gene transfer, researchers can estimate the distances between genes and the total length of the chromosome.

Chromosome Length Calculation

The total length of the chromosome can be calculated by summing the time intervals for the transfer of all genes from the oriT to the last gene transferred. Each time interval represents the distance between genes, and by adding these intervals, one can derive the total length of the chromosome in minutes, which can be converted to physical distance if needed.