LABORATORY INVESTIGATION : How Do Molecules of DNA Replicate?

LABORATORY INVESTIGATION How Do Molecules of DNA Replicate? INTRODUCTION In 1953, Francis Crick and James Watson discovered the structure of the DNA molecule. This discovery enabled other researchers to begin to determine how molecules of DNA function. Like many other polymers, a strand of DNA is made of many individual connected subunits. A molecule of DNA consists of two parallel strands. The subunits on one strand are connected to the subunits on the other strand. It is the connections between the two strands, which can be broken and reformed, that enable DNA to make a copy of, or replicate, itself. In this laboratory investigation, you will use paper “models” of DNA subunits and portions of DNA strands to learn how this remarkable chemical is constructed and can be copied. MATERIALS Diagrams for steps 1–4 (from Teacher’s Manual), scissors, blank paper, tape PROCEDURE Step 1. Cut out the six nucleotides in the handout. Assemble them into a double strand of DNA that consists of three pairs of nucleotides that are joined together. (Hint: The phosphates attached to the sugars point up on one strand and down on the other strand. Biochemists say the two strands are “antiparallel.”) Tape the pieces into place on a sheet of paper. Step 2. Cut out the four nucleotides from the second handout. Match them up correctly according to rules for nitrogenous-base pairing. Remember that the hydrogen bonds (dotted lines) coming from one base must match up with the hydrogen bonds in its partner. Tape the pieces into place on a sheet of paper. Step 3. Refer to the diagram for step 3. Use the rules that govern the pairing of bases. Write the complementary base on the unlettered strand that will bond with the base shown on the lettered strand. Step 4. Refer to the diagram for step 4. Show the replication of the 14- base-pair DNA molecule. Write in the nitrogenous base pairs after the strands have separated. Then show all the base pairs for molecules A and B. Compare these two molecules to the original. INTERPRETIVE QUESTIONS 1. From the results in step 2, determine the two reasons why each base can pair with only one of the three types of nitrogenous bases. 2. From the results of all four steps, write a paragraph that describes how DNA is a well-organized molecule. Explain why this organization makes it possible for DNA to be the genetic material for all organisms.