WORKED PROBLEMS

Problem 1

DNA from a strain of bacteria with genotype a⁺ b⁺ c⁺ d⁺ e⁺ was isolated and used to transform a strain of bacteria that was a⁻ b⁻ c⁻ d⁻ e⁻. The transformed cells were tested for the presence of donated genes. The following genes were cotransformed:

What is the order of genes a, b, c, d, and e on the bacterial chromosome?

Solution Strategy

What information is required in your answer to the problem?

The order of genes a, b, c, d, and e on the bacterial chromosome.

What information is provided to solve the problem?

• The donor cells were a⁺ b⁺ c⁺ d⁺ e⁺ and the recipient cells were a⁻ b⁻ c⁻ d⁻ e⁻.
• The combinations of genes that were cotransformed.

For help with this problem, review:

Transformation in Bacteria in Section 9.2.

Solution Steps

In this transformation experiment, gene c⁺ is cotransformed with both genes e⁺ and d⁺, but genes e⁺ and d⁺ are not cotransformed; therefore the c locus must be between the d and e loci:

Gene e⁺ is also cotransformed with gene b⁺; so the e and b loci must be located close together. Locus b could be on either side of locus e. To determine whether locus b is on the same side of e as locus c, we look to see whether genes b⁺ and c⁺ are cotransformed. They are not; so locus b must be on the opposite side of e from c:

Gene a⁺ is cotransformed with gene d⁺; so they must be located close together. If locus a were located on the same side of d as locus c, then genes a⁺ and c⁺ would be cotransformed. Because these genes display no cotransformation, locus a must be on the opposite side of locus d:

Problem 2

Consider three genes in E. coli: thr⁺ (the ability to synthesize threonine), ara⁺ (the ability to metabolize arabinose), and leu⁺ (the ability to synthesize leucine). All three of these genes are close together on the E. coli chromosome. Phages are grown in a thr⁺ ara⁺ leu⁺ strain of bacteria (the donor strain). The phage lysate is collected and used to infect a strain of bacteria that is thr⁻ ara⁻ leu⁻. The recipient bacteria are then tested on medium lacking leucine. Bacteria that grow and form colonies on this medium (leu⁺ transductants) are then replica plated onto medium lacking threonine and onto medium lacking arabinose to see which are thr⁺ and which are ara⁺.

Another group of recipient bacteria are tested on medium lacking threonine. Bacteria that grow and form colonies on this medium (thr⁺ transductants) are then replica plated onto medium lacking leucine and onto medium lacking arabinose to see which are ara⁺ and which are leu⁺. Results from these experiments are as follows:

How are the loci arranged on the chromosome?

Solution Strategy

What information is required in your answer to the problem?

The order of genes thr, leu, and ara on the bacterial chromosome.

What information is provided to solve the problem?

• The genes are located close together on the E. coli chromosome.
• The donor strain is thr⁺ ara⁺ leu⁺ and the recipient strain is thr⁻ ara⁻ leu⁻.
• The percent of cells with cotransduced genes.

For help with this problem, review:

Transduction: Using Phages to Map Bacterial Genes in Section 9.3.

Solution Steps

Notice that, when we select for leu⁺ (the top half of the table), most of the selected cells are also ara⁺. This finding indicates that the leu and ara genes are located close together, because they are usually cotransduced. In contrast, thr⁺ is only rarely cotransduced with leu⁺, indicating that leu and thr are much farther apart. On the basis of these observations, we know that leu and ara are closer together than are leu and thr, but we don’t yet know the order of three genes—whether thr is on the same side of ara as leu or on the opposite side, as shown here:

Notice that, although the cotransduction frequency for thr and leu also is 3%, no thr⁺ ara⁺ cotransductants are observed. This finding indicates that thr is closer to leu than to ara, and therefore thr must be to the left of leu, as shown here: