Passage VI

DNA molecules contain a linear sequence of base pairs (2 nitrogenous bases bonded together). There are

2 kinds of base pairs. Different DNA molecules have different base-pair sequences.

A restriction enzyme (RE) can cut a DNA molecule at a specific base-pair sequence, producing shorter DNA fragments. Three studies were done with DNA and REs. (In the 3 studies, the action of each RE did not affect the action of any other RE, and each RE cuts DNA at a different sequence. )

Study 1

A series of trials were performed with 2 REs and a DNA molecule that was 100 base pairs (bp) in length. Table 1 shows the RE (s) used in each trial, the number of DNA fragments produced after the original DNA molecule was exposed to the RE (s), and the length of the DNA fragments.

Using this data, a map of the original DNA molecule was drawn to show the location of the RE cuts (see Figure 1).

Study 2

Study 1 was repeated with a different 100 bp DNA molecule, the 2 REs used in Study 1, and 4 additional REs. The results appear in Table 2.

Study 3

Study 2 was repeated with a 100 bp DNA molecule that partially overlaps with the DNA molecule used in Study 2 and includes the DNA sites that were cut by RE1 and RE4 in Study 2. Figure 2 shows a map of this molecule, indicating the location of the RE cuts.

33. A researcher discovered that when the DNA molecule used in Study 3 is exposed to a seventh RE (RE7), a single cut is made that is 15 bp from the cut made by RE1 and 10 bp from the cut made by RE3. If this DNA molecule is exposed to only RE7, the lengths of the 2 DNA fragments produced would be:

Answer and Explanation

Your Answer is

Correct Answer is C

Explanation

See Figure 2, the tangent points of RS1 and RS3 are at 85 and 60 respectively, and the distance is 85-60=25. If the cut point of RS7 is 15 bp away from the cut point of RS1, and 10 bp away from the cut point of RS3, it means that the cut point of RS7 should be exactly at position 70, between the cut points of RS3 and RS1. Therefore, the lengths of the two fragments cut out by RS7 should be 70 and 30 respectively.