Student Prior Knowledge
It is expected that students have a working knowledge of cell types, fundamental structures, and DNA replication. Students have a basic understanding of the differences between prokaryotic and eukaryotic cells to the extent that prokaryotes house their DNA within the cytoplasm rather than in the enclosed nucleus. Students study the differences between bacterial, plant, and animal cells; identifying the differences in structures and functions present in each. Relevant structures covered include: cytoplasm, ribosomes, nucleus, cell membrane, and cell wall. The eighth grade curriculum covers DNA replication and therefore is only revisited during the unit and not explicitly taught.
Students must understand that double-stranded DNA is systematically split apart by the enzyme DNA polymerase in order to produce two new complement strands. DNA polymerase “reads” the nucleotides on the template strands and incorporates base pairs onto the new growing strands.1 DNA is always read moving from the 5’ to 3’ end. This creates a leading strand and a lagging strand. On the leading strand, the new strand of DNA is synthesized in the same direction that the replication fork is traveling; on the lagging strand, the new strand is produced in the opposite direction by creating short DNA fragments, Okazaki fragments, which must be fused together by ligase to form a continuous strand.2 DNA replication takes place at multiple locations along a single strand of DNA. Replication is initiated at numerous points of origin based upon specific sequences of DNA and catalyzed by DNA polymerase and other proteins.3 This process yields two identical copies of DNA that remain joined at the centromere within the nucleus until mitosis, at which time the two copies are split apart.
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