From Gene to Protein—A Historical Perspective
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Teaching Tips
Ask the students to come up with an analogy for the phenomenon described above. For
example, a student could use a simple scenario: Susie Smith, a student in AP Bio, is trying
to ride up an escalator at the local mall. If the escalator is moving upward and Susie rides
it upward, she travels in the same direction the escalator is moving; she is riding up the
“leading” strand. However, if Susie tries to ride up an escalator that is moving downward,
she will have trouble traveling up this “lagging” strand.
Follow up by asking how Susie can travel up an escalator that is moving down. Can she
ever make it to the top? (Yes, if she skips steps or travels upward in leaps or bounds.)
In contrast to the leading strand, which elongates continuously, the lagging strand is
synthesized as a series of segments—the “leaps and bounds” of the analogy above. As the
helix uncoils, DNA polymerase assembles short segments of nucleotides called Okazaki
fragments, named for the Japanese scientist who discovered them in the 1960s (Campbell
and Reece, 302–303). Another enzyme, ligase, ultimately joins the segments together,
forming a single new strand of DNA.
DNA polymerases cannot initiate the synthesis of a polynucleotide but, rather, can
only add nucleotides to the 3ʹ end of a preexisting chain. e rst nucleotides of the
leading strand and each Okazaki fragment of the lagging strand are initiated by RNA
primase, which, unlike DNA polymerase, can start an RNA chain from scratch. Primase
joins RNA nucleotides together one at a time, making a short segment of RNA primer
complementary to the DNA template strand where replication begins; DNA polymerase
then adds a DNA nucleotide to the 3ʹ end of RNA primer and continues adding
nucleotides to the growing strand according to the base-pairing rules. Only one RNA
primer is required for DNA polymerase to begin synthesizing the leading strand because
there is only one 3ʹ attachment site. However, each Okazaki fragment is primed separately.
Ultimately, appropriate DNA nucleotides replace RNA primers.
Inquiry for Students
Using the molecule of DNA you constructed as part of the embedded classroom assessment,
“From Gene to Protein” (Appendix A), can you model the process of DNA replication?
Materials
In addition to the previously constructed molecule of DNA, students will need construction
paper, scissors, markers, or other media to construct several additional DNA nucleotides
consisting of deoxyribose, phosphate groups, and the nitrogenous bases adenine, thymine,
cytosine, and guanine. Students will also need to construct representations of the enzymes
used in DNA replication, including helicase, DNA polymerase, and ligase. e teacher may
also ask students to construct RNA primers and primase.