Lynn Gordon And Jane Obbink
1993 Woodrow Wilson Biology Institute


Biotechnology, particularly genetic engineering, focuses on manipulating tiny structures in the cell. While carrying out the procedures, many concepts and standard techniques can be difficult to comprehend. Students may not understand the main idea of the procedure if they are not familar with the technique beforehand. Short analogies, stories, and/or models have been developed to help students visualize the overall concept or technique. These analogies are designed to stand alone, and may or may not include a laboratory activity.


Regular or Honors Biology


Five to ten minutes per scenario


Written Scenario only.


The analogies are designed to help students understand some basic concepts of biotechnology at the time the concepts are introduced to the students. It is suggested that the analogies be used before any wet or dry laboratory work with the concept.



Suppose you have a 10 foot banner of which you want to make an exact copy. You would take it to a copy machine, right? But will the entire banner fit on the copy machine? No, only a small portion will (either an 8 X 11 or 8 X 14 inch piece). You can make as many copies as you want, but the copy machine will only copy the selected region of the banner. Polymerase Chain Reaction or PCR works in the same manner. The banner represents a chromosome; it can be copied as many times as you want (called cycles), but only for a small region of the chromosome. Typically PCR does 30 cycles, which will copy the small region of the chromosome 210 times making over a billion copies. Now you have many copies of that chromosome region for procedures such as cloning into vectors, determining tissue types (for organ transplants), and criminal investigations.

(See Scientific American April 1990 for additional background information on the PCR method)


Have you noticed how the same recipe will turn out differently when two different people make it? Use this phenomenon to show how the environment plays a role in determining phenotype.

Distribute one recipe for chocolate chip cookies to each of the students. This is the cookie's genotype and must be identical from student to student. Allow several days for the students to bake the cookies. The students should bring one dozen cookies to class on a specified date. Keeping each student's cookies on a separate plate, put 3-4 plates of cookies on tables scattered around the room. Divide the students into groups of 3-4 and put them at each table. Have the groups look at the cookies from each individual plate and then compare cookies between plates. Questions for students to answer could be:

  1. Do all the cookies on a plate come from the same recipe?

  2. How do the cookies from one plate look similar?

  3. How do the cookies from one plate look different?

  4. What might be some reasons for your responses to Questions 2 and 3?

  5. Name some cookies that are most commonly associated with an ethnic group and/or geographical location. Why might this be so?

The students will discover that both cookies on the same plate and between plates are different. Since the cookies started with the same recipe (genotype), the student bakers (environment) played a role in their final appearance (phenotype).


  • Try this activity with 4 different cookie recipes to represent some genetic diversity.



    Imagine an oval plate with plain spaghetti, Spaghetti O's, and tomato sauce on it. The spaghetti is only one strand, but very long and coiled up on top of itself. The spaghetti strand is attached at one point to the edge of the plate. The Spaghetti O's are smaller circular spaghetti pieces made up of the same material as the longer strand. There are only a couple of Spaghetti O's in the plate and are not attached, but free floating in the tomato sauce.

    With this image in mind, you are imaging a bacterial cell. The oval plate represents the rigid capsule of the bacteria. Inside the bacteria cell, the long strand of spaghetti attached to the edge is the bacteria's main chromosome all coiled up. The Spaghetti O's represent the circular plasmids. As spaghetti and Spaghetti O's are made of the same material, likewise the bacterial chromosome and plasmids are both made up of DNA. The spaghetti and Spaghetti O's float in a tomato sauce, just as the DNA floats in the bacteria cell's cytoplasm.


    Imagine that you work in a manufacturing plant that produces wooden ladders. The company makes their ladders really, really long and has the rungs of the ladder painted with one of four colors: Red, Blue, Green, and Yellow. The color pattern of the rungs is random. It is your job to cut the really, really long ladder into smaller ladders. You are given the color sequence of where to cut the ladder, usually in a six color pattern. For example, every time you find the rung pattern:

    Red, Red, Yellow, Yellow, Blue, Green

    you are to cut the ladder. As in other types of millwork, there are two styles of cutting:

    1. Cut straight across the sides of the ladder to completely sever the ladder. This is called a "Blunt end" cut. (Called a Butt end in lumber terms)

    2. Cut through one side of the ladder and down the middle of the rungs and back out another side of the ladder. This is called a "Sticky end" cut (called a Lap end in lumber terms).

    Your job is to walk along the really, really long ladder and look for the RRYYBG color pattern. When you find it, you cut the ladder (usually with the "Sticky end" pattern) to make a separate, smaller piece. You continue searching down the ladder for this color sequence and repeat the cutting step until you reach the end of the ladder. Now you have cut the one really, really long ladder into many pieces, probably of different lengths.

    A Restriction Enzyme does the same task of cutting a really, really long ladder of DNA. The restriction enzyme moves down the DNA Ladder looking for a specific sequence of bases, rather than colors. When it finds that specific base sequence, the restriction enzyme cuts the DNA (using a Blunt or Sticky end pattern) and a smaller piece is made. The restriction enzyme continues moving down the DNA, cutting at the specific sequence until it has reached the end of the DNA Ladder. Now the DNA is cut into many pieces, probably of different lengths.


    Your next assignment in the wooden ladder manufacturing company is to work out the specifications for making a new ladder. Your boss tells you to make a hybrid ladder that is a combination of two ladders. So you go to work. You use 2 different types of ladders: the really, really long ladder (see How Do Restriction Enzymes Work? scenario for a review) and a circular ladder. You cut both ladders using the same color sequence pattern and "Sticky End" cut-style. You notice that the circular ladder has only one cutting sequence and becomes linear when it is cut. The really, really long ladder is cut up into many pieces. You can't use each piece of this cut ladder, so you decide to use your favorite, smaller piece (called Your Favorite Ladder or YFL). Now you connect YFL with the (linear) circle ladder so the Sticky ends of both pieces match up. Using superglue, you glue the two pieces of ladder together. The hybrid ladder becomes circular again.

    The same scenario occurs for making Recombinant Molecules. You cut a really, really long piece of DNA from an organism with a restriction enzyme. Use the same restriction enzyme to cut a circular plasmid, and change it into a linear piece. Select a favorite piece of DNA from the long-cut ladder (called Your Favorite Gene or YFG). Now you connect YFG with the linearized plasmid so the Sticky Ends of both pieces match up. Using a superglue called Ligase, you glue the two pieces of DNA together. The hybrid DNA becomes circular again and is called a Recombinant Molecule.

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