Cracking the AP Biology Exam

15

Laboratory

LAB 6: MOLECULAR BIOLOGY

In this lab, the principles of genetic engineering were studied. This technology permits the manipulation of genetic material.

What are the general concepts you really need to know?

• This is one lab ETS loves to test. We already know that bacteria can accept fragmnets of foreign DNA and incorporate them into their own chromosomes. In Chapter 9, we mentioned three ways in which bacteria accomplish this: conjugation, transformation, and transduction. In addition, DNA can also be inserted into bacteria using plasmids. Plasmids are small circular DNA fragments that can serve as a vector—a vehicle—to incorporate genes into the host’s chromosome. Plasmids are key elements in genetic engineering.
• One way to incorporate specific genes into a plasmid is to use restriction enzymes, which cut the foreign DNA at specific sites, producing DNA fragments. A specific fragment can then be mixed together with a plasmid. The resulting recombinant plasmid is then taken up by E. coli.
• Plasmids can give a transformed cell a selective advantage. For example, if a plasmid carries genes that confer resistance to antibiotics, such as ampicillin, it can transfer these genes to the bacteria. This bacteria is then said to be transformed. That means if ampicillin is in the culture, only transformed cells will grow.
• Be familiar with the technique of electrophoresis. This technique is used in genetic engineering to separate and identify DNA fragments. First, the DNA is cut with various restriction enzymes. Then the fragments are “loaded” into wells on a special gel called an agarose gel. As electricity courses through the gel, the fragments move across it according to their molecular weights.

• In the end, the distance of each fragment is recorded. This technique is especially helpful in identifying a mutant gene for such diseases as sickle-cell anemia.
• The bottom line is this: You can use plasmids to transform nonresistant bacteria into resistant bacteria.